Multifunctional nanocomposite assessment using carbon nanotube fiber sensors

Multifunctional nanocomposite assessment using carbon nanotube fiber sensors | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Multifunctional nanocomposite assessment using carbon nanotube fiber sensors Hassaan A. Butt, Dmitry V. Krasnikov, Vladislav A. Kondrashov, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4376476/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Here, we propose a novel application of carbon nanotube fibers (CNTFs) for the one-step, dual-stage, non-destructive monitoring of multifunctional conductive nanocomposites. Hierarchical nanocomposites were created by embedding CNTFs into carbon nanotube (CNT) - modified matrices during their manufacturing to assess production variables. CNTFs are then left embedded in the structure for monitoring during nanocomposite application. We investigated the dependence of detection sensitivity and reliability on the CNTF diameter (~ 40–700 µm), electrical conductivity (~ 10 2 -10 4 S/m), and the choice of measurement technique (2- and 4-point) for single-walled and multiwalled CNT fillers at different concentrations. The sensors showed promising sensitivity to CNT type and concentration, the results were independent of CNTF diameter and contact resistance, and showed low noise. For application monitoring, nanocomposites electrical and mechanical (tensile and cyclic) properties were tested to determine sensitivity to static and dynamic conditions. CNTFs did not cause any reduction in mechanical properties, unlike the losses observed for metallic electrodes (up to 60% reduction in ultimate tensile strength). CNTF-based evaluation of the electrical resistivity (between 10 2 — 10 6 Ohm∙cm) and dynamic electrical response (gauge factor between ~ 2 — 12) matched values from a standard electrode material. Microstructural analysis proved that this unique performance was due to the surface and internal volume infiltration of the nanocomposite matrices into the CNTFs, causing interconnection of the CNTs of the matrix and CNTFs. These findings show that CNTFs may be used to accurately monitor nanocomposite multifunctional properties both during manufacturing and application using one-step integration, regardless of the sample size and manufacturing technology. Materials Engineering Nanocomposite multifunctional properties carbon nanotubes carbon nanotube fiber Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction Composite materials, particularly those incorporating nanoparticles, may be susceptible to particle agglomeration and formation of defects during manufacturing [ 1 , 2 ] or during their usage lifecycle [ 3 – 5 ]. With the inclusion of nanomaterials, the range of possible manufacturing defects broadens. These include mechanisms such as nanoparticle filtration, “dead spaces”, as well as a lowered degree of polymer curing [ 5 – 7 ]. Identifying such defects is commonly done with techniques such as visible or acoustic microscopy, infrared imagery, ultrasonic and X-ray inspection, as well as FTIR, NMR, DSC, and TGA methods [ 8 , 9 ]. These methods are often time and resource-consuming, not feasible for forming a complete picture due to their specificity, and may not be representative of the entire part being manufactured [ 8 ]. Furthermore, most techniques for manufacturing and post-manufacturing (in service) defect detection cannot be integrated to provide information for both stages. To address these challenges, recent studies have focused on the usage of embedded sensor systems that can provide real-time information during the manufacturing of composites and then can remain within the material to double up as sensors for structural health monitoring in service [ 10 – 12 ]. Although such technology has promising potential, embedded sensors within composites and composite structures may affect their mechanical properties and deformation behavior [ 13 – 15 ]. In addition, the issue of nanoparticle filtration is yet to be addressed in detail in the context of embeddable sensors [ 7 , 16 ]. The range (not exhaustive) of embeddable sensor technologies for composite monitoring includes fiber optics [ 17 ], piezoelectric/piezoresistive materials [ 18 ], dielectric [ 19 ], acoustic [ 15 ] as well as electromagnetic sensors [ 20 ]. These sensors usually possess rather distinct material properties with respect to the composites they are embedded within. This mechanical property mismatch may undermine the performance and lead to premature failure due to the lack of adhesion and bonding with the host material and the introduction of defects and give rise to stress concentration in the vicinity of inclusions [ 21 – 24 ]. Fiber optic sensors have been shown to allow parameter tracking during manufacturing [ 25 ] and post-manufacturing structural health monitoring [ 26 ]. However, they possess additional drawbacks, namely, these systems require specialized and expensive equipment that is difficult to use during regular composite production and monitoring, and also encounter difficulties with CNT nanocomposites in which the transmittance properties deteriorate [ 27 ]. Piezoelectric/resistive sensors have also shown the ability for monitoring both stages, but their dimensions, coupled with electric wiring, cause regions of inhomogeneity within the composite [ 28 ]. Dielectric, acoustic, and electromagnetic sensors have similar drawbacks mentioned above [ 19 , 29 , 30 ]. Currently, no embeddable sensor technology been successfully developed and upscaled that (1) does not cause mechanical performance loss, (2) is simple and economic to operate, and (3) can provide in situ monitoring of materials from manufacturing to usage. Carbon nanotube fibers (CNTFs) can offer a solution for the embedded monitoring of polymer nanocomposites. CNTFs are fiber-shaped macro-porous structures obtained from the assembly of CNTs. They show exceptional flexibility in combination with electrical, thermal, mechanical, and piezoresistive properties [ 31 – 33 ]. CNTFs can be produced by both wet and dry techniques [ 34 , 35 ], allowing the tailoring of properties and performance through changes in the density, porosity, and surface morphology. Previous studies have used CNTFs for monitoring the manufacturing stage of polymers and composites. They showed sensitivity to the changes in thermoset resin and chemical reagent concentrations [ 36 ]. The porosity and flexibility of the CNTFs allow infiltration of the resin into the fiber, resulting in distinct piezoresistive changes during each of the various stages of the polymerization reaction. The same principle is used for their application in structural health monitoring, where the application of stress and strain causes changes in the percolative connections between the nanotubes within the CNTF, resulting in a distinctly detectable electrical signal capable of showing correlation with the material health status and damage [ 37 ]. Although composite monitoring using CNTFs has been separately studied for manufacturing and post-manufacturing, to the best of the authors’ knowledge there has been no attempt to carry out a combined study of the detection of manufacturing parameters, structural health, and functional property changes. CNTFs can also offer the possibility of a sensing solution for detecting different concentrations of conductive nanoparticles, which can allow identifying the filtration effect in fiber-reinforced nanocomposites during manufacturing [ 16 , 38 ]. To experimentally verify the feasibility of CNTFs as embedded sensors for functional properties and defect detection, the present study creates a hierarchical nanocomposite using CNTs as the matrix-filling material for providing multifunctional properties and CNTFs as the embedded electrodes. The CNTFs were produced via the wet pulling technique utilizing SWCNT thin films and were studied from a variety of aspects. Their ability of functional property monitoring, sensitivity to MWCNTs and SWCNTs concentrations in the matrix, and to the effect that CNTF dimensions may have on the detection performance and mechanical properties of the nanocomposite were investigated. The properties were then compared to the standard methods employed in reported scientific practice for comparative analysis. Overall, herein we propose a one-step solution for multifunctional composite monitoring utilizing embeddable CNTFs. The proposed system performs as well as the currently utilized standard techniques, and in some cases exceeds their performance. We reveal the mechanisms of interaction between CNTFs and the dispersed CNTs within a thermoset matrix, delving into an area of research that currently remains underrepresented in literature. The present study also appears to be the first to show the feasibility of the wet pulling technique [ 31 ] and the use of SWCNT-based CNTFs for scalable manufacturing and post-manufacturing monitoring. The aim is to pave the way for the use of CNTFs as novel electrodes for CNT-based multifunctional nanocomposite property sensors. 2. Experimental Section 2.1 Materials CNTFs utilized in this study were produced from SWCNT thin films synthesized using the aerosol (floating catalyst) CVD technique, as described previously [ 39 ], and the films had a thickness of ~ 53 nm. For the creation of multifunctional thermoset nanocomposites, commercial SWCNT and MWCNT masterbatches (see Table 1 for details) were utilized. The polymer matrix used for nanocomposite manufacturing consisted of a bis-a-phenol epoxy matrix produced by Axson Sika (Baar, CH) and sold under the tradename CR-82, with the associated hardener CH 80 − 2. Table 1 Details of the CNTs used for nanocomposite manufacturing CNT type Length (µm) Diameter (nm) Aspect Ratio Manufacturer SWCNT > 5 1.6 ± 0.4 ~ 3000 OCSiAl MWCNT 0.1–10 10–15 6–1000 Arkema 2.2 Methods This work required the production of two separate materials, which were then integrated together. The first is the multifunctional thermoset nanocomposite, which is intended to be measured during the manufacturing process and mechanical loading. The second are the novel CNTF electrodes, which are embedded into the nanocomposite during the manufacturing stage to measure the multifunctional properties without affecting the final mechanical performance. Once the multifunctional nanocomposites were produced with the various electrode systems, they were tested for their electrical and piezoresistive properties. The thermoset nanocomposites were produced using a standardized manufacturing route [ 6 ], utilizing a combination of ultrasonication and high-speed shear mixing. In the first stage, a certain amount of masterbatch was weighed according to the intended weight percentage of the nanocomposite. For example, for a 100 g nanocomposite batch of 0.25% wt. MWCNTs, 1 g of the masterbatch (25% wt.) was used. This amount was then soaked in 5 ml of acetone overnight to ensure that the dense masterbatch could be dispersed during the following processing stages. The soaked masterbatch was then subjected to ultrasonication in an ultrasonic bath for 30 minutes to help soften the masterbatches and create a pre-dispersion. Once this was completed, a weighed amount of thermoset resin, without hardener, was added and the mixture was then shear mixed using an IKA T-25 Ultra Turrax homogenizer (Staufen, DE). The mixture was first homogenized at the lowest rate of 3200 RPM for 3 minutes, followed by mixing at 7500 RPM for 45 minutes and finally at 10000 RPM for 15 minutes. Once the homogenizing step was completed, the mixture was ultrasonicated again for 1 hour to help improve dispersion while degassing the mixture. Following this, the mixture was placed in a vacuum chamber and degassed for 30 minutes before the weighed amount of hardener was added. The mixture was then slowly hand mixed for 10 minutes, followed by a second degassing step of 10 minutes. Finally, the nanocomposite mixture was hand poured into silicon molds corresponding to ISO 527. Samples were cured at room temperature for 24 hours before being post-cured at 60°C for 12 hours in a laboratory oven. The SWCNT thin film used for CNTF production was characterized prior to CNTF fabrication. Nanotube I G /I D ratio (quality assessment) was measured using a Thermo Fisher Scientific DXRxi Raman Imaging microscope (Waltham, MA, USA), thickness was calculated using a Perkin Elmer Lambda 1050 UV–vis–NIR spectrometer (Waltham, MA, USA) [ 40 ] and sheet resistance was measured using a Jandel 4-point probe RM3000 (Leighton Buzzard, UK). The measured ratio of G and D intensities in the Raman spectra was ~ 37, the thickness corresponded to ~ 53 nm and the sheet resistance measured was 48 Ω/sq. CNTFs were produced from these thin films using the wet-pulling technique, which involves the dry transfer of CNT thin films to a substrate, their wetting with a volatile liquid, and subsequent mechanical pulling, which converts the thin films into fibers. The fabrication procedure has been detailed in previous works [ 31 – 33 ]. In this work, ethanol was used as the liquid for the wetting of the SWCNT film and its subsequent densification under evaporation. Once the fibers were produced, they were placed on glass substrates and their ends fixed using conductive silver glue. Their dimensions were measured using an optical microscope and their electrical properties were characterized with a Keithley 2000 multimeter (Beaverton, OR, USA). Their nominal conductivity, that is inversely proportional to resistivity, was calculated using the standard formula R = ρ \(\frac{l}{a}\) , where R is the measured resistance, ρ is resistivity, l is the length between electrodes and a is the nominal cross sectional area [ 32 ]. To manufacture samples with embedded and surface applied electrodes, the metallic and CNTF-based electrodes were carefully placed within the gauge section of the sample molds prior to nanocomposite molding, at a distance of 5 mm from each other. 2 and 4 electrodes were embedded within the samples to allow measurements using the 2 and 4-point technique, as shown in Fig. 1 . - Copper wiring, with a diameter of 125–150 µm and conductivity of ~ 2 ×10 7 S/m was used to make up the metallic embedded electrodes, since wiring of a smaller caliber was difficult to accomplish and broke during demolding and additional wiring attachment. Such wiring is common for use with multifunctional nanocomposites [ 41 ]. Samples with externally applied silver glue-based electrodes, as is common in literature with multifunctional nanocomposites [ 42 – 44 ], followed the same design principle with lines of the silver glue being made on the sample surfaces in the same region all along the sample periphery. Nanocomposite electrical resistivity was calculated using the same method used for the CNTF electrical conductivity. The piezoresistive response of the samples was measured under two conditions, the first being uniaxial tensile loading and the second being uniaxial cyclic loading. For piezoresistive measurements, the resistance change of the samples was measured during mechanical loading by attaching the embedded electrodes to the Keithley multimeter. The gauge factor (GF) of the samples was calculated using the following equation, where R is the resistance value measured, R 0 is the initial resistance of the sample and ɛ is the strain: $$GF= \frac{\frac{R-{R}_{0}}{{R}_{0}}}{\varepsilon }$$ 1 Tensile loading was carried out with a traverse head speed of 1 mm/min on an Intron 5969 universal testing machine (High Wycombe, UK). Strain measurements were made using a digital image correlation system, LIMESS (Correlated Solutions, Irmo, SC, USA). 