{"paper_id":"35d60874-2a1c-47c2-9cd3-6f8efce9e9d7","body_text":"Remarkable mechanical performance at low temperatures of hydroxy-terminated polybutadiene enhanced by hyperbranched polysiloxane | 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 Remarkable mechanical performance at low temperatures of hydroxy-terminated polybutadiene enhanced by hyperbranched polysiloxane Junshan Yuan, Xiaoying Huang, Rui Wang, Weixu Feng, Hongxia Yan This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3932592/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract The inadequate mechanical properties and limited low temperature adaptability of Hydroxy-terminated polybutadiene (HTPB) impose constraints on its practical utilization in solid propellant applications. In the present investigation, a pioneering approach involved the synthesis of a novel hyperbranched polysiloxane, denoted as HBPSi-NH 2 , which encompasses -NH 2 groups and Si-O-C chains. The HBPSi-NH 2 with its unique flexible Si-O-C segments, serving as the soft component in the crosslinked network, in conjunction with the curing agent TDI as the hard component, achieves a synergistic balance of rigidity and flexibility. The resulting HTPB composites not only demonstrate enhanced mechanical properties but also exhibit excellent low temperature adaptability. Remarkably, the HTPB composites exhibit excellent mechanical properties at both 25°C (0.74 MPa ~ 2.08 MPa) and − 40°C (1.77 MPa ~ 12.49 MPa). This enhancement can be ascribed to the abundant presence of functional groups, namely -OH and -NH 2 . These active groups significantly augment the cross-linking density within the HTPB system, also promote the formation of numerous hydrogen bonds, enhancing the strength of HTPB. Simultaneously, the abundant presence of Si-O-C flexible chain segments within HBPSi-NH 2 enhances the reactivity of the HTPB molecular chains, not only improving the toughness of HTPB but also significantly reducing its T g (-65.95°C to -75.62°C). Furthermore, this study establishes a pivotal direction for the design and synthesis of high-performance HTPB-PU materials. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction HTPB is a liquid rubber renowned for its favorable attributes, encompassing low viscosity, excellent transparency, resistance to aging, and remarkable processing capabilities [ 1 – 4 ]. Through the reaction between HTPB and diisocyanate, HTPB-based polyurethane (HTPB-PU) is derived, boasting exceptional mechanical properties, chemical stability, and an outstanding resistance to solvents[ 5 ]. Its extensive range of applications spans various domains, such as biocompatible materials[ 6 ], coatings [ 7 ], adhesives [ 8 ], etc. Notably, HTPB-PU plays a pivotal role as a polymer binder in solid rocket propellants [ 9 – 11 ]. Consequently, there exists substantial value in enhancing the mechanical properties and low-temperature performance of HTPB specifically for solid propellant applications [ 12 ]. Diverse strategies have been pursued to augment the mechanical performance of polyurethane, such as nanoparticle incorporation[ 13 ], coordination with metal ion[ 14 ] and construction of synergetic multiple dynamic bonds[ 15 ]. Among these approaches, hyperbranched polymers (HBPs) have garnered considerable attention from both academia and industry due to their capacity to enhance the mechanical properties of polyurethane composites[ 16 , 17 ]. Hyperbranched polymers (HBPs), with their unique topology, have garnered significant attention from researchers due to their enhanced fluidity, processability, reduced viscosity, distinctive cavity structure, and a profusion of active end groups[ 18 , 19 ]. Within the realm of HBPs, hyperbranched polysiloxanes (HBPSi) stand out as a remarkable category, showcasing exceptional thermal properties, chemical stability, low viscosity, abundant terminal functional groups, and facile grafting modification[ 20 – 23 ]. Notably, HBPSi has demonstrated the ability to enhance friction reduction, wear resistance, mechnical properties, and flame retardancy of polymers[ 24 – 26 ]. Unlike the intricate synthesis processes associated with dendrimers, the preparation method for HBPSi is comparatively straightforward and environmentally friendly [ 27 ]. Moreover, the presence of abundant Si-O-C chain segments within HBPSi holds the potential to enhance the compatibility of HTPB with other constituents and theoretically lower its glass transition temperature (T g ). This opens up promising avenues for the further modification and application of HTPB. In this investigation, a one-step method was employed to synthesize hyperbranched polysiloxanes with terminal amino groups (HBPSi-NH 2 ), followed by their copolymerization with HTPB. The influence of HBPSi-NH 2 content on the mechanical and thermal properties of the resulting composites was thoroughly examined. Remarkably, the inclusion of HBPSi-NH 2 led to a substantial enhancement in both tensile strength and toughness of the HBPSi-NH 2 /HTPB composites, particularly under low-temperature conditions. Additionally, these composites exhibited a lowered T g . Through a comprehensive analysis of the mechanical properties of the HBPSi-NH 2 /HTPB composites, a potential mechanism for the reinforcement was proposed. 2. Materials and experiments 2.1. Materials (3-Aminopropyl)triethoxysilane (APTES) was acquired from Jingzhou Jianghan Fine Chemical Co., Ltd. The procurement of 1,3-propylene glycol (PDO) and toluene diisocyanate (TDI) was facilitated by Xi'an Haotian Chemical Glass Instrument Co., Ltd. HTPB was sourced from Shenzhen Hongyuan Chemical New Material Technology Co., Ltd. Anhydrous ethanol and glass instruments, along with other experimental materials, were generously provided by Xi'an Haotian Chemical Glass Instrument Co., Ltd. 2.2. Preparation of HBPSi-NH 2 The amine-terminated hyperbranched polysiloxane (HBPSi-NH 2 ) was successfully synthesized through a one-pot copolymerization reaction, as depicted in Fig. 1 . To prevent gelation during the reaction, the reactant feed ratio was meticulously calculated using the Flory-Carothers equation. In a 250 mL three-necked flask equipped with a stirring rod, thermometer, nitrogen flask, and condensate reflux device, 35.42 g (0.16 mol) of KH-550 and 26.63 g (0.35 mol) of PDO were added. The mixture was gradually heated to 120°C and held at this temperature until distillation of a byproduct occurred. Subsequently, the distillate temperature was carefully controlled to not exceed 65℃, while gradually increasing the temperature to 160℃. Stirring and heating were ceased once the distillate temperature dropped to approximately 50 ℃. The weight of the collected distillate was determined to be 18.37 g. 2.3. Fabrication of HBPSi-NH 2 /HTPB composities HBPSi-NH 2 /HTPB polyurethane was synthesized by combining HTPB (with a hydroxyl value of 0.66 mmol/g), toluene diisocyanate, and HBPSi-NH 2 , and the reaction mechanism is shown in Figure S1 . The ratio of the curing agent was strictly calculated to maintain a specific ratio of functional groups (-NCO: -OH = 1.1). Initially, HTPB was placed in a 100 mL beaker, and the appropriate amounts of HBPSi-NH 2 (0wt%, 1.0wt%, 2.0wt%, 4.0wt%, 6.0wt%) and toluene diisocyanate were added (referred to as HTPB-0, HTPB-1, HTPB-2, HTPB-4, HTPB-6). The mixture was stirred for 10 minutes. Subsequently, the mixture was degassed in a vacuum oven at 50°C for 15 minutes. It was then poured into a pre-heated PTFE mold and cured for 80×96 hours. Finally, the cured HBPSi-NH 2 /HTPB polyurethane was demolded and prepared for subsequent testing. 2.4. Characterization 1 H NMR spectra were acquired on the Bruker Avance 400 MHz spectrometer (U.S.A) by using d-DMSO as solvent. Fourier transform infrared spectra (FT-IR) were recorded in the region 400 to 4000cm − 1 on the Nicolet FT-IR 5700 spectrometer (U.S.A). The gel permeation chromatography (GPC) was conducted by using the Agilent 1260 system (U.S.A). The storage modulus and tanδ were acquired on the NETZSCH DMA 242E (Germany), and the heating rate was set at 5℃·min − 1 from − 100℃ to 0℃. Thermogravimetric (TG) analysis was conducted by using PerkinElmer TGA8000 (U.S.A), and the heating rate was set at 20 ℃·min − 1 from 30 ℃ to 800 ℃ under N 2 atmosphere. The morphology of tensile fracture surface analysis were performed on the TESCAN VEGA 3 LMH Scanning Electron Microscope (SEM). The content of -OH group was determined according to GB/T 7383 − 2007 (Chinese standard). The cross-linking density of the HBPSi-NH 2 /HTPB composites was determined on the low field NMR analyzer (NIUMAG, PQ001, China). The mechanical property were measured according to GB/T 528–2009 (Chinese standard) by electro-mechanical universal testing machine (CMT 6303). The adhesion property was carried out according to GB/T 7124 − 2008 (Chinese standard) by electro-mechanical universal testing machine (CMT 7204). 3. Results and discussion 3.1. Characterization of HBPSi-NH 2 The structure of the synthesized HBPSi-NH 2 was characterized using FT-IR and 1 H-NMR techniques. Figure 2 displays the FT-IR spectra of HBPSi-NH 2 and the raw materials. In the IR spectrum of PDO, the -OH stretching vibration peak is observed at 3320 cm − 1 . The peaks at 2943 cm − 1 and 2882 cm − 1 correspond to the stretching vibration of -CH 2 -, which is also evident in the IR spectra of KH-550 and HBPSi-NH 2 . The -CH 2 - bending vibration is observed at 1421 cm − 1 . The peak at 1062 cm − 1 corresponds to the stretching vibration of -OH. In the IR spectrum of KH-550, the -NH 2 stretching vibration peaks are observed at 3376 cm − 1 and 3298 cm − 1 , while the -NH 2 bending vibrations are observed at 1600 cm − 1 and 958 cm − 1 , respectively. The stretching vibration of -CH 3 is observed at 2975 cm − 1 . Additionally, the peaks at 1082 cm − 1 and 781 cm − 1 are assigned to the stretching vibrations of Si-O and Si-C, respectively. In the IR spectrum of HBPSi-NH 2 , the stretching vibration peak of -OH is not prominently observed, likely due to its overlap with the -NH 2 peak. Additionally, the presence of hydrogen bonding between -NH 2 and -OH results in an elongation of the chemical bond length and a reduction in the chemical bond strength of -NH 2 . This, in turn, causes a red-shift in the bending vibrations. Furthermore, the stretching vibration peak of Si-C undergoes a red-shift owing to the inductive effect of -NH 2 , as supported by previous studies [ 28 , 29 ]. These findings align with the proposed structure of HBPSi-NH 2 . Notably, the IR spectrum of the collected distillate (as depicted in Figure S2) provides crucial evidence affirming the successful synthesis of HBPSi-NH 2 . The structure of HBPSi-NH 2 was determined using 1 H NMR spectroscopy, as illustrated in Fig. 3 . In the 1 H NMR spectrum of PDO (Fig. 3 a), the signals observed at 3.71–3.19 and 1.64–1.47 ppm (labeled as 2 and 1, respectively) correspond to the protons of HO-CH 2 -CH 2 -CH 2 - and HO-CH 2 -CH 2 -CH 2 -. The 3.71–3.19 ppm signal arises from the proton of HO-CH 2 -. In the 1 H NMR spectrum of KH-550 (Fig. 3 b), signals at 3.93–3.66, 2.64–2.41, 1.52–1.29, and 0.64 − 0.45 ppm (labeled as 1, 2, 4, and 3, respectively) correspond to the protons of CH 3 -CH 2 -, NH 2 -CH 2 -, -CH 2 -CH 2 -CH 2 -, and -CH 2 -CH 2 -Si-, respectively. The protons of methyl groups and amino groups are observed at 1.24–0.97 ppm. In the 1 H NMR spectrum of HBPSi-NH 2 (Fig. 3 c), signals at 4.05–3.93, 3.83–3.70, 3.51–3.36,1.94–1.70, 1.68–1.60, 1.61–1.50, 1.48–1.31, and 0.64 − 0.45 ppm (labeled as 9, 7, 2, 5, 8, 6, 3, and 4, respectively) correspond to the protons of methylene groups. The -NH 2 protons are observed at 1.09–1.01 ppm. Notably, the signals at 3.83–3.7 ppm are noticeably weaker compared to those observed in KH-550. This observation suggests that the oxyethyl groups of KH-550 are not fully consumed by the excessive hydroxyl groups of PDO, likely due to the increasing steric hindrance effect during the reaction [ 28 ]. The 1 H NMR spectra of HBPSi-NH 2 and the raw materials provide compelling evidence confirming the successful synthesis of HBPSi-NH 2 . Furthermore, the GPC spectrum of HBPSi-NH 2 , depicted in Figure S3 and Table S1 , supports this conclusion. Additionally, the hydroxyl value of HBPSi-NH 2 was determined using titration methods, and the results are presented in Table S2. 