5-megapixel cameras were used for image capturing and VIC-3D software was used for calculating strain. Cyclic loading was carried out on an Instron Electropulse 3000 at 10 Hz at 60% of the ultimate tensile strength (UTS) measured during the tensile testing. This testing frequency was chosen because they are usually set as the upper limit for the cyclic testing of polymers to avoid sample heating. These testing procedures allowed comparative quantitative assessment of how embedded electrodes adversely affected the mechanical properties. After mechanical testing, samples were visually inspected at sites of breakage, followed by SEM visualization using a Helios G4 PFIB dual-beam microscope (Thermo Fisher Scientific, Waltham MA, USA). 3. Results and Discussion 3.1 CNTF characterization The diameter and electrical resistance values of the CNTFs are compiled in Fig. 2 (a), and (b) displays a SEM visualization of a CNTF. The CNTFs were produced with films of two different widths, the first being 0.5 cm and the second being 1.0 cm. This created fibers with a noticeable diameter difference to determine if it affected sensing performance or the mechanical properties later on. As the results show, the thinner width of film resulted in CNTFs with a smaller diameter and higher conductivity and that a substantial difference in diameter was present between the two groups. The values are in a good correlation (~ 10 3 -10 4 S/m) with previous works that reported the diameter and electrical conductivity of CNTFs produced by the wet-pulling technique [ 31 , 32 ], and were therefore deemed suitable for investigating as embeddable electrodes. For embedding and functional property detection of the nanocomposites, electrodes were paired depending on the diameter and conductivity shown (i.e., for 2- and 4-point measurements, we respectively used 2 and 4 similar electrodes). 3.2 Monitoring nanocomposite manufacturing using CNTFs A single-step sensor incorporation approach was used to study the feasibility of using CNTFs as embedded electrodes for dual-stage monitoring. The CNTFs were placed in the gauge section of the molds at equidistant spacing. Continuous electrical measurement of the samples was conducted during the molding process, with measurements running for 24 hours until the polymer mixture was cured at room temperature. The CNTFs were not removed from the samples once curing had completed and were used as electrical connections for multifunctional property measurements during the post-manufacturing stage. Two types of nanocomposites were monitored to determine the sensitivity to concentration and CNT type, one based on SWCNTs and another on MWCNTs. For both cases, two concentrations of CNTs were studied (0.25% and 0.75%). 3.2.1 Electrical monitoring and the effect of diameter on property measurement During the manufacturing stage, both 2- and 4-point electrical measurements were performed. 2-point measurements were conducted with CNTFs produced from both widths of thin films (0.5 and 1.0 cm) which allowed the identification of diameter-related differences. These were then compared to 4-point measurements to determine if contact resistance was present and how it manifests during monitoring. Silver glue was not used during this stage due to the fact that the base of the glue is a polymer and may result in additional deviation in the measurements. In addition, the usage of the silver glue as embedded electrodes would not be feasible on an industrial scale and would be impractical for a one-step embedding process since the contacts would be inaccessible after matrix coverage. Figure 3 (a) is an example of the differences in the measured electrical readings seen between CNTF and metallic electrodes during 2-point measurements. Figure 3 (b) presents typical manufacturing stage resistance evolution curves obtained from 2-point measurements made with different diameter CNTFs. The full experimental data set is shown in Figures SI 1–2. The manufacturing stage monitoring of the nanocomposites, for both 2- and 4-point schemes, shows remarkable features. Regardless of the diameter of the CNTFs used for monitoring, the measured electrical resistance values are almost identical for the same type of nanocomposite mixture, as can be seen in Fig. 3 (b). This is seen when measuring both SWCNT and MWCNT nanocomposite mixtures at 0.25% and 0.75% by weight, respectively. 4-point measurements with CNTFs matched the values in orders of magnitude, with minor differences between electrodes of the same type. This is due to the fact that each experiment required a separate batch of nanocomposite mixture to be produced, which resulted in variations in inherent electrical properties. The behavior has already been documented in batch production with the materials under study [ 6 , 45 ]. For the embedded metallic electrodes, however, the 2-point measurements showed considerable contact resistance in the form of noise and higher reading magnitude, with values being 1–2 orders larger in all cases, as seen in Fig. 3 and Figures SI 1–2. The 4-point measurements reduced the electrical noise and variance for these electrodes (up to a factor of 2), but the contact resistance could not be eliminated, as seen in Figure SI 3. During the curing process, the electrical resistance for all samples and electrodes shows a slight tendency to increase with time for both the metallic and CNTF electrodes. This behavior is attributed to the increase in resistance experienced by CNT-thermoset nanocomposite systems during the process of curing [ 46 , 47 ]. These results show that the CNTFs are sensitive to CNT type, show similar results regardless of diameter, and register electrical matrix changes, as is reported in literature. The findings show that the CNTFs are more sensitive to percolation network dependent properties of the nanocomposite matrix than metallic electrodes, making the fibers an ideal low-noise candidate for measuring the filtration effect, which is agglomeration due to flow against physical boundaries, that may take place when nanocomposite matrices are used to create fiber reinforced nanocomposites. In short, the CNTF embedded electrodes provide a more stable, reliable and noise and contact resistance-free electrical measurement of the nanocomposite mixtures, regardless of the diameter, concentration of CNTs in the nanocomposite as well as type of CNTs used in the nanocomposite matrix. 3.2.2 Electrical resistivity measurements Since electrical resistivity or conductivity is one of the main areas where CNT multifunctional nanocomposites are at the forefront and attract a large amount of industrial and research attention [ 48 , 49 ], this was the first property determined after manufacturing. Selected results showing the general trends obtained with different electrode systems have been compiled in Fig. 4 , with full data sets for all the nanocomposite matrices present in Figure SI 4. The nanocomposites produced and tested with the silver-based glue acted as the benchmark and standard test method for measurement, as it has been used in various publications [ 33 , 43 , 50 ]. These show electrical resistivity values consistent with those found in literature. For the SWCNT nanocomposites measured using silver standard electrodes, the resistivity values for the systems with 0.25% and 0.75% by weight respectively were in the range of ~ 10 3 and ~ 10 2 Ohm∙cm, respectively, which matches literature values [ 6 ]. For the same weight percentages, the MWCNT nanocomposites show electrical resistivities of ~ 10 6 and ~ 10 3 Ohm∙cm, respectively, which also coincide with or are better than those reported in literature [ 51 , 52 ]. This shows that the nanocomposites incorporating the novel CNTF electrodes deliver the expected performance and are thus suitable for examining their detection capability. In Fig. 4 and SI 4, the trend of showing lower resistivity values for both SWCNT and MWCNT nanocomposites by the CNTF electrodes continues, regardless of the use of 2- or 4-point measurement schemes. Compared to the metallic electrodes, the CNTFs show a 1–2 orders of magnitude value smaller for all nanocomposite weight percentages tested for both schemes. We observe this phenomenon regardless of the diameter of the CNTFs, further confirming that they display negligible contact resistance when used to measure the multifunctional nanocomposite matrices. When compared with the silver standard electrodes, the resistivity values detected by the CNTFs show no significant difference for any of the batches. The results show that the CNTFs are making a stronger electrical contact and connection with the percolation network of the nanocomposites compared to embedded metallic electrodes, whereas their performance matches that of a standard electrode material. Hence, the electrical testing and manufacturing monitoring showed that CNTF-based electrodes are less susceptible to contact resistance compared to their metallic counterparts, regardless of the measurement technique. The reasons underlying these enhanced properties is further explained in the section devoted to the microstructural examination of the materials. Besides confirming the advantages listed above, the results show that the CNTFs are more versatile than embedded metallic electrodes as the more readily accessible two-point scheme provides reliable readings. This provides benefits in terms of the ease of installation, reliability and reduced areas of inhomogeneity where embedded electrodes may cause mechanical property loss. The CNTFs have the advantage of being suitable for embedding in the nanocomposites during production and then being used for lifecycle measurements. They also provide the added benefits of reduced cost with the detection performance matching that of commonly used standard materials, as well as a simpler application route. 3.3 Post-manufacturing nanocomposite monitoring using CNTFs As the aim of this work was to determine whether CNTFs are a suitable alternative for one-step incorporation and monitoring, the post-manufacturing testing consisted of measuring the sensitivity to piezoresistive response. Measurements were made both during uniaxial tensile straining as well as under uniaxial cyclic testing for measuring their performance and reliability. 3.3.1 Tensile piezoresistive response One of the main multifunctional properties exhibited by CNT-thermoset nanocomposites is their piezoresistive response that makes the material a prime candidate for smart materials applications such as structural health monitoring [ 38 , 53 ]. As the CNTFs were intended for embedding, it was necessary to determine whether they introduce any mechanical property reduction when combined with the nanocomposites. Figure 5 displays the tensile piezoresistive response of the nanocomposites with different electrodes. The tensile testing curves are presented in Figure SI 5 and are compiled in Table 2 . Table 2 Piezoresistive performance and mechanical properties for different matrices and electrodes Materials Ultimate tensile strength (MPa) Gauge factor Young’s modulus (GPa) Poisson Ratio Epoxy 71.8 ± 1.3 - 3.15 ± 0.97 0.33 ± 0.02 Epoxy + CNTF 72.3 ± 2.1 - 3.37 ± 0.15 0.36 ± 0.01 Epoxy + metallic 59.6 ± 3.2 - 3.59 ± 0.19 0.34 ± 0.02 0.25% MWCNT + silver glue 68.2 ± 0.5 11.6 ± 0.8 3.18 ± 0.03 0.39 ± 0.01 0.25% MWCNT + CNTF 61.5 ± 6.2 12.5 ± 0.4 2.93 ± 0.28 0.40 ± 0.01 0.25% MWCNT + metallic 52.3 ± 6.8 17.1 ± 2.7 3.07 ± 0.26 0.40 ± 0.02 0.75% MWCNT + silver glue 59.8 ± 0.5 6.6 ± 0.5 2.89 ± 0.15 0.39 ± 0.01 0.75% MWCNT + CNTF 60.7 ± 1.1 6.4 ± 1.2 3.13 ± 0.35 0.41 ± 0.03 0.75% MWCNT + metallic 47.4 ± 2.2 8.8 ± 1.5 2.60 ± 0.43 0.41 ± 0.01 0.25% SWCNT + silver glue 22.3 ± 0.3 4.3 ± 0.7 0.44 ± 0.04 0.43 ± 0.05 0.25% SWCNT + CNTF 22.6 ± 3.8 5.8 ± 1.3 0.36 ± 0.05 0.41 ± 0.02 0.25% SWCNT + metallic 9.6 ± 2.7 6.9 ± 0.7 0.46 ± 0.09 0.42 ± 0.01 0.75% SWCNT + silver glue 2.5 ± 0.3 3.1 ± 0.3 0.03 ± 0.01 0.44 ± 0.04 0.75% SWCNT + CNTF 2.2 ± 0.3 2.9 ± 1.7 0.03 ± 0.01 0.44 ± 0.02 0.75% SWCNT + metallic 1.6 ± 0.4 3.7 ± 0.6 0.03 ± 0.01 0.45 ± 0.01 To understand the piezoresistive performance of the electrode systems it is necessary to begin with the base material mechanical characteristics. It was observed for all nanocomposites that the addition of CNTs causes a modification of mechanical properties. This is reflected in the decrease of the ultimate tensile stress (UTS), Young’s modulus, and an increased Poisson’s ratio. The nanocomposites tend towards a plastic or viscoelastic response with the progressive addition of CNTs to the polymer. This has been well reported and attributed to the CNTs interfering with the polymerization reaction, eventually leading to a decrease in the overall degree of cure [ 9 , 54 , 55 ]. It is important to note here that each batch of material and electrodes has shown similar values of Young’s modulus and Poisson’s ratio, meaning that the base material behavior remained essentially the same, so that any significant difference in the UTS may be attributed to the inclusion of electrodes. The initial difference in UTS in evident from testing with plain epoxy, with the metallic electrodes showing a ~ 17% loss, and CNTF electrodes showing no significant difference. For the nanocomposite with 0.25% wt. MWCNT addition, the metallic electrodes show a UTS value drop of ~ 24% and a piezoresistive gauge factor of 17 that was ~ 55% higher than that for silver glue and CNTF samples. The higher gauge factor is expected as the metallic electrodes register a consistently higher resistivity value due to the presence of contact resistance. In comparison, the samples containing CNTFs show a UTS and gauge factor which is almost the same as the samples with silver glue. For nanocomposites with 0.75% wt. MWCNTs, the CNTF containing samples show no significant changes in the UTS and the measured gauge factor, whereas the metallic electrodes display a slightly overestimated gauge factor and a ~ 21% loss in UTS. For the MWCNT nanocomposites, at both weight percentages, the CNTF electrodes showed performance almost identical to that of the commonly used surface applied silver standard electrodes, but surprisingly showed no loss in tensile mechanical properties. Nanocomposites manufactured with SWCNTs on the other hand, showed severe mechanical degradation, even at the low wt.% used in this study. The property loss is again attributed to the changes in polymerization, as cited previously. This provided the opportunity to study the CNTF electrodes with a material which has relatively high conductivity and also shows high propensity for plastic or viscoelastic behavior. Nanocomposite batches manufactured with 0.25% SWCNTs showed the same trend as seen with MWCNTs, whereas the CNTF electrodes caused no significant changes in the piezoresistive strain detection or UTS as compared to the silver standard electrodes. The samples incorporating embedded metallic electrodes, however, showed slightly higher gauge factors combined with a ~ 60% loss in UTS. The nanocomposites with 0.75% wt. SWCNT loading narrowed the piezoresistive detection difference, with the metallic electrodes performing no different than the CNTF and silver glue-based samples when variance was taken into account. However, they did show a 37% decrease in the UTS. For nanocomposites with SWCNTs, the electrical resistivity values were extremely low, which, considering the magnitude of the difference seen, may be the reason why a large difference in gauge factors is not present as compared to nanocomposites containing MWCNTs. During this stage of testing, the feasibility of CNTFs as embedded electrodes for the sensing of multifunctional properties during uniaxial strain was verified. The CNTFs caused no discernable property loss, performed on par with a standard electrode material, and provided no overestimated piezoresistive response. Although metallic embedded electrodes seem to be suitable for the monitoring of highly conductive nanocomposites, their negative influence on the mechanical properties cannot be overlooked, as shown in Fig. 6 . It is fundamental to note that during this testing, all samples containing metallic electrodes failed at the location of embedding, often following the electrode path. This was not the case for embedded CNTFs or surface applied electrodes. This is further discussed in the microstructural and mechanism section and example images of these failures are provided in Fig. 7 . An additional interesting find was that although samples with embedded metallic electrodes contained regions of inhomogeneity which were conducive to sample fracture and failure, the surface applied silver electrodes would detach upon the extreme shock experienced during breakage. Single frames from DIC are shown in Figure SI 6 showing this taking place. Although the surface applied silver-glue is suitable for static material measurement, this work shows that they may not be suitable for applications where a high rate loading is experienced by the nanocomposite such that the bonding strength of the adhesive is exceeded and overcome. This limits their usage in large-scale applications in the real-world, unlike the CNTF-based electrodes. 3.3.2 Cyclic piezoresistive response and properties Cyclic testing of the nanocomposites was conducted to determine whether cyclic loading would affect the piezoresistive sensing capability by determining whether any of the electrodes may fail and/or cause failure under alternating loads. Representative sample cyclic response is shown in Fig. 8 , with the full sample sets presented in Figures SI 7–8. The cyclic response of the nanocomposites showed results that corresponded well to the trends seen during monotonic tensile testing. For nanocomposites made with 0.25% wt. MWCNTs, the CNTF electrodes show a similar response to that of the standard silver electrodes whereas the metallic electrodes show a heightened response at a slightly lower applied force. The same is noted for the nanocomposites with 0.75% wt. MWCNTs, where on average, the response of the metallic electrodes was higher than the counterparts. The higher response of the metallic electrodes ties in with the microstructural analysis, showing that the poor interface between the nanocomposite matrix and electrodes causes higher contact and tunneling resistance. It should be noted that for both these weight percentages, the nanocomposites showed no major drift, highlighting that the base nanocomposite may be a reliable material for structural health monitoring. Samples with embedded metallic electrodes again failed at the site of the electrode placement, which was not the case for samples containing CNTFs or with standard silver contacts. Also, samples with embedded metallic electrodes failed at a lower number of cycles as compared to their counterparts. Although the metallic electrodes may provide a heightened response and sensitivity, they provide a location for stress concentration and accelerate failure. This was not the case with embedded CNTFs. The SWCNT nanocomposites, showing electrical resistivity values which differ by only one order of magnitude as a function of weight percentage, showed similar and subdued responses. Given the low testing force (based on the UTS) and the more pronounced viscoelastic properties of the matrix, all samples for both weight percentages completed 1 million cycles without failure. To facilitate visualization, a segment of their responses is shown in Fig. 8 , with the full response cycles present in Figures SI 7–8. For the samples created with 0.25% SWCNTs, the responses of the different electrodes become similar, with the metallic electrodes showing a slightly higher response. This is not unexpected, since the higher the conductivity of CNT nanocomposites, the lower the overall piezoresistive response becomes [ 6 , 56 , 57 ]. As the weight percentage of SWCNTs is increased to 0.75%, a very slow cyclic response is noted from all materials, with the degree of sensitivity that is almost the same. Considering the microstructure seen in the SEM analysis (provided further), the response is not surprising since the percolation network formed in the nanocomposites is extremely dense. It provides a large number of interconnections between the SWCNTs due to the high aspect ratio, a high dispersion degree and relatively large weight percentage. Again, no major drift was seen in any of the samples from the different electrode types, showing that the base material was being measured in a similar way by all electrodes. The cyclic testing allowed some important conclusions to be made regarding the usage of the CNTF electrodes. It showed that for nanocomposites made with either MWCNTs or SWCNTs, the CNTFs are able to pick up cyclic changes similar to silver standard electrodes, meaning that the measurements made are comparable to standard utilized techniques. The same trend of similarity is shown by the CNTFs at both high and low forces of testing, for both relatively stiff as well as for more compliant viscoelastic nanocomposites. This makes the CNTFs as versatile as the existing embedded and surface-applied electrode systems, without the drawback of mechanical property loss. Also, the CNTFs have been shown to register changes in nanocomposites that have relatively high conductivity or lie at the border of being electrically insulating, giving them a broad range of applicability for a number of nanocomposites and applications. Last but not least, these findings combined with the fact that they can be embedded during the manufacturing process to provide dual-stage monitoring, confers them with an advantage over contemporary measurement systems. 3.3.4 Microstructural analysis and the working mechanism To understand if the multi-functional sensing performance of the CNTFs is rooted in the microstructure of the hierarchical nanocomposite, optical and SEM analyses were conducted. Initial optical imagery showed that the CNTFs displayed an internal infiltration of the nanocomposite matrices into the fiber. Figure 9 shows the interface between a droplet of nanocomposite matrix and CNTF. The matrix can be seen to infiltrate the fiber through its surface porosity and dented irregular surface paths at places, as seen in Fig. 2 . The irregular surface provides paths within the volume of the fiber for the flow of the nanocomposite matrix, allowing an enhanced interaction between the CNTs of both materials. It was noted during initial testing that the fibers were completely wetted by the matrix (both SWCNT and MWCNT) in areas where the matrix was placed. It was also noted that the fibers obtained a stiffness, similar to a rigid fiber, without any matrix present on the surface in regions near where the infiltration of the droplet was present. In comparison, the metallic electrodes did not show a similar mechanism when examined with optical microscopy, but rather displayed regions of inhomogeneous connection with the nanocomposite matrices as well as regions of separation of matrix constituents. It is postulated that the porous nature of the CNTFs allows for enhanced interaction and mechanical interlocking with the dispersed CNTs, and through the infiltration effect, maintains a volume of CNTs at the interface due to the affinity between the CNTs from the matrix and the CNTs in the fibers. This mechanism is most likely responsible for the lack of contact resistance seen during electrical testing, whereas the inhomogeneous connection of the metallic electrodes with the matrix contributes to and is responsible for the measurable contact resistance. To examine these characteristics further, SEM was conducted on the fracture surfaces of the samples which were used in piezoresistive testing. Since the samples with embedded CNTF electrodes did not break at the point of insertion or along the length of the electrodes, they were carefully cut using a handheld Dremel saw at the point of insertion. Samples prepared from surface applied silver electrodes were used as the baseline for microstructural comparison and images are provided in Figure SI 9, while the images of samples with embedded electrodes are provided in Figs. 10 and 11 . The SEM results elucidated the interaction of the nanocomposite matrices with the embedded electrodes and confirmed that both the functional properties and the piezoresistive measurement capability were highly influenced by the interface. As seen in Fig. 10 , the embedded metallic electrodes display poor adhesion to both the MWCNT and SWCNT nanocomposites, which contributes to them acting as locations of stress concentration that promote mechanical failure. The surfaces of the electrodes seem pristine, indicating that regardless of the microcracks, the adhesion between the materials was poor, providing fracture paths for delamination along the length of the electrodes. A careful inspection of the interface shows the presence of CNTs at the interface between the electrodes and matrices without CNT bridging in place. This also indicates a lack of strong contact between the CNTs and the electrodes and is most likely the cause of the contact resistance and higher electrical values seen during the earlier phases of the study. In homogeneously dispersed nanocomposites where the CNTs form a highly electrically conductive percolation network surrounded by a strong dielectric material, as is the case in this study, the CNTs are considered to be solely responsible for the electrical conductivity through ohmic contacts and the tunneling mechanism [ 58 , 59 ]. A lack of connection with this percolation network, especially when CNTs are often covered by the polymer making an insulating layer, results in an overestimation of both the electrical resistivity and piezoresistive response. The samples with the CNTF electrodes provided the most interesting microstructural properties, explaining why these materials displayed no loss in mechanical properties and a capability for multifunctional property measurement closer to standard surface applied electrodes. As shown in Fig. 11 , the CNTFs are completely wetted by the nanocomposite matrices, with no clear boundary visible between the two materials except for the directional density of CNTs forming the CNTF. This indicates that the infiltration mechanism gleaned from the optical microscopy was identified correctly, and that the surface of the CNTFs shows a stronger adhesion to the nanocomposite matrices due to their porous surface (additional images of surface porosity provided in Figure SI 10). Furthermore, in both MWCNT and SWCNT samples, CNT concentration near the interface of the electrodes and matrices was present and visible, with CNT bridging taking place between the CNTs of the matrices and the CNTF surface (highlighted with yellow circles). In Fig. 11 (c), complete large bundles can be seen adhering to the CNTF at the interface. This points to the fact that there is a strong interfacial connection between the electrodes and CNTs in the matrices which explains the absence of significant loss in mechanical properties. This interaction of the two components of the nanocomposite allows the elimination of contact resistance seen during multifunctional property measurement and enables more accurate and precise measurement through the reduction of tunneling distance and the promotion of ohmic contacts. Hence, the working mechanism and performance of the embedded CNTF electrodes can be explained by the enhanced microstructural interaction with the nanocomposite matrices. The infiltration of the nanomodified matrices into the internal volume of the fibers through surface defects (as seen in Fig. 2 , 11 and SI 10) along with surface infiltration through porosity allows for strong interfacial bonding and reduces the areas of inhomogeneity, preserving the mechanical performance. At the same time, the enhanced interaction allows a higher contact area and CNT bridging between the CNTs in the electrodes and the nanocomposites, which in turn causes the contact resistance between the materials to be insignificant. This results in more accurate and precise measurements of the multifunctional properties during both stages of the material lifecycle. 