3.2. Cross-linking density and mechanical properties of HBPSi-NH 2 /HTPB composites The presence of abundant hydroxyl and amino groups in HBPSi-NH 2 enables chemical cross-linking with isocyanate groups, facilitating the formation of covalent bonds between HBPSi-NH 2 and TDI. This cross-linking mechanism enhances the overall cross-linking density of the composites, resulting in a significant improvement in their strength. The relaxation time-dependent variation of NMR signal intensity in HBPSi-NH 2 /HTPB composites was analyzed to characterize their properties (Fig. 4 (a)). Additionally, the cross-linking density and the average molecular weight between cross-linking points were determined using low-field nuclear magnetic resonance (NMR) (Fig. 4 (b)). As depicted in Fig. 4 (a), the decrease in NMR signal intensity becomes more pronounced with increasing HBPSi-NH 2 content, indicating an enhanced binding effect and cross-linking density in HBPSi-NH 2 /HTPB composites [ 30 ]. This observation is also supported by Fig. 4 (b). Comparing HTPB-0 with HTPB-6, the cross-linking density increased by 26.3% (3.65 × 10 − 4 mol·mL − 1 ), while the average molecular weight between cross-linking points decreased by 25.97% (2.58 kg·mol − 1 ). These changes can be attributed to the incorporation of a significant number of active amino and hydroxyl groups from HBPSi-NH 2 into HTPB. These functional groups react with the isocyanate, actively participating in the curing process of HTPB and resulting in an improved cross-linking density in the HBPSi-NH 2 /HTPB composites. Based on the aforementioned analysis, it is evident that the incorporation of HBPSi-NH 2 enhances the cross-linking density of the composites, which may potentially compromise their mechanical properties. The results of the tensile tests are presented in Fig. 5 . As depicted in Fig. 5 , the addition of HBPSi-NH 2 leads to simultaneous improvements in the tensile strength and elongation at break of HTPB. The tensile strength increases with increasing HBPSi-NH 2 content, with HTPB-6 exhibiting the highest tensile strength of 2.08 MPa, representing a remarkable 181.08% increase compared to HTPB-0 (0.74 MPa). Meanwhile, the elongation at break gradually reaches its peak at 625.24% when the loading of HBPSi-NH 2 reaches 2.0wt%, demonstrating a 72.79% increase compared to HTPB-0 (361.86%). In the cross-linked network of HTPB, the hyperbranched structure of HBPSi-NH 2 acts as the soft component, while the benzene ring structure of TDI serves as the hard component, significantly improving the tensile properties of HTPB through a synergistic effect of flexibility and rigidity. The abundant functional groups in HBPSi-NH 2 facilitate the formation of multiple hydrogen bonds within the composites and increase the number of covalent bonds, thereby enhancing the cross-linking density. Furthermore, the unique spherical structural network of HBPSi-NH 2 possesses a significant amount of free volume, enabling it to absorb energy from external sources and generate new voids when subjected to external forces [ 23 ]. All of these factors contribute to the absorption and dissipation of a considerable amount of failure energy when HTPB is subjected to external stress, resulting in an improvement in the tensile strength. Typically, an increase in cross-linking density and tensile strength leads to a reduction in the elongation at break [ 31 ]. However, in the case of HBPSi-NH 2 /HTPB composites, an increase in the elongation at break is observed. This can be attributed to the presence of abundant Si-O-C flexible chain segments in HBPSi-NH 2 , which enhance the flexibility of the molecular chains by facilitating the rotation of chemical bonds and chain segments. However, as the amount of HBPSi-NH 2 increases, the cross-linking density becomes excessively high, impeding the movement of molecular chain segments and gradually reducing the improvement in elongation at break. In order to validate the underlying mechanism of HBPSi-NH 2 enhancement, the tensile fracture surfaces of HBPSi-NH 2 /HTPB composites were examined using SEM imaging, as illustrated in Fig. 6 The SEM image depicted demonstrates a gradual transition from a flat and smooth surface to a rough and heterogeneous morphology upon the addition of HBPSi-NH 2 . In the case of HTPB-0 (Fig. 6 (a)), the fracture surface appears flat and smooth, devoid of any noticeable phase separation. As the quantity of HBPSi-NH 2 increases, the fracture morphology of the composites progressively becomes rough, displaying the presence of rough filaments known as \"dimples,\" which are indicative of toughness fracture. In this condition, stress can be effectively distributed, thereby showcasing relatively outstanding mechanical properties. Nonetheless, if the dosage of HBPSi-NH 2 becomes excessively high (Fig. 6 (d)), it can lead to the aggregation in HTPB, resulting in a more heterogeneous and irregular fracture surface. This, in turn, leads to the concentration of stress in specific regions, potentially affecting the mechanical properties of the composites. To explore the behavior of HBPSi-NH 2 /HTPB composites under low temperature conditions, a comparative analysis of their mechanical properties was conducted at both 25°C and − 40°C. The results, as presented in Fig. 7 , reveal that the HBPSi-NH 2 /HTPB composites exhibit enhanced tensile strength and elongation at break when subjected to a -40°C environment. Specifically, HTPB-4 demonstrates a tensile strength of 9.7 MPa and an elongation at break of 