4. Conclusions In this work, we examined the novel application of CNTFs made through the wet pulling technique. This appears to be the first report on the combination of CNTFs with CNT matrices, both SWCNTs and MWCNTs, for one-step, dual-stage, non-destructive multifunctional property measurement. During manufacturing, the CNTF electrodes showed sensitivity to the concentration of both types of CNTs, proving that they may be utilized for the detection of different concentrations within a nanocomposite matrix. Manufacturing resistance values ranged from 10 6 -10 3 and 10 3 -10 2 Ohm for MWCNT- and SWCNT nanocomposites at 0.25% and 0.75% wt., respectively. The electrodes showed almost identical readings with negligible variance, regardless of 2- or 4-point measurements and diameter. This presents the advantages of reducing the number of electrodes needed for monitoring, reducing chances for regions of inhomogeneity which may cause mechanical failure, and allowing an inexpensive material-based manufacturing monitoring solution. In comparison, embedded metallic electrodes have the disadvantage of high noise, contact resistance, and inconsistency between batches. The lifecycle multifunctional property monitoring showed the CNTFs measure electrical resistivity values with no significant difference (within sample to sample and batch to batch variance) to standard silver electrodes. Metallic electrode values were 1–2 orders of magnitude larger, with a generally higher variance. Tensile testing revealed that CNTFs detect piezoresistive responses similar to standard silver electrodes (GFs ~ 3–13) without mechanical property loss, whereas the samples with metallic electrodes showed higher responses at lower force values (GFs ~ 4–17). Metallic electrodes also adversely affected the mechanical properties, with samples failing at points of insertion as they acted as regions of stress concentration and inhomogeneity (up to 24% and 60% loss in UTS for MW- and SWCNT nanocomposites, respectively). Cyclic testing showed that samples with CNTFs sustained more cycles at higher strains than the samples with metallic electrodes and piezoresistive response was not overestimated. Microstructural analysis proved that the superior performance of the CNTFs was due to matrix infiltration, both through the porous surface and into the volume of the fiber through surface artefacts, which allowed for an enhanced connection between the CNTs of the nanocomposites and the CNTFs. We believe that this work lays the foundation for further in-depth investigations where additional factors such as CNT film type and quality, density, porosity, thickness as well as CNT and CNTF type and additional functionalization should be investigated to further optimize the performance of CNTFs for advanced hierarchical nanocomposite application. Declarations Acknowledgements The authors would like to acknowledge Veronika A. Dmitrieva, Dr. Anastasia Goldt, Ilya Krupatin, and Dr. Yaroslava Shakhova from The Skolkovo Institute of Science and Technology for providing SEM imagery. Contributions Hassaan Butt : Designed the concept and methods, conducted experimentation, validated, analyzed and visualized data, and wrote the main manuscript. Dmitry Krasnikov : Designed the concept, methods and experimentation, validated, analyzed and visualized data, wrote the main manuscript, and provided supervision, funding, resources and project management. Vladislav Kondrashov : Designed testing methods and custom software, validated data, and wrote the main manuscript. Boris Voloskov : Conducted experimentation and data processing. Stepan Konev : Conducted experimentation and data processing. Anna Vershinina : Conducted experimentation and wrote the main manuscript. Sergey Shandakov : Supervision, resources and funding. Zeyu Wang : Conceptual explanation and development. Alexander Korsunsky : Conceptual explanation and development. Ivan Sergeichev : Experimentation, supervision and resources. Albert Nasibulin : Designed the concept, methods and experimentation, validated, analyzed and visualized data, wrote the main manuscript, and provided supervision, project management, funding, and resources. All authors reviewed and edited the manuscript. Data Availability Statement The data for this study may be provided upon reasonable request. Statements and Declarations The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding Hassaan Butt, Vladislav Kondrashov and Dmitry Krasnikov acknowledge the Russian Science Foundation project No. 20-73-10256 (synthesis and characterization of carbon nanotube thin films). 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Abaimov, Conductive CNT-polymer nanocomposites digital twins for self-diagnostic structures: Sensitivity to CNT parameters, Composite Structures. (2022). V. Kostopoulos, A. Masouras, A. Baltopoulos, A. Vavouliotis, G. Sotiriadis, L. Pambaguian, A critical review of nanotechnologies for composite aerospace structures, CEAS Space J. 9 (2017) 35–57. https://doi.org/10.1007/s12567-016-0123-7. Additional Declarations The authors declare no competing interests. Supplementary Files Supplementary.docx Supplementary Information: Multifunctional nanocomposite assessment using carbon nanotube fiber sensors Graphicalabstract.png Graphical Abstract Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Butt","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYBACAxTGB4YEBj4gzUykFmbGxhlALWxspGhp5iFGizn78ccfftTcYTCXyD/+2OZPmhybfPPBzwUM9xIbcGix7Mkxk+w59ozBckYyY3NuW44xGxtbsvQMhmKcWgwO5LAx8LAdZjC4AdLSUJHYxsZjxgx0IW4t558//vjnH1SLxR+QFv5v+LXcSDCQ5m2DamFgywHZwkZAyxszadm+wzyWPY8NZ/a2pQH9kmYszWOQYIzbYemPP775dljOnD3xwYcff5Ll+JkPP/zMU5Egi0sLDPCgG0VA/SgYBaNgFIwCvAAASelSr1WOTi4AAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-4834-1969","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Hassaan","middleName":"A.","lastName":"Butt","suffix":""},{"id":299220767,"identity":"e99f0704-7303-4d1b-a005-6d0cae9d6d05","order_by":1,"name":"Dmitry V. Krasnikov","email":"","orcid":"","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Dmitry","middleName":"V.","lastName":"Krasnikov","suffix":""},{"id":299220768,"identity":"1490428f-f2bd-4dcc-a2f7-63e83f67787f","order_by":2,"name":"Vladislav A. Kondrashov","email":"","orcid":"","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Vladislav","middleName":"A.","lastName":"Kondrashov","suffix":""},{"id":299220769,"identity":"3d35ada8-96f4-45f7-8896-dd837d93b3d3","order_by":3,"name":"Boris V. Voloskov","email":"","orcid":"","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Boris","middleName":"V.","lastName":"Voloskov","suffix":""},{"id":299220770,"identity":"450b954a-4d69-4483-8637-2f2e8c68411e","order_by":4,"name":"Stepan D. Konev","email":"","orcid":"","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Stepan","middleName":"D.","lastName":"Konev","suffix":""},{"id":299220771,"identity":"b2802763-9973-4648-96b9-7bcd1c56c4e6","order_by":5,"name":"Anna I. Vershinina","email":"","orcid":"","institution":"Kemerovo State University","correspondingAuthor":false,"prefix":"","firstName":"Anna","middleName":"I.","lastName":"Vershinina","suffix":""},{"id":299220772,"identity":"4e91e8aa-4264-4d22-9a6e-2fe2370fc722","order_by":6,"name":"Sergey D. Shandakov","email":"","orcid":"","institution":"Kemerovo State University","correspondingAuthor":false,"prefix":"","firstName":"Sergey","middleName":"D.","lastName":"Shandakov","suffix":""},{"id":299220773,"identity":"a5c4d2f8-154a-474a-9a7f-fd5df371a69d","order_by":7,"name":"Zeyu Wang","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Zeyu","middleName":"","lastName":"Wang","suffix":""},{"id":299220774,"identity":"fe9be055-dcdb-4cc9-90c3-b4ecdb174df9","order_by":8,"name":"Alexander M. Korsunsky","email":"","orcid":"","institution":"Trinity College, Oxford","correspondingAuthor":false,"prefix":"","firstName":"Alexander","middleName":"M.","lastName":"Korsunsky","suffix":""},{"id":299220775,"identity":"1292c048-f74a-42f6-a255-8ef4234e1626","order_by":9,"name":"Ivan V. Sergeichev","email":"","orcid":"","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Ivan","middleName":"V.","lastName":"Sergeichev","suffix":""},{"id":299220776,"identity":"6069cb6b-a1d8-407d-8364-54b7eba7e0ec","order_by":10,"name":"Albert G. Nasibulin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3UlEQVRIie3RsQrCMBCA4ZNAXM52LVTbV0gpdLEvI0JnHd0EoZPg6uOkFOrSB6i0gy7OloJEcPCUbkLo6JB/SuA+khAAk+lvExIsBqMLSJh+9mwQ4QyYIILABxEapUHuDCL2dpK1atUs0vGk2KgyRvB3WbfSEEdaSxfFbZEyK6mxSugUvnSP2iuhcEHkRDCq4Z4TwYihRvgSw6fqyVp9if3QEiExcrAngNX3FK4lQY7RnN4SEgldLBPkPKGFhninfXhWr2Z2sMugVUXs2Sy/djry+wVcN20ymUymYb0BhI5AwvM2/n0AAAAASUVORK5CYII=","orcid":"","institution":"Skolkovo Institute of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Albert","middleName":"G.","lastName":"Nasibulin","suffix":""}],"badges":[],"createdAt":"2024-05-06 11:30:26","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-4376476/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4376476/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":55984809,"identity":"887ae4c9-3b32-49f9-9176-96a379a02e2c","added_by":"auto","created_at":"2024-05-07 07:41:09","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":43915,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of ISO 527 samples with (a) 2-point and (b) 4-point embedded schemes for (1) CNTFs, (2) Cu wire and (3) silver glue\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/065532575ee3f4d9caf5f996.png"},{"id":55985497,"identity":"50fad704-e3ec-41de-ba64-0b6d97008cd2","added_by":"auto","created_at":"2024-05-07 07:49:09","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2343541,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The correlation between the diameter and electrical conductivity of CNTFs made of SWCNT films of two different widths and (b) a SEM image of a typical CNTF\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/e2ffb822dc0cd47948bfa5e3.png"},{"id":55984808,"identity":"05475829-b7d9-40e7-be31-d099244d3fdd","added_by":"auto","created_at":"2024-05-07 07:41:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":216574,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Typical resistance monitoring curves measured during the curing of nanocomposite with CNTF and metallic electrodes, and (b) similar responses shown by electrodes for different CNTF diameters\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/10a0d8591f70426cdb064426.png"},{"id":55984816,"identity":"4f5c1da7-e09f-484e-820b-3ac403254333","added_by":"auto","created_at":"2024-05-07 07:41:09","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":174236,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative electrical resistivity measurements made with different electrode materials (CNTF – black, metallic – orange, and silver glue – grey) and schemes (solid – 4-point scheme, vertical dash – 2-point lower diameter set, horizontal dash – 2- point higher diameter set) for (a) MWCNT and (b) SWCNT-based nanocomposites\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/fc2baf61feba9f7d9e288a12.png"},{"id":55986173,"identity":"649920f9-3edc-4713-850a-1ffb872b426c","added_by":"auto","created_at":"2024-05-07 07:57:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":316494,"visible":true,"origin":"","legend":"\u003cp\u003ePiezoresistive curves of the relative resistance change vs applied macroscopic strain for the nanocomposites measured using different electrode systems. (a) and (b) show MWCNT, while (c) and (d) show SWCNT nanocomposites at 0.25% and 0.75% wt., respectively\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/c3a93d296f8bcfff44397bb5.png"},{"id":55986175,"identity":"88dff6bd-25fa-47ef-9de4-08ca16984923","added_by":"auto","created_at":"2024-05-07 07:57:10","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":303033,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative image of measured gauge factors and relative change in UTS for different types of electrodes. Red cloud is metallic electrodes, blue CNTF and green standard silver glue. Shapes represent matrix type; Squares are pure epoxy, circles are 0.25% MWCNT/epoxy, upward pointing triangles are 0.75% MWCNT/epoxy, downward pointing triangles are 0.25% SWCNT/epoxy and diamonds are 0.75% SWCNT/epoxy\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/a3fdc0ef3b3b0bc8d347f42e.png"},{"id":55985505,"identity":"16307542-3736-4608-a5f6-284a90c52842","added_by":"auto","created_at":"2024-05-07 07:49:09","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":169555,"visible":true,"origin":"","legend":"\u003cp\u003eSamples showing fracture path along metallic embedded electrodes for (a) pure thermoset polymer, (b) MWCNT and (C) SWCNT samples\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/f11d588b5bfeb8d9c581ccfb.jpeg"},{"id":55985494,"identity":"2c5dc7c2-e648-4c3d-910b-318f7c1f093e","added_by":"auto","created_at":"2024-05-07 07:49:09","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":267462,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic piezoresistive response measured using different electrode systems for (a), (b) MWCNT and (c), (d) SWCNT nanocomposites of 0.25% and 0.75% wt., respectively\u003c/p\u003e","description":"","filename":"Fig8.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/51ddabde3ac350d3952f6b97.png"},{"id":55984817,"identity":"1d5e32f7-c977-444d-a900-8abeb743fde5","added_by":"auto","created_at":"2024-05-07 07:41:09","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":5858071,"visible":true,"origin":"","legend":"\u003cp\u003eOptical images showing (a) the infiltration of the matrix along the CNTF and (b) inhomogeneous regions of connection between the metallic electrode and matrix\u003c/p\u003e","description":"","filename":"Fig9.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/c8efa823a4bd7737a26ff16f.png"},{"id":55984819,"identity":"279e7822-6f9e-4fa3-84e0-a6a783d4c159","added_by":"auto","created_at":"2024-05-07 07:41:09","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":6195726,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of embedded metallic electrodes within nanocomposites with MWCNTs at (a) 2000X, (b) 20,000X and SWCNTs at (c)3000X and (d) 18000X\u003c/p\u003e","description":"","filename":"Fig10.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/a34ee87442f1f321972240bc.png"},{"id":55984811,"identity":"99c6cfb5-a7de-466e-ae2f-da54e1c47738","added_by":"auto","created_at":"2024-05-07 07:41:09","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":6559289,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of CNTFs embedded within nanocomposites with MWCNTs at (a) 500X, (b) 10000X and SWCNTs at (c) 60000X and (d) 16000X. Yellow circles indicate CNT bridging from CNTs in the matrix\u003c/p\u003e","description":"","filename":"Fig11.