455.54%, representing a remarkable increase of 448.02% and 26.41%, respectively, compared to HTPB-0. It is noteworthy that the modification effect of HBPSi-NH 2 on the tensile strength of HTPB is particularly pronounced at low temperatures, surpassing that observed at room temperature, as illustrated in Fig. 7 . However, the effect on elongation at break is slightly diminished due to the higher cross-linking density. These results suggest that the HBPSi-NH 2 /HTPB composites maintain elevated mechanical strength under low temperature conditions. Nevertheless, as the temperature decreases, the mobility of material molecules is restricted, leading to reduced activity and flexibility of the molecular chains. Consequently, the malleability and elongation at break of the composites decrease. Overall, the HBPSi-NH 2 /HTPB composites exhibit commendable mechanical strength at low temperatures, showcasing exceptional adaptability in low-temperature environments. Based on the extensive investigation conducted on the mechanical properties of HBPSi-NH 2 /HTPB composites, we have elucidated the mechanism behind their enhancement, as depicted in Fig. 8 . Firstly, the active -OH and -NH 2 groups present in HBPSi-NH 2 play a crucial role in forming multiple hydrogen bonds within the composites, thereby augmenting the intermolecular forces. Additionally, these groups can undergo reactions with isocyanates, leading to the formation of covalent bonds and a subsequent increase in the cross-linking density of the composites. This substantial enhancement in the cross-linking density significantly reinforces the strength of HTPB. Furthermore, the abundant presence of flexible Si-O-C segments within HBPSi-NH 2 contributes to the improved mobility of molecular chains, particularly at low temperatures. This enhanced mobility imparts greater toughness to HTPB, rendering it more resilient in low-temperature environments. In summary, the remarkable mechanical properties and adaptability of HBPSi-NH 2 /HTPB composites can be attributed to the synergistic effects of multiple factors, as described above. 3.3. Dynamic thermomechanical analyses of HBPSi-NH 2 /HTPB composites DMA was carried out to investigate the effect of HBPSi-NH 2 on the storage modulus and T g of HTPB composites, as shown in Fig. 9 . Figure 9 (a) illustrates the storage modulus curves. In comparison to HTPB-0, HTPB-4 demonstrates a notable increase in its storage modulus. This enhancement signifies that the inclusion of HBPSi-NH2 substantially augments the rigidity of HTPB, consequently improving the material's stability and load-bearing capacity in practical applications. The principal factor contributing to this effect is the presence of abundant active functional groups in the HBPSi-NH 2 . These functional groups actively participate in the curing process of HTPB, leading to an increase in the composites' crosslink density and rigidity. Figure 9 (b) illustrates the tanδ curves, which exhibit a single peak over the entire temperature testing range, with the peak temperature corresponding to the glass transition temperature (T g ). The symmetrical peak shape indicates that HBPSi-NH 2 /HTPB composites possess a homogeneous structure, and there is strong compatibility observed between HBPSi-NH 2 and HTPB. Furthermore, it is evident that the T g of HTPB-4 has decreased to -75.62°C, which is 9.67°C lower than that of HTPB-0. This reduction can be primarily attributed to the presence of flexible Si-O-C segments within HBPSi-NH 2 . These segments diminish the intermolecular attractive forces, facilitating molecular chain movement at lower temperatures and resulting in a lower T g . Additionally, the highly branched structure of HBPSi-NH 2 may introduce additional gaps between HTPB molecules, reducing their close packing and further contributing to the lowered T g . These factors collectively benefit HTPB by enabling it to maintain relatively stable performance at low temperatures while preserving its elasticity and toughness. 3.3. Thermal analyses of HBPSi-NH 2 /HTPB composites The thermal properties of HTPB in the presence of HBPSi-NH 2 were investigated using the TGA method. As depicted in Fig. 10 (a), both HTPB-0 and HTPB-4 exhibit similar thermal decomposition processes, suggesting that the addition of HBPSi-NH 2 does not alter the underlying thermal decomposition mechanism of HTPB. Although the initial decomposition temperatures(T 5% , corresponding to 5wt% loss) of HTPB-4 and HTPB-0 are similar, the maximum decomposition temperature (T 50% , corresponding to 50wt% loss) of HTPB-4 is significantly higher. This disparity can be attributed to the presence of raw monomer and residual ethanol in HBPSi-NH 2 , leading to a comparatively marginal elevation in the initial decomposition temperature of the composites. However, as the temperature rises, the beneficial impact of chemical bonds such as -OH, -O-, -CH 2 -CH 2 -, and others present in HBPSi-NH 2 , become conspicuous in augmenting the thermal stability of HTPB. Despite an increase in the decomposition rate, Fig. 10 (b) illustrates that the decomposition peak of HTPB-4 shifts to higher temperature. The TGA results demonstrate that the HBPSi-NH 2 effectively preserves the basic thermal stability of HTPB composites. The tailed thermal decomposition data are presented in Table S3. 4. Conclusions In this study, we successfully synthesized a novel hyperbranched modifier called HBPSi-NH 2 through a straightforward polycondensation reaction. The primary objective was to enhance both the mechanical properties and low-temperature adaptability of HTPB, reducing its glass transition temperature (T g ) simultaneously. Remarkably, the addition of HBPSi-NH 2 led to substantial improvements in the tensile strength and elongation at break of HTPB-4, with increases of 125.68% and 62.66% compared to HTPB-0, respectively. Interestingly, even at -40°C, HTPB-4 still exhibited impressive tensile strength and elongation at break, with values maintained at 447.02% and 26.41% compared to room temperature, respectively. Furthermore, the considerable number of flexible Si-O-C chains in HBPSi-NH 2 effectively improved the mobility of molecular chains, thus enhancing the flexibility and adaptability of HTPB at low temperatures. Consequently, this results in a reduction of its T g (-65.95°C to -75.62°C), while leaving the thermal stability of HTPB unaffected. This comprehensive investigation provides valuable insights into the strengthening mechanism of HBPSi-NH 2 on HTPB, establishing a solid foundation for future research in this area. Moreover, the proposed approach offers a novel pathway for the preparation of HTPB-PU with outstanding low-temperature adaptability, presenting promising opportunities for various applications. Declarations Credit authorship contribution statement Junshan Yuan: Conceptualization, Formal analysis, Writing - original draft. Xiaoying Huang: Investigation. Rui Wang: Review. Wei Tian: Supervision. Weixu Feng: Supervision. Hongxia Yan: Conceptualization, Writing - review & editing. Conflicts of interest 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. Acknowledgements This work is sponsored by the National Natural Science Foundation of China (22175143) and Foundation of Science and Technology on Combustion and Explosion Laboratory (J-JK-JJ-2201/6142603032210). Thanks to the Analytical & Testing Center of Northwestern Polytechnical University for test assistance. References Yarmohammadi M, Shahidzadeh M, Ramezanzadeh B (2018) Designing an elastomeric polyurethane coating with enhanced mechanical and self-healing properties: The influence of disulfide chain extender. Prog Org Coat 121:45–52 Shahidzadeh M, Varkaneh ZK, Ramezanzadeh B, Pedram MZ, Yarmohammadi M (2020) Self-healing dual cured polyurethane elastomeric coatings prepared by orthogonal reactions. Prog Org Coat 140:105503 Toosi FS, Shahidzadeh M, Ramezanzadeh B (2015) An investigation of the effects of pre-polymer functionality on the curing behavior and mechanical properties of HTPB-based polyurethane. 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Supplementary Files SupportInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 06 May, 2024 Reviews received at journal 15 Apr, 2024 Reviewers agreed at journal 01 Apr, 2024 Reviews received at journal 11 Feb, 2024 Reviewers agreed at journal 08 Feb, 2024 Reviewers invited by journal 08 Feb, 2024 Editor assigned by journal 06 Feb, 2024 Submission checks completed at journal 06 Feb, 2024 First submitted to journal 05 Feb, 2024 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-3932592\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Research Article\",\"associatedPublications\":[],\"authors\":[{\"id\":271312243,\"identity\":\"915d6bd4-b16f-412a-841f-6229112c1997\",\"order_by\":0,\"name\":\"Junshan Yuan\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Northwestern Polytechnical University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Junshan\",\"middleName\":\"\",\"lastName\":\"Yuan\",\"suffix\":\"\"},{\"id\":271312244,\"identity\":\"cae0ee96-ceb6-4231-a435-33f3b6c5afef\",\"order_by\":1,\"name\":\"Xiaoying Huang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Northwestern Polytechnical University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Xiaoying\",\"middleName\":\"\",\"lastName\":\"Huang\",\"suffix\":\"\"},{\"id\":271312245,\"identity\":\"abbd2a86-e20e-48af-8be4-dcbf4d6d35ab\",\"order_by\":2,\"name\":\"Rui Wang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Northwestern Polytechnical University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Rui\",\"middleName\":\"\",\"lastName\":\"Wang\",\"suffix\":\"\"},{\"id\":271312246,\"identity\":\"822b3d49-8e70-4033-ad6e-717b86c22466\",\"order_by\":3,\"name\":\"Weixu Feng\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Northwestern Polytechnical University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Weixu\",\"middleName\":\"\",\"lastName\":\"Feng\",\"suffix\":\"\"},{\"id\":271312247,\"identity\":\"2e0b5b00-0bd5-4f0c-9186-8a45bff61136\",\"order_by\":4,\"name\":\"Hongxia Yan\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA70lEQVRIiWNgGAWjYBACAwTJfAAqlkC0FjaYUqK0gAEPjE1Aizl77+HXPAV2if2ze7495t1xmIGfPceA4ecO3Fose86lWc4wSE6ccefsdmPeM4cZJHveGDD2nsHjsBs5ZgYfDJgTG27kbpPObTsMEjFgZmwjoCXBoD5x/o2cZ2At9kRoMX7wweBw4oYbOWwQWyQIaTlzxoxxhsFx44030syk/55J55E486zgYC8+Lcd7jD/z/KmWnXcj+ZnkzB3WcvztyRsf/MSjBQjYJICEYwOIydjAwAOiD+DVAEwoH4CEPQNUyygYBaNgFIwCDAAAEhdVH7nY7S8AAAAASUVORK5CYII=\",\"orcid\":\"\",\"institution\":\"Northwestern Polytechnical University\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Hongxia\",\"middleName\":\"\",\"lastName\":\"Yan\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2024-02-06 03:18:15\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-3932592/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-3932592/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":50862364,\"identity\":\"01911617-8b2b-4e4f-8392-8fd9b657f609\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:17\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":67862,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003ePreparation route of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/970d496617c4dfd793fbcb27.png\"},{\"id\":50862365,\"identity\":\"1a6d7380-4cba-45d6-a53d-1e457984e5cb\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:17\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":79003,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eFT-IR spectra of PDO, KH-550 and synthesized HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/e03f9dbb47dd45b4ce7cae2a.png\"},{\"id\":50862366,\"identity\":\"5316366a-32c5-4adb-a00a-b607cf3a779a\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":195179,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectra of PDO (a), KH-550 (b) and HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e (c).