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/cd3130421f1985c566eaa85b.png"},{"id":55986880,"identity":"9dda2a85-2e70-4c6f-8fd0-24c7982f0323","added_by":"auto","created_at":"2024-05-07 08:05:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5576564,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/43beec72-f3ff-43f9-9181-170cbe2ad99c.pdf"},{"id":55984812,"identity":"9d04ce91-723a-4997-899a-922a5a0888e9","added_by":"auto","created_at":"2024-05-07 07:41:09","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":8043303,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information: Multifunctional nanocomposite assessment using carbon nanotube fiber sensors\u003c/p\u003e","description":"","filename":"Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/079578221e119ef2c9820472.docx"},{"id":55986174,"identity":"9b8add2d-4359-470f-b138-90f8baa019bc","added_by":"auto","created_at":"2024-05-07 07:57:09","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":514468,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical Abstract\u003c/p\u003e","description":"","filename":"Graphicalabstract.png","url":"https://assets-eu.researchsquare.com/files/rs-4376476/v1/7f7c1cf3092aa2299a7a66ce.png"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eMultifunctional nanocomposite assessment using carbon nanotube fiber sensors\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eComposite materials, particularly those incorporating nanoparticles, may be susceptible to particle agglomeration and formation of defects during manufacturing [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] or during their usage lifecycle [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. With the inclusion of nanomaterials, the range of possible manufacturing defects broadens. These include mechanisms such as nanoparticle filtration, \u0026ldquo;dead spaces\u0026rdquo;, as well as a lowered degree of polymer curing [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Identifying such defects is commonly done with techniques such as visible or acoustic microscopy, infrared imagery, ultrasonic and X-ray inspection, as well as FTIR, NMR, DSC, and TGA methods [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. These methods are often time and resource-consuming, not feasible for forming a complete picture due to their specificity, and may not be representative of the entire part being manufactured [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Furthermore, most techniques for manufacturing and post-manufacturing (in service) defect detection cannot be integrated to provide information for both stages. To address these challenges, recent studies have focused on the usage of embedded sensor systems that can provide real-time information during the manufacturing of composites and then can remain within the material to double up as sensors for structural health monitoring in service [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Although such technology has promising potential, embedded sensors within composites and composite structures may affect their mechanical properties and deformation behavior [\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. In addition, the issue of nanoparticle filtration is yet to be addressed in detail in the context of embeddable sensors [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe range (not exhaustive) of embeddable sensor technologies for composite monitoring includes fiber optics [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], piezoelectric/piezoresistive materials [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], dielectric [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], acoustic [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] as well as electromagnetic sensors [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. These sensors usually possess rather distinct material properties with respect to the composites they are embedded within. This mechanical property mismatch may undermine the performance and lead to premature failure due to the lack of adhesion and bonding with the host material and the introduction of defects and give rise to stress concentration in the vicinity of inclusions [\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Fiber optic sensors have been shown to allow parameter tracking during manufacturing [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] and post-manufacturing structural health monitoring [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. However, they possess additional drawbacks, namely, these systems require specialized and expensive equipment that is difficult to use during regular composite production and monitoring, and also encounter difficulties with CNT nanocomposites in which the transmittance properties deteriorate [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Piezoelectric/resistive sensors have also shown the ability for monitoring both stages, but their dimensions, coupled with electric wiring, cause regions of inhomogeneity within the composite [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Dielectric, acoustic, and electromagnetic sensors have similar drawbacks mentioned above [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Currently, no embeddable sensor technology been successfully developed and upscaled that (1) does not cause mechanical performance loss, (2) is simple and economic to operate, and (3) can provide \u003cem\u003ein situ\u003c/em\u003e monitoring of materials from manufacturing to usage.\u003c/p\u003e \u003cp\u003eCarbon nanotube fibers (CNTFs) can offer a solution for the embedded monitoring of polymer nanocomposites. CNTFs are fiber-shaped macro-porous structures obtained from the assembly of CNTs. They show exceptional flexibility in combination with electrical, thermal, mechanical, and piezoresistive properties [\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. CNTFs can be produced by both wet and dry techniques [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], allowing the tailoring of properties and performance through changes in the density, porosity, and surface morphology. Previous studies have used CNTFs for monitoring the manufacturing stage of polymers and composites. They showed sensitivity to the changes in thermoset resin and chemical reagent concentrations [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The porosity and flexibility of the CNTFs allow infiltration of the resin into the fiber, resulting in distinct piezoresistive changes during each of the various stages of the polymerization reaction. The same principle is used for their application in structural health monitoring, where the application of stress and strain causes changes in the percolative connections between the nanotubes within the CNTF, resulting in a distinctly detectable electrical signal capable of showing correlation with the material health status and damage [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAlthough composite monitoring using CNTFs has been separately studied for manufacturing and post-manufacturing, to the best of the authors\u0026rsquo; knowledge there has been no attempt to carry out a combined study of the detection of manufacturing parameters, structural health, and functional property changes. CNTFs can also offer the possibility of a sensing solution for detecting different concentrations of conductive nanoparticles, which can allow identifying the filtration effect in fiber-reinforced nanocomposites during manufacturing [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. To experimentally verify the feasibility of CNTFs as embedded sensors for functional properties and defect detection, the present study creates a hierarchical nanocomposite using CNTs as the matrix-filling material for providing multifunctional properties and CNTFs as the embedded electrodes. The CNTFs were produced via the wet pulling technique utilizing SWCNT thin films and were studied from a variety of aspects. Their ability of functional property monitoring, sensitivity to MWCNTs and SWCNTs concentrations in the matrix, and to the effect that CNTF dimensions may have on the detection performance and mechanical properties of the nanocomposite were investigated. The properties were then compared to the standard methods employed in reported scientific practice for comparative analysis.\u003c/p\u003e \u003cp\u003eOverall, herein we propose a one-step solution for multifunctional composite monitoring utilizing embeddable CNTFs. The proposed system performs as well as the currently utilized standard techniques, and in some cases exceeds their performance. We reveal the mechanisms of interaction between CNTFs and the dispersed CNTs within a thermoset matrix, delving into an area of research that currently remains underrepresented in literature. The present study also appears to be the first to show the feasibility of the wet pulling technique [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] and the use of SWCNT-based CNTFs for scalable manufacturing and post-manufacturing monitoring. The aim is to pave the way for the use of CNTFs as novel electrodes for CNT-based multifunctional nanocomposite property sensors.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eCNTFs utilized in this study were produced from SWCNT thin films synthesized using the aerosol (floating catalyst) CVD technique, as described previously [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], and the films had a thickness of ~\u0026thinsp;53 nm. For the creation of multifunctional thermoset nanocomposites, commercial SWCNT and MWCNT masterbatches (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for details) were utilized. The polymer matrix used for nanocomposite manufacturing consisted of a bis-a-phenol epoxy matrix produced by Axson Sika (Baar, CH) and sold under the tradename CR-82, with the associated hardener CH 80\u0026thinsp;\u0026minus;\u0026thinsp;2.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDetails of the CNTs used for nanocomposite manufacturing\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCNT type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLength (\u0026micro;m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiameter (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAspect Ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eManufacturer\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSWCNT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e~\u0026thinsp;3000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eOCSiAl\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMWCNT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.1\u0026ndash;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u0026ndash;15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eArkema\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Methods\u003c/h2\u003e \u003cp\u003eThis work required the production of two separate materials, which were then integrated together. The first is the multifunctional thermoset nanocomposite, which is intended to be measured during the manufacturing process and mechanical loading. The second are the novel CNTF electrodes, which are embedded into the nanocomposite during the manufacturing stage to measure the multifunctional properties without affecting the final mechanical performance. Once the multifunctional nanocomposites were produced with the various electrode systems, they were tested for their electrical and piezoresistive properties.\u003c/p\u003e \u003cp\u003eThe thermoset nanocomposites were produced using a standardized manufacturing route [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], utilizing a combination of ultrasonication and high-speed shear mixing. In the first stage, a certain amount of masterbatch was weighed according to the intended weight percentage of the nanocomposite. For example, for a 100 g nanocomposite batch of 0.25% wt. MWCNTs, 1 g of the masterbatch (25% wt.) was used. This amount was then soaked in 5 ml of acetone overnight to ensure that the dense masterbatch could be dispersed during the following processing stages. The soaked masterbatch was then subjected to ultrasonication in an ultrasonic bath for 30 minutes to help soften the masterbatches and create a pre-dispersion. Once this was completed, a weighed amount of thermoset resin, without hardener, was added and the mixture was then shear mixed using an IKA T-25 Ultra Turrax homogenizer (Staufen, DE). The mixture was first homogenized at the lowest rate of 3200 RPM for 3 minutes, followed by mixing at 7500 RPM for 45 minutes and finally at 10000 RPM for 15 minutes. Once the homogenizing step was completed, the mixture was ultrasonicated again for 1 hour to help improve dispersion while degassing the mixture. Following this, the mixture was placed in a vacuum chamber and degassed for 30 minutes before the weighed amount of hardener was added. The mixture was then slowly hand mixed for 10 minutes, followed by a second degassing step of 10 minutes. Finally, the nanocomposite mixture was hand poured into silicon molds corresponding to ISO 527. Samples were cured at room temperature for 24 hours before being post-cured at 60\u0026deg;C for 12 hours in a laboratory oven.\u003c/p\u003e \u003cp\u003eThe SWCNT thin film used for CNTF production was characterized prior to CNTF fabrication. Nanotube I\u003csub\u003eG\u003c/sub\u003e/I\u003csub\u003eD\u003c/sub\u003e ratio (quality assessment) was measured using a Thermo Fisher Scientific DXRxi Raman Imaging microscope (Waltham, MA, USA), thickness was calculated using a Perkin Elmer Lambda 1050 UV\u0026ndash;vis\u0026ndash;NIR spectrometer (Waltham, MA, USA) [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e] and sheet resistance was measured using a Jandel 4-point probe RM3000 (Leighton Buzzard, UK). The measured ratio of G and D intensities in the Raman spectra was ~\u0026thinsp;37, the thickness corresponded to ~\u0026thinsp;53 nm and the sheet resistance measured was 48 Ω/sq.\u003c/p\u003e \u003cp\u003eCNTFs were produced from these thin films using the wet-pulling technique, which involves the dry transfer of CNT thin films to a substrate, their wetting with a volatile liquid, and subsequent mechanical pulling, which converts the thin films into fibers. The fabrication procedure has been detailed in previous works [\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. In this work, ethanol was used as the liquid for the wetting of the SWCNT film and its subsequent densification under evaporation. Once the fibers were produced, they were placed on glass substrates and their ends fixed using conductive silver glue. Their dimensions were measured using an optical microscope and their electrical properties were characterized with a Keithley 2000 multimeter (Beaverton, OR, USA). Their nominal conductivity, that is inversely proportional to resistivity, was calculated using the standard formula \u003cem\u003eR\u0026thinsp;=\u0026thinsp;ρ\u003c/em\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{l}{a}\\)\u003c/span\u003e\u003c/span\u003e, where \u003cem\u003eR\u003c/em\u003e is the measured resistance, \u003cem\u003eρ\u003c/em\u003e is resistivity, \u003cem\u003el\u003c/em\u003e is the length between electrodes and \u003cem\u003ea\u003c/em\u003e is the nominal cross sectional area [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo manufacture samples with embedded and surface applied electrodes, the metallic and CNTF-based electrodes were carefully placed within the gauge section of the sample molds prior to nanocomposite molding, at a distance of 5 mm from each other. 2 and 4 electrodes were embedded within the samples to allow measurements using the 2 and 4-point technique, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. -\u003c/p\u003e \u003cp\u003eCopper wiring, with a diameter of 125\u0026ndash;150 \u0026micro;m and conductivity of ~\u0026thinsp;2 \u0026times;10\u003csup\u003e7\u003c/sup\u003e S/m was used to make up the metallic embedded electrodes, since wiring of a smaller caliber was difficult to accomplish and broke during demolding and additional wiring attachment. Such wiring is common for use with multifunctional nanocomposites [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Samples with externally applied silver glue-based electrodes, as is common in literature with multifunctional nanocomposites [\u003cspan additionalcitationids=\"CR43\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e], followed the same design principle with lines of the silver glue being made on the sample surfaces in the same region all along the sample periphery.\u003c/p\u003e \u003cp\u003eNanocomposite electrical resistivity was calculated using the same method used for the CNTF electrical conductivity. The piezoresistive response of the samples was measured under two conditions, the first being uniaxial tensile loading and the second being uniaxial cyclic loading. For piezoresistive measurements, the resistance change of the samples was measured during mechanical loading by attaching the embedded electrodes to the Keithley multimeter. The gauge factor (GF) of the samples was calculated using the following equation, where \u003cem\u003eR\u003c/em\u003e is the resistance value measured, \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the initial resistance of the sample and \u003cem\u003eɛ\u003c/em\u003e is the strain:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$GF= \\frac{\\frac{R-{R}_{0}}{{R}_{0}}}{\\varepsilon }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eTensile loading was carried out with a traverse head speed of 1 mm/min on an Intron 5969 universal testing machine (High Wycombe, UK). Strain measurements were made using a digital image correlation system, LIMESS (Correlated Solutions, Irmo, SC, USA). 5-megapixel cameras were used for image capturing and VIC-3D software was used for calculating strain. Cyclic loading was carried out on an Instron Electropulse 3000 at 10 Hz at 60% of the ultimate tensile strength (UTS) measured during the tensile testing. This testing frequency was chosen because they are usually set as the upper limit for the cyclic testing of polymers to avoid sample heating. These testing procedures allowed comparative quantitative assessment of how embedded electrodes adversely affected the mechanical properties. After mechanical testing, samples were visually inspected at sites of breakage, followed by SEM visualization using a Helios G4 PFIB dual-beam microscope (Thermo Fisher Scientific, Waltham MA, USA).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1 CNTF characterization\u003c/h2\u003e\n\u003cp\u003eThe diameter and electrical resistance values of the CNTFs are compiled in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e (a), and (b) displays a SEM visualization of a CNTF. The CNTFs were produced with films of two different widths, the first being 0.5 cm and the second being 1.0 cm. This created fibers with a noticeable diameter difference to determine if it affected sensing performance or the mechanical properties later on. As the results show, the thinner width of film resulted in CNTFs with a smaller diameter and higher conductivity and that a substantial difference in diameter was present between the two groups. The values are in a good correlation (~\u0026thinsp;10\u003csup\u003e3\u003c/sup\u003e-10\u003csup\u003e4\u003c/sup\u003e S/m) with previous works that reported the diameter and electrical conductivity of CNTFs produced by the wet-pulling technique [\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e], and were therefore deemed suitable for investigating as embeddable electrodes. For embedding and functional property detection of the nanocomposites, electrodes were paired depending on the diameter and conductivity shown (i.e., for 2- and 4-point measurements, we respectively used 2 and 4 similar electrodes).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2 Monitoring nanocomposite manufacturing using CNTFs\u003c/h2\u003e\n\u003cp\u003eA single-step sensor incorporation approach was used to study the feasibility of using CNTFs as embedded electrodes for dual-stage monitoring. The CNTFs were placed in the gauge section of the molds at equidistant spacing. Continuous electrical measurement of the samples was conducted during the molding process, with measurements running for 24 hours until the polymer mixture was cured at room temperature. The CNTFs were not removed from the samples once curing had completed and were used as electrical connections for multifunctional property measurements during the post-manufacturing stage. Two types of nanocomposites were monitored to determine the sensitivity to concentration and CNT type, one based on SWCNTs and another on MWCNTs. For both cases, two concentrations of CNTs were studied (0.25% and 0.75%).\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\n\u003ch2\u003e3.2.1 Electrical monitoring and the effect of diameter on property measurement\u003c/h2\u003e\n\u003cp\u003eDuring the manufacturing stage, both 2- and 4-point electrical measurements were performed. 2-point measurements were conducted with CNTFs produced from both widths of thin films (0.5 and 1.0 cm) which allowed the identification of diameter-related differences. These were then compared to 4-point measurements to determine if contact resistance was present and how it manifests during monitoring. Silver glue was not used during this stage due to the fact that the base of the glue is a polymer and may result in additional deviation in the measurements. In addition, the usage of the silver glue as embedded electrodes would not be feasible on an industrial scale and would be impractical for a one-step embedding process since the contacts would be inaccessible after matrix coverage.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (a) is an example of the differences in the measured electrical readings seen between CNTF and metallic electrodes during 2-point measurements. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (b) presents typical manufacturing stage resistance evolution curves obtained from 2-point measurements made with different diameter CNTFs. The full experimental data set is shown in Figures SI 1\u0026ndash;2.\u003c/p\u003e\n\u003cp\u003eThe manufacturing stage monitoring of the nanocomposites, for both 2- and 4-point schemes, shows remarkable features. Regardless of the diameter of the CNTFs used for monitoring, the measured electrical resistance values are almost identical for the same type of nanocomposite mixture, as can be seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (b). This is seen when measuring both SWCNT and MWCNT nanocomposite mixtures at 0.25% and 0.75% by weight, respectively. 4-point measurements with CNTFs matched the values in orders of magnitude, with minor differences between electrodes of the same type. This is due to the fact that each experiment required a separate batch of nanocomposite mixture to be produced, which resulted in variations in inherent electrical properties. The behavior has already been documented in batch production with the materials under study [\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e]. For the embedded metallic electrodes, however, the 2-point measurements showed considerable contact resistance in the form of noise and higher reading magnitude, with values being 1\u0026ndash;2 orders larger in all cases, as seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e and Figures SI 1\u0026ndash;2. The 4-point measurements reduced the electrical noise and variance for these electrodes (up to a factor of 2), but the contact resistance could not be eliminated, as seen in Figure SI 3. During the curing process, the electrical resistance for all samples and electrodes shows a slight tendency to increase with time for both the metallic and CNTF electrodes. This behavior is attributed to the increase in resistance experienced by CNT-thermoset nanocomposite systems during the process of curing [\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eThese results show that the CNTFs are sensitive to CNT type, show similar results regardless of diameter, and register electrical matrix changes, as is reported in literature. The findings show that the CNTFs are more sensitive to percolation network dependent properties of the nanocomposite matrix than metallic electrodes, making the fibers an ideal low-noise candidate for measuring the filtration effect, which is agglomeration due to flow against physical boundaries, that may take place when nanocomposite matrices are used to create fiber reinforced nanocomposites. In short, the CNTF embedded electrodes provide a more stable, reliable and noise and contact resistance-free electrical measurement of the nanocomposite mixtures, regardless of the diameter, concentration of CNTs in the nanocomposite as well as type of CNTs used in the nanocomposite matrix.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\n\u003ch2\u003e3.2.2 Electrical resistivity measurements\u003c/h2\u003e\n\u003cp\u003eSince electrical resistivity or conductivity is one of the main areas where CNT multifunctional nanocomposites are at the forefront and attract a large amount of industrial and research attention [\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e], this was the first property determined after manufacturing. Selected results showing the general trends obtained with different electrode systems have been compiled in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, with full data sets for all the nanocomposite matrices present in Figure SI 4.\u003c/p\u003e\n\u003cp\u003eThe nanocomposites produced and tested with the silver-based glue acted as the benchmark and standard test method for measurement, as it has been used in various publications [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e]. These show electrical resistivity values consistent with those found in literature. For the SWCNT nanocomposites measured using silver standard electrodes, the resistivity values for the systems with 0.25% and 0.75% by weight respectively were in the range of ~\u0026thinsp;10\u003csup\u003e3\u003c/sup\u003e and ~\u0026thinsp;10\u003csup\u003e2\u003c/sup\u003e Ohm∙cm, respectively, which matches literature values [\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e]. For the same weight percentages, the MWCNT nanocomposites show electrical resistivities of ~\u0026thinsp;10\u003csup\u003e6\u003c/sup\u003e and ~\u0026thinsp;10\u003csup\u003e3\u003c/sup\u003e Ohm∙cm, respectively, which also coincide with or are better than those reported in literature [\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e]. This shows that the nanocomposites incorporating the novel CNTF electrodes deliver the expected performance and are thus suitable for examining their detection capability.\u003c/p\u003e\n\u003cp\u003eIn Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e and SI 4, the trend of showing lower resistivity values for both SWCNT and MWCNT nanocomposites by the CNTF electrodes continues, regardless of the use of 2- or 4-point measurement schemes. Compared to the metallic electrodes, the CNTFs show a 1\u0026ndash;2 orders of magnitude value smaller for all nanocomposite weight percentages tested for both schemes. We observe this phenomenon regardless of the diameter of the CNTFs, further confirming that they display negligible contact resistance when used to measure the multifunctional nanocomposite matrices. When compared with the silver standard electrodes, the resistivity values detected by the CNTFs show no significant difference for any of the batches. The results show that the CNTFs are making a stronger electrical contact and connection with the percolation network of the nanocomposites compared to embedded metallic electrodes, whereas their performance matches that of a standard electrode material.\u003c/p\u003e\n\u003cp\u003eHence, the electrical testing and manufacturing monitoring showed that CNTF-based electrodes are less susceptible to contact resistance compared to their metallic counterparts, regardless of the measurement technique. The reasons underlying these enhanced properties is further explained in the section devoted to the microstructural examination of the materials. Besides confirming the advantages listed above, the results show that the CNTFs are more versatile than embedded metallic electrodes as the more readily accessible two-point scheme provides reliable readings. This provides benefits in terms of the ease of installation, reliability and reduced areas of inhomogeneity where embedded electrodes may cause mechanical property loss. The CNTFs have the advantage of being suitable for embedding in the nanocomposites during production and then being used for lifecycle measurements. They also provide the added benefits of reduced cost with the detection performance matching that of commonly used standard materials, as well as a simpler application route.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3 Post-manufacturing nanocomposite monitoring using CNTFs\u003c/h2\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eAs the aim of this work was to determine whether CNTFs are a suitable alternative for one-step incorporation and monitoring, the post-manufacturing testing consisted of measuring the sensitivity to piezoresistive response. Measurements were made both during uniaxial tensile straining as well as under uniaxial cyclic testing for measuring their performance and reliability.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\n\u003ch2\u003e3.3.1 Tensile piezoresistive response\u003c/h2\u003e\n\u003cp\u003eOne of the main multifunctional properties exhibited by CNT-thermoset nanocomposites is their piezoresistive response that makes the material a prime candidate for smart materials applications such as structural health monitoring [\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e]. As the CNTFs were intended for embedding, it was necessary to determine whether they introduce any mechanical property reduction when combined with the nanocomposites. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e displays the tensile piezoresistive response of the nanocomposites with different electrodes. The tensile testing curves are presented in Figure SI 5 and are compiled in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003ePiezoresistive performance and mechanical properties for different matrices and electrodes\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMaterials\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eUltimate tensile strength (MPa)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eGauge factor\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eYoung\u0026rsquo;s modulus (GPa)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003ePoisson Ratio\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEpoxy\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e71.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e3.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.97\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEpoxy\u0026thinsp;+\u0026thinsp;CNTF\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e72.3\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e3.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.36\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEpoxy\u0026thinsp;+\u0026thinsp;metallic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e59.6\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e3.