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/eb3f16e0c20eeb6479667356.png\"},{\"id\":50862374,\"identity\":\"fd55321f-774d-41ea-8ac6-f38262f2b58a\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":130584,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe variation of NMR signal intensity with relaxation time (a), cross-linking density and the average molecular weight between the cross-linking density points (b) of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/f01db2254df9978083de8f63.png\"},{\"id\":50862375,\"identity\":\"eef27769-c445-4fda-9af0-e453b37c36f6\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":151795,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe tensile test curve (a) and tensile strength, elongation at break (b) of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/d6f366d1c5211db8c9d5ece0.png\"},{\"id\":50862368,\"identity\":\"f9985568-b671-469d-af62-7ba84884a8be\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":400993,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eTensile fracture surfaces of HTPB-0(a), HTPB-1(b), HTPB-4(c) and HTPB-6(d).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage6.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/6cc34b9c2470322ea9896297.png\"},{\"id\":50862599,\"identity\":\"9f530a9c-782a-471d-b9d9-8d19cab074ac\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:12:18\",\"extension\":\"png\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":114790,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe tensile strength (a) and elongation at break (b) of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites at 25℃ and -40℃.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage7.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/22fbb02f4f39b95c549b1a07.png\"},{\"id\":50862367,\"identity\":\"3ff32828-0d38-447a-bb0d-17df00057ccc\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":8,\"title\":\"Figure 8\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":244763,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSchematic diagram of enhancement mechanism for the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage8.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/b009938ee5b2e7c4a7701301.png\"},{\"id\":50862373,\"identity\":\"618cbfb4-afa5-4cb5-8f91-5d901797ce9c\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":9,\"title\":\"Figure 9\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":102711,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe storage modulus (a) and tanδ (b) curves of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage9.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/c2428fff4ad1fe2b0b610638.png\"},{\"id\":50862369,\"identity\":\"86cac400-9a19-47cc-8444-6752f5623193\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"png\",\"order_by\":10,\"title\":\"Figure 10\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":199887,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe TGA (a) and DTG (b) curves of HTPB-0 and HTPB-4.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage10.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/9eecc44bed6588923d4d1464.png\"},{\"id\":50862849,\"identity\":\"c97b137b-553f-4d2f-b73c-d423eff6c04b\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:20:19\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":1977604,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/bd7208a1-66a6-4ee3-8c40-e61a6fa39192.pdf\"},{\"id\":50862372,\"identity\":\"008e12f0-c25a-4ae7-bc75-ac3e497d1ed0\",\"added_by\":\"auto\",\"created_at\":\"2024-02-08 14:04:18\",\"extension\":\"docx\",\"order_by\":2,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":350818,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"SupportInformation.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-3932592/v1/4cc5eaa9b906eeb06c5430aa.docx\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Remarkable mechanical performance at low temperatures of hydroxy-terminated polybutadiene enhanced by hyperbranched polysiloxane\",\"fulltext\":[{\"header\":\"1. Introduction\",\"content\":\"\\u003cp\\u003eHTPB is a liquid rubber renowned for its favorable attributes, encompassing low viscosity, excellent transparency, resistance to aging, and remarkable processing capabilities [\\u003cspan class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e\\u0026ndash;\\u003cspan class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e]. Through the reaction between HTPB and diisocyanate, HTPB-based polyurethane (HTPB-PU) is derived, boasting exceptional mechanical properties, chemical stability, and an outstanding resistance to solvents[\\u003cspan class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e]. Its extensive range of applications spans various domains, such as biocompatible materials[\\u003cspan class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e], coatings [\\u003cspan class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e], adhesives [\\u003cspan class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e], etc. Notably, HTPB-PU plays a pivotal role as a polymer binder in solid rocket propellants [\\u003cspan class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e\\u0026ndash;\\u003cspan class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e]. Consequently, there exists substantial value in enhancing the mechanical properties and low-temperature performance of HTPB specifically for solid propellant applications [\\u003cspan class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e]. Diverse strategies have been pursued to augment the mechanical performance of polyurethane, such as nanoparticle incorporation[\\u003cspan class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e], coordination with metal ion[\\u003cspan class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e] and construction of synergetic multiple dynamic bonds[\\u003cspan class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e]. Among these approaches, hyperbranched polymers (HBPs) have garnered considerable attention from both academia and industry due to their capacity to enhance the mechanical properties of polyurethane composites[\\u003cspan class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e].\\u003c/p\\u003e\\n\\u003cp\\u003eHyperbranched polymers (HBPs), with their unique topology, have garnered significant attention from researchers due to their enhanced fluidity, processability, reduced viscosity, distinctive cavity structure, and a profusion of active end groups[\\u003cspan class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e]. Within the realm of HBPs, hyperbranched polysiloxanes (HBPSi) stand out as a remarkable category, showcasing exceptional thermal properties, chemical stability, low viscosity, abundant terminal functional groups, and facile grafting modification[\\u003cspan class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e\\u0026ndash;\\u003cspan class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e]. Notably, HBPSi has demonstrated the ability to enhance friction reduction, wear resistance, mechnical properties, and flame retardancy of polymers[\\u003cspan class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e\\u0026ndash;\\u003cspan class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e]. Unlike the intricate synthesis processes associated with dendrimers, the preparation method for HBPSi is comparatively straightforward and environmentally friendly [\\u003cspan class=\\\"CitationRef\\\"\\u003e27\\u003c/span\\u003e]. Moreover, the presence of abundant Si-O-C chain segments within HBPSi holds the potential to enhance the compatibility of HTPB with other constituents and theoretically lower its glass transition temperature (T\\u003csub\\u003eg\\u003c/sub\\u003e). This opens up promising avenues for the further modification and application of HTPB.\\u003c/p\\u003e\\n\\u003cp\\u003eIn this investigation, a one-step method was employed to synthesize hyperbranched polysiloxanes with terminal amino groups (HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e), followed by their copolymerization with HTPB. The influence of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e content on the mechanical and thermal properties of the resulting composites was thoroughly examined. Remarkably, the inclusion of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e led to a substantial enhancement in both tensile strength and toughness of the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites, particularly under low-temperature conditions. Additionally, these composites exhibited a lowered T\\u003csub\\u003eg\\u003c/sub\\u003e. Through a comprehensive analysis of the mechanical properties of the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites, a potential mechanism for the reinforcement was proposed.\\u003c/p\\u003e\"},{\"header\":\"2. Materials and experiments\",\"content\":\"\\u003cdiv id=\\\"Sec2\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e2.1. Materials\\u003c/h2\\u003e\\n\\u003cp\\u003e(3-Aminopropyl)triethoxysilane (APTES) was acquired from Jingzhou Jianghan Fine Chemical Co., Ltd. The procurement of 1,3-propylene glycol (PDO) and toluene diisocyanate (TDI) was facilitated by Xi'an Haotian Chemical Glass Instrument Co., Ltd. HTPB was sourced from Shenzhen Hongyuan Chemical New Material Technology Co., Ltd. Anhydrous ethanol and glass instruments, along with other experimental materials, were generously provided by Xi'an Haotian Chemical Glass Instrument Co., Ltd.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e2.2. Preparation of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e\\u003c/h2\\u003e\\n\\u003cp\\u003eThe amine-terminated hyperbranched polysiloxane (HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e) was successfully synthesized through a one-pot copolymerization reaction, as depicted in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e. To prevent gelation during the reaction, the reactant feed ratio was meticulously calculated using the Flory-Carothers equation. In a 250 mL three-necked flask equipped with a stirring rod, thermometer, nitrogen flask, and condensate reflux device, 35.42 g (0.16 mol) of KH-550 and 26.63 g (0.35 mol) of PDO were added. The mixture was gradually heated to 120\\u0026deg;C and held at this temperature until distillation of a byproduct occurred. Subsequently, the distillate temperature was carefully controlled to not exceed 65℃, while gradually increasing the temperature to 160℃. Stirring and heating were ceased once the distillate temperature dropped to approximately 50 ℃. The weight of the collected distillate was determined to be 18.37 g.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003cdiv id=\\\"Sec4\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e2.3. Fabrication of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composities\\u003c/h2\\u003e\\n\\u003cp\\u003eHBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB polyurethane was synthesized by combining HTPB (with a hydroxyl value of 0.66 mmol/g), toluene diisocyanate, and HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, and the reaction mechanism is shown in Figure \\u003cspan class=\\\"InternalRef\\\"\\u003eS1\\u003c/span\\u003e. The ratio of the curing agent was strictly calculated to maintain a specific ratio of functional groups (-NCO: -OH\\u0026thinsp;=\\u0026thinsp;1.1). Initially, HTPB was placed in a 100 mL beaker, and the appropriate amounts of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e (0wt%, 1.0wt%, 2.0wt%, 4.0wt%, 6.0wt%) and toluene diisocyanate were added (referred to as HTPB-0, HTPB-1, HTPB-2, HTPB-4, HTPB-6). The mixture was stirred for 10 minutes. Subsequently, the mixture was degassed in a vacuum oven at 50\\u0026deg;C for 15 minutes. It was then poured into a pre-heated PTFE mold and cured for 80\\u0026times;96 hours. Finally, the cured HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB polyurethane was demolded and prepared for subsequent testing.