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25% MWCNT\u0026thinsp;+\u0026thinsp;silver glue\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e68.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e11.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e3.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25% MWCNT\u0026thinsp;+\u0026thinsp;CNTF\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e61.5\u0026thinsp;\u0026plusmn;\u0026thinsp;6.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.93\u0026thinsp;\u0026plusmn;\u0026thinsp;0.28\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25% MWCNT\u0026thinsp;+\u0026thinsp;metallic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e52.3\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e17.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e3.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.75% MWCNT\u0026thinsp;+\u0026thinsp;silver glue\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e59.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.89\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.75% MWCNT\u0026thinsp;+\u0026thinsp;CNTF\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e60.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e3.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.75% MWCNT\u0026thinsp;+\u0026thinsp;metallic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e47.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e8.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.43\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25% SWCNT\u0026thinsp;+\u0026thinsp;silver glue\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e22.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25% SWCNT\u0026thinsp;+\u0026thinsp;CNTF\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e22.6\u0026thinsp;\u0026plusmn;\u0026thinsp;3.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e5.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.36\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25% SWCNT\u0026thinsp;+\u0026thinsp;metallic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e9.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.46\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.42\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.75% SWCNT\u0026thinsp;+\u0026thinsp;silver glue\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.75% SWCNT\u0026thinsp;+\u0026thinsp;CNTF\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.75% SWCNT\u0026thinsp;+\u0026thinsp;metallic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.45\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eTo understand the piezoresistive performance of the electrode systems it is necessary to begin with the base material mechanical characteristics. It was observed for all nanocomposites that the addition of CNTs causes a modification of mechanical properties. This is reflected in the decrease of the ultimate tensile stress (UTS), Young\u0026rsquo;s modulus, and an increased Poisson\u0026rsquo;s ratio. The nanocomposites tend towards a plastic or viscoelastic response with the progressive addition of CNTs to the polymer. This has been well reported and attributed to the CNTs interfering with the polymerization reaction, eventually leading to a decrease in the overall degree of cure [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e55\u003c/span\u003e]. It is important to note here that each batch of material and electrodes has shown similar values of Young\u0026rsquo;s modulus and Poisson\u0026rsquo;s ratio, meaning that the base material behavior remained essentially the same, so that any significant difference in the UTS may be attributed to the inclusion of electrodes.\u003c/p\u003e\n\u003cp\u003eThe initial difference in UTS in evident from testing with plain epoxy, with the metallic electrodes showing a\u0026thinsp;~\u0026thinsp;17% loss, and CNTF electrodes showing no significant difference. For the nanocomposite with 0.25% wt. MWCNT addition, the metallic electrodes show a UTS value drop of ~\u0026thinsp;24% and a piezoresistive gauge factor of 17 that was ~\u0026thinsp;55% higher than that for silver glue and CNTF samples. The higher gauge factor is expected as the metallic electrodes register a consistently higher resistivity value due to the presence of contact resistance. In comparison, the samples containing CNTFs show a UTS and gauge factor which is almost the same as the samples with silver glue. For nanocomposites with 0.75% wt. MWCNTs, the CNTF containing samples show no significant changes in the UTS and the measured gauge factor, whereas the metallic electrodes display a slightly overestimated gauge factor and a\u0026thinsp;~\u0026thinsp;21% loss in UTS. For the MWCNT nanocomposites, at both weight percentages, the CNTF electrodes showed performance almost identical to that of the commonly used surface applied silver standard electrodes, but surprisingly showed no loss in tensile mechanical properties.\u003c/p\u003e\n\u003cp\u003eNanocomposites manufactured with SWCNTs on the other hand, showed severe mechanical degradation, even at the low wt.% used in this study. The property loss is again attributed to the changes in polymerization, as cited previously. This provided the opportunity to study the CNTF electrodes with a material which has relatively high conductivity and also shows high propensity for plastic or viscoelastic behavior. Nanocomposite batches manufactured with 0.25% SWCNTs showed the same trend as seen with MWCNTs, whereas the CNTF electrodes caused no significant changes in the piezoresistive strain detection or UTS as compared to the silver standard electrodes. The samples incorporating embedded metallic electrodes, however, showed slightly higher gauge factors combined with a\u0026thinsp;~\u0026thinsp;60% loss in UTS. The nanocomposites with 0.75% wt. SWCNT loading narrowed the piezoresistive detection difference, with the metallic electrodes performing no different than the CNTF and silver glue-based samples when variance was taken into account. However, they did show a 37% decrease in the UTS. For nanocomposites with SWCNTs, the electrical resistivity values were extremely low, which, considering the magnitude of the difference seen, may be the reason why a large difference in gauge factors is not present as compared to nanocomposites containing MWCNTs.\u003c/p\u003e\n\u003cp\u003eDuring this stage of testing, the feasibility of CNTFs as embedded electrodes for the sensing of multifunctional properties during uniaxial strain was verified. The CNTFs caused no discernable property loss, performed on par with a standard electrode material, and provided no overestimated piezoresistive response. Although metallic embedded electrodes seem to be suitable for the monitoring of highly conductive nanocomposites, their negative influence on the mechanical properties cannot be overlooked, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. It is fundamental to note that during this testing, all samples containing metallic electrodes failed at the location of embedding, often following the electrode path. This was not the case for embedded CNTFs or surface applied electrodes. This is further discussed in the microstructural and mechanism section and example images of these failures are provided in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAn additional interesting find was that although samples with embedded metallic electrodes contained regions of inhomogeneity which were conducive to sample fracture and failure, the surface applied silver electrodes would detach upon the extreme shock experienced during breakage. Single frames from DIC are shown in Figure SI 6 showing this taking place. Although the surface applied silver-glue is suitable for static material measurement, this work shows that they may not be suitable for applications where a high rate loading is experienced by the nanocomposite such that the bonding strength of the adhesive is exceeded and overcome. This limits their usage in large-scale applications in the real-world, unlike the CNTF-based electrodes.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\n\u003ch2\u003e3.3.2 Cyclic piezoresistive response and properties\u003c/h2\u003e\n\u003cp\u003eCyclic testing of the nanocomposites was conducted to determine whether cyclic loading would affect the piezoresistive sensing capability by determining whether any of the electrodes may fail and/or cause failure under alternating loads. Representative sample cyclic response is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e, with the full sample sets presented in Figures SI 7\u0026ndash;8.\u003c/p\u003e\n\u003cp\u003eThe cyclic response of the nanocomposites showed results that corresponded well to the trends seen during monotonic tensile testing. For nanocomposites made with 0.25% wt. MWCNTs, the CNTF electrodes show a similar response to that of the standard silver electrodes whereas the metallic electrodes show a heightened response at a slightly lower applied force. The same is noted for the nanocomposites with 0.75% wt. MWCNTs, where on average, the response of the metallic electrodes was higher than the counterparts. The higher response of the metallic electrodes ties in with the microstructural analysis, showing that the poor interface between the nanocomposite matrix and electrodes causes higher contact and tunneling resistance. It should be noted that for both these weight percentages, the nanocomposites showed no major drift, highlighting that the base nanocomposite may be a reliable material for structural health monitoring. Samples with embedded metallic electrodes again failed at the site of the electrode placement, which was not the case for samples containing CNTFs or with standard silver contacts. Also, samples with embedded metallic electrodes failed at a lower number of cycles as compared to their counterparts. Although the metallic electrodes may provide a heightened response and sensitivity, they provide a location for stress concentration and accelerate failure. This was not the case with embedded CNTFs.\u003c/p\u003e\n\u003cp\u003eThe SWCNT nanocomposites, showing electrical resistivity values which differ by only one order of magnitude as a function of weight percentage, showed similar and subdued responses. Given the low testing force (based on the UTS) and the more pronounced viscoelastic properties of the matrix, all samples for both weight percentages completed 1\u0026nbsp;million cycles without failure. To facilitate visualization, a segment of their responses is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e, with the full response cycles present in Figures SI 7\u0026ndash;8. For the samples created with 0.25% SWCNTs, the responses of the different electrodes become similar, with the metallic electrodes showing a slightly higher response. This is not unexpected, since the higher the conductivity of CNT nanocomposites, the lower the overall piezoresistive response becomes [\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e]. As the weight percentage of SWCNTs is increased to 0.75%, a very slow cyclic response is noted from all materials, with the degree of sensitivity that is almost the same. Considering the microstructure seen in the SEM analysis (provided further), the response is not surprising since the percolation network formed in the nanocomposites is extremely dense. It provides a large number of interconnections between the SWCNTs due to the high aspect ratio, a high dispersion degree and relatively large weight percentage. Again, no major drift was seen in any of the samples from the different electrode types, showing that the base material was being measured in a similar way by all electrodes.\u003c/p\u003e\n\u003cp\u003eThe cyclic testing allowed some important conclusions to be made regarding the usage of the CNTF electrodes. It showed that for nanocomposites made with either MWCNTs or SWCNTs, the CNTFs are able to pick up cyclic changes similar to silver standard electrodes, meaning that the measurements made are comparable to standard utilized techniques. The same trend of similarity is shown by the CNTFs at both high and low forces of testing, for both relatively stiff as well as for more compliant viscoelastic nanocomposites. This makes the CNTFs as versatile as the existing embedded and surface-applied electrode systems, without the drawback of mechanical property loss. Also, the CNTFs have been shown to register changes in nanocomposites that have relatively high conductivity or lie at the border of being electrically insulating, giving them a broad range of applicability for a number of nanocomposites and applications. Last but not least, these findings combined with the fact that they can be embedded during the manufacturing process to provide dual-stage monitoring, confers them with an advantage over contemporary measurement systems.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\n\u003ch2\u003e3.3.4 Microstructural analysis and the working mechanism\u003c/h2\u003e\n\u003cp\u003eTo understand if the multi-functional sensing performance of the CNTFs is rooted in the microstructure of the hierarchical nanocomposite, optical and SEM analyses were conducted. Initial optical imagery showed that the CNTFs displayed an internal infiltration of the nanocomposite matrices into the fiber. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e shows the interface between a droplet of nanocomposite matrix and CNTF. The matrix can be seen to infiltrate the fiber through its surface porosity and dented irregular surface paths at places, as seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The irregular surface provides paths within the volume of the fiber for the flow of the nanocomposite matrix, allowing an enhanced interaction between the CNTs of both materials. It was noted during initial testing that the fibers were completely wetted by the matrix (both SWCNT and MWCNT) in areas where the matrix was placed. It was also noted that the fibers obtained a stiffness, similar to a rigid fiber, without any matrix present on the surface in regions near where the infiltration of the droplet was present. In comparison, the metallic electrodes did not show a similar mechanism when examined with optical microscopy, but rather displayed regions of inhomogeneous connection with the nanocomposite matrices as well as regions of separation of matrix constituents. It is postulated that the porous nature of the CNTFs allows for enhanced interaction and mechanical interlocking with the dispersed CNTs, and through the infiltration effect, maintains a volume of CNTs at the interface due to the affinity between the CNTs from the matrix and the CNTs in the fibers. This mechanism is most likely responsible for the lack of contact resistance seen during electrical testing, whereas the inhomogeneous connection of the metallic electrodes with the matrix contributes to and is responsible for the measurable contact resistance.\u003c/p\u003e\n\u003cp\u003eTo examine these characteristics further, SEM was conducted on the fracture surfaces of the samples which were used in piezoresistive testing. Since the samples with embedded CNTF electrodes did not break at the point of insertion or along the length of the electrodes, they were carefully cut using a handheld Dremel saw at the point of insertion. Samples prepared from surface applied silver electrodes were used as the baseline for microstructural comparison and images are provided in Figure SI 9, while the images of samples with embedded electrodes are provided in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eThe SEM results elucidated the interaction of the nanocomposite matrices with the embedded electrodes and confirmed that both the functional properties and the piezoresistive measurement capability were highly influenced by the interface. As seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e, the embedded metallic electrodes display poor adhesion to both the MWCNT and SWCNT nanocomposites, which contributes to them acting as locations of stress concentration that promote mechanical failure. The surfaces of the electrodes seem pristine, indicating that regardless of the microcracks, the adhesion between the materials was poor, providing fracture paths for delamination along the length of the electrodes. A careful inspection of the interface shows the presence of CNTs at the interface between the electrodes and matrices without CNT bridging in place. This also indicates a lack of strong contact between the CNTs and the electrodes and is most likely the cause of the contact resistance and higher electrical values seen during the earlier phases of the study. In homogeneously dispersed nanocomposites where the CNTs form a highly electrically conductive percolation network surrounded by a strong dielectric material, as is the case in this study, the CNTs are considered to be solely responsible for the electrical conductivity through ohmic contacts and the tunneling mechanism [\u003cspan class=\"CitationRef\"\u003e58\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e59\u003c/span\u003e]. A lack of connection with this percolation network, especially when CNTs are often covered by the polymer making an insulating layer, results in an overestimation of both the electrical resistivity and piezoresistive response.\u003c/p\u003e\n\u003cp\u003eThe samples with the CNTF electrodes provided the most interesting microstructural properties, explaining why these materials displayed no loss in mechanical properties and a capability for multifunctional property measurement closer to standard surface applied electrodes. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e, the CNTFs are completely wetted by the nanocomposite matrices, with no clear boundary visible between the two materials except for the directional density of CNTs forming the CNTF. This indicates that the infiltration mechanism gleaned from the optical microscopy was identified correctly, and that the surface of the CNTFs shows a stronger adhesion to the nanocomposite matrices due to their porous surface (additional images of surface porosity provided in Figure SI 10). Furthermore, in both MWCNT and SWCNT samples, CNT concentration near the interface of the electrodes and matrices was present and visible, with CNT bridging taking place between the CNTs of the matrices and the CNTF surface (highlighted with yellow circles). In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e (c), complete large bundles can be seen adhering to the CNTF at the interface. This points to the fact that there is a strong interfacial connection between the electrodes and CNTs in the matrices which explains the absence of significant loss in mechanical properties. This interaction of the two components of the nanocomposite allows the elimination of contact resistance seen during multifunctional property measurement and enables more accurate and precise measurement through the reduction of tunneling distance and the promotion of ohmic contacts.\u003c/p\u003e\n\u003cp\u003eHence, the working mechanism and performance of the embedded CNTF electrodes can be explained by the enhanced microstructural interaction with the nanocomposite matrices. The infiltration of the nanomodified matrices into the internal volume of the fibers through surface defects (as seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e and SI 10) along with surface infiltration through porosity allows for strong interfacial bonding and reduces the areas of inhomogeneity, preserving the mechanical performance. At the same time, the enhanced interaction allows a higher contact area and CNT bridging between the CNTs in the electrodes and the nanocomposites, which in turn causes the contact resistance between the materials to be insignificant. This results in more accurate and precise measurements of the multifunctional properties during both stages of the material lifecycle.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this work, we examined the novel application of CNTFs made through the wet pulling technique. This appears to be the first report on the combination of CNTFs with CNT matrices, both SWCNTs and MWCNTs, for one-step, dual-stage, non-destructive multifunctional property measurement. During manufacturing, the CNTF electrodes showed sensitivity to the concentration of both types of CNTs, proving that they may be utilized for the detection of different concentrations within a nanocomposite matrix. Manufacturing resistance values ranged from 10\u003csup\u003e6\u003c/sup\u003e-10\u003csup\u003e3\u003c/sup\u003e and 10\u003csup\u003e3\u003c/sup\u003e-10\u003csup\u003e2\u003c/sup\u003e Ohm for MWCNT- and SWCNT nanocomposites at 0.25% and 0.75% wt., respectively. The electrodes showed almost identical readings with negligible variance, regardless of 2- or 4-point measurements and diameter. This presents the advantages of reducing the number of electrodes needed for monitoring, reducing chances for regions of inhomogeneity which may cause mechanical failure, and allowing an inexpensive material-based manufacturing monitoring solution. In comparison, embedded metallic electrodes have the disadvantage of high noise, contact resistance, and inconsistency between batches.\u003c/p\u003e \u003cp\u003eThe lifecycle multifunctional property monitoring showed the CNTFs measure electrical resistivity values with no significant difference (within sample to sample and batch to batch variance) to standard silver electrodes. Metallic electrode values were 1\u0026ndash;2 orders of magnitude larger, with a generally higher variance. Tensile testing revealed that CNTFs detect piezoresistive responses similar to standard silver electrodes (GFs\u0026thinsp;~\u0026thinsp;3\u0026ndash;13) without mechanical property loss, whereas the samples with metallic electrodes showed higher responses at lower force values (GFs\u0026thinsp;~\u0026thinsp;4\u0026ndash;17). Metallic electrodes also adversely affected the mechanical properties, with samples failing at points of insertion as they acted as regions of stress concentration and inhomogeneity (up to 24% and 60% loss in UTS for MW- and SWCNT nanocomposites, respectively). Cyclic testing showed that samples with CNTFs sustained more cycles at higher strains than the samples with metallic electrodes and piezoresistive response was not overestimated. Microstructural analysis proved that the superior performance of the CNTFs was due to matrix infiltration, both through the porous surface and into the volume of the fiber through surface artefacts, which allowed for an enhanced connection between the CNTs of the nanocomposites and the CNTFs.\u003c/p\u003e \u003cp\u003eWe believe that this work lays the foundation for further in-depth investigations where additional factors such as CNT film type and quality, density, porosity, thickness as well as CNT and CNTF type and additional functionalization should be investigated to further optimize the performance of CNTFs for advanced hierarchical nanocomposite application.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to acknowledge Veronika A. Dmitrieva, Dr. Anastasia Goldt, Ilya Krupatin, and Dr. Yaroslava Shakhova from The Skolkovo Institute of Science and Technology for providing SEM imagery.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eHassaan Butt\u003c/em\u003e: Designed the concept and methods, conducted experimentation, validated, analyzed and visualized data, and wrote the main manuscript. \u003cem\u003eDmitry Krasnikov\u003c/em\u003e: Designed the concept, methods and experimentation, validated, analyzed and visualized data, wrote the main manuscript, and provided supervision, funding, resources and project management. \u003cem\u003eVladislav Kondrashov\u003c/em\u003e: Designed testing methods and custom software, validated data, and wrote the main manuscript. \u003cem\u003eBoris Voloskov\u003c/em\u003e: Conducted experimentation and data processing. \u003cem\u003eStepan Konev\u003c/em\u003e: Conducted experimentation and data processing. \u003cem\u003eAnna Vershinina\u003c/em\u003e: Conducted experimentation and wrote the main manuscript. \u003cem\u003eSergey Shandakov\u003c/em\u003e: Supervision, resources and funding. \u003cem\u003eZeyu Wang\u003c/em\u003e: Conceptual explanation and development. \u003cem\u003eAlexander Korsunsky\u003c/em\u003e: Conceptual explanation and development. \u003cem\u003eIvan Sergeichev\u003c/em\u003e: Experimentation, supervision and resources. \u003cem\u003eAlbert Nasibulin\u003c/em\u003e: Designed the concept, methods and experimentation, validated, analyzed and visualized data, wrote the main manuscript, and provided supervision, project management, funding, and resources. All authors reviewed and edited the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data for this study may be provided upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStatements and Declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHassaan Butt, Vladislav Kondrashov and Dmitry Krasnikov acknowledge the Russian Science Foundation project No. 20-73-10256 (synthesis and characterization of carbon nanotube thin films). We also recognize the Ministry of Science and Higher Education of the Russian Federation (project No. FZSR-2020-0007 \u0026ndash; FZSR-2024-0004).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eR. Talreja, Manufacturing defects in composites and their effects on performance, in: Polymer Composites in the Aerospace Industry, Elsevier, 2020: pp. 83\u0026ndash;97. https://doi.org/10.1016/B978-0-08-102679-3.00004-6.\u003c/li\u003e\n\u003cli\u003eY.K. Hamidi, M.C. Altan, Process Induced Defects in Liquid Molding Processes of Composites, International Polymer Processing. 32 (2017) 527\u0026ndash;544. https://doi.org/10.3139/217.3444.\u003c/li\u003e\n\u003cli\u003eS.Z.H. Shah, S. Karuppanan, P.S.M. Megat-Yusoff, Z. Sajid, Impact resistance and damage tolerance of fiber reinforced composites: A review, Composite Structures. 217 (2019) 100\u0026ndash;121. https://doi.org/10.1016/j.compstruct.2019.03.021.\u003c/li\u003e\n\u003cli\u003eH. Altenbach, A. 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Pambaguian, A critical review of nanotechnologies for composite aerospace structures, CEAS Space J. 9 (2017) 35\u0026ndash;57. https://doi.org/10.1007/s12567-016-0123-7.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[{"identity":"e00a7d30-cc55-4808-9b17-99d23fd346e9","identifier":"10.13039/501100006769","name":"Russian Science Foundation","awardNumber":"20-73-10256 ","order_by":0},{"identity":"c63bf799-f878-4b52-8fc3-8152a5bcd68a","identifier":"10.13039/501100012190","name":"Ministry of Science and Higher Education of the Russian Federation","awardNumber":"FZSR-2020-0007 – FZSR-2024-0004","order_by":1}],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Skolkovo Institute of Science and Technology","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Nanocomposite, multifunctional properties, carbon nanotubes, carbon nanotube fiber","lastPublishedDoi":"10.21203/rs.3.rs-4376476/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4376476/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHere, we propose a novel application of carbon nanotube fibers (CNTFs) for the one-step, dual-stage, non-destructive monitoring of multifunctional conductive nanocomposites. Hierarchical nanocomposites were created by embedding CNTFs into carbon nanotube (CNT) - modified matrices during their manufacturing to assess production variables. CNTFs are then left embedded in the structure for monitoring during nanocomposite application. We investigated the dependence of detection sensitivity and reliability on the CNTF diameter (~\u0026thinsp;40\u0026ndash;700 \u0026micro;m), electrical conductivity (~\u0026thinsp;10\u003csup\u003e2\u003c/sup\u003e-10\u003csup\u003e4\u003c/sup\u003e S/m), and the choice of measurement technique (2- and 4-point) for single-walled and multiwalled CNT fillers at different concentrations. The sensors showed promising sensitivity to CNT type and concentration, the results were independent of CNTF diameter and contact resistance, and showed low noise. For application monitoring, nanocomposites electrical and mechanical (tensile and cyclic) properties were tested to determine sensitivity to static and dynamic conditions. CNTFs did not cause any reduction in mechanical properties, unlike the losses observed for metallic electrodes (up to 60% reduction in ultimate tensile strength). CNTF-based evaluation of the electrical resistivity (between 10\u003csup\u003e2\u003c/sup\u003e \u0026mdash; 10\u003csup\u003e6\u003c/sup\u003e Ohm∙cm) and dynamic electrical response (gauge factor between ~\u0026thinsp;2 \u0026mdash; 12) matched values from a standard electrode material. Microstructural analysis proved that this unique performance was due to the surface and internal volume infiltration of the nanocomposite matrices into the CNTFs, causing interconnection of the CNTs of the matrix and CNTFs. These findings show that CNTFs may be used to accurately monitor nanocomposite multifunctional properties both during manufacturing and application using one-step integration, regardless of the sample size and manufacturing technology.\u003c/p\u003e","manuscriptTitle":"Multifunctional nanocomposite assessment using carbon nanotube fiber sensors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-07 07:41:02","doi":"10.21203/rs.3.rs-4376476/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b36dccd5-dcba-4c18-9a21-59575a877e94","owner":[],"postedDate":"May 7th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":31565127,"name":"Materials Engineering"}],"tags":[],"updatedAt":"2024-05-07T07:41:03+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-07 07:41:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4376476","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4376476","identity":"rs-4376476","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}