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003cdiv id=\\\"Sec5\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e2.4. Characterization\\u003c/h2\\u003e\\n\\u003cp\\u003e\\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectra were acquired on the Bruker Avance 400 MHz spectrometer (U.S.A) by using d-DMSO as solvent. Fourier transform infrared spectra (FT-IR) were recorded in the region 400 to 4000cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e on the Nicolet FT-IR 5700 spectrometer (U.S.A). The gel permeation chromatography (GPC) was conducted by using the Agilent 1260 system (U.S.A). The storage modulus and tan\\u0026delta; were acquired on the NETZSCH DMA 242E (Germany), and the heating rate was set at 5℃\\u0026middot;min\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e from \\u0026minus;\\u0026thinsp;100℃ to 0℃. Thermogravimetric (TG) analysis was conducted by using PerkinElmer TGA8000 (U.S.A), and the heating rate was set at 20 ℃\\u0026middot;min\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e from 30 ℃ to 800 ℃ under N\\u003csub\\u003e2\\u003c/sub\\u003e atmosphere. The morphology of tensile fracture surface analysis were performed on the TESCAN VEGA 3 LMH Scanning Electron Microscope (SEM). The content of -OH group was determined according to GB/T 7383\\u0026thinsp;\\u0026minus;\\u0026thinsp;2007 (Chinese standard). The cross-linking density of the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites was determined on the low field NMR analyzer (NIUMAG, PQ001, China). The mechanical property were measured according to GB/T 528\\u0026ndash;2009 (Chinese standard) by electro-mechanical universal testing machine (CMT 6303). The adhesion property was carried out according to GB/T 7124\\u0026thinsp;\\u0026minus;\\u0026thinsp;2008 (Chinese standard) by electro-mechanical universal testing machine (CMT 7204).\\u003c/p\\u003e\\n\\u003c/div\\u003e\"},{\"header\":\"3. Results and discussion\",\"content\":\"\\u003cdiv id=\\\"Sec7\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e3.1. Characterization of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e\\u003c/h2\\u003e\\n\\u003cp\\u003eThe structure of the synthesized HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e was characterized using FT-IR and \\u003csup\\u003e1\\u003c/sup\\u003eH-NMR techniques. Figure\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e displays the FT-IR spectra of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e and the raw materials. In the IR spectrum of PDO, the -OH stretching vibration peak is observed at 3320 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e. The peaks at 2943 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e and 2882 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e correspond to the stretching vibration of -CH\\u003csub\\u003e2\\u003c/sub\\u003e-, which is also evident in the IR spectra of KH-550 and HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e. The -CH\\u003csub\\u003e2\\u003c/sub\\u003e- bending vibration is observed at 1421 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e. The peak at 1062 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e corresponds to the stretching vibration of -OH. In the IR spectrum of KH-550, the -NH\\u003csub\\u003e2\\u003c/sub\\u003e stretching vibration peaks are observed at 3376 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e and 3298 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, while the -NH\\u003csub\\u003e2\\u003c/sub\\u003e bending vibrations are observed at 1600 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e and 958 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, respectively. The stretching vibration of -CH\\u003csub\\u003e3\\u003c/sub\\u003e is observed at 2975 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e. Additionally, the peaks at 1082 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e and 781 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e are assigned to the stretching vibrations of Si-O and Si-C, respectively.\\u003c/p\\u003e\\n\\u003cp\\u003eIn the IR spectrum of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, the stretching vibration peak of -OH is not prominently observed, likely due to its overlap with the -NH\\u003csub\\u003e2\\u003c/sub\\u003e peak. Additionally, the presence of hydrogen bonding between -NH\\u003csub\\u003e2\\u003c/sub\\u003e and -OH results in an elongation of the chemical bond length and a reduction in the chemical bond strength of -NH\\u003csub\\u003e2\\u003c/sub\\u003e. This, in turn, causes a red-shift in the bending vibrations. Furthermore, the stretching vibration peak of Si-C undergoes a red-shift owing to the inductive effect of -NH\\u003csub\\u003e2\\u003c/sub\\u003e, as supported by previous studies [\\u003cspan class=\\\"CitationRef\\\"\\u003e28\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e29\\u003c/span\\u003e]. These findings align with the proposed structure of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e. Notably, the IR spectrum of the collected distillate (as depicted in Figure S2) provides crucial evidence affirming the successful synthesis of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e.\\u003c/p\\u003e\\n\\u003cp\\u003eThe structure of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e was determined using \\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectroscopy, as illustrated in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e. In the \\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectrum of PDO (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ea), the signals observed at 3.71\\u0026ndash;3.19 and 1.64\\u0026ndash;1.47 ppm (labeled as 2 and 1, respectively) correspond to the protons of HO-CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e- and HO-CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-. The 3.71\\u0026ndash;3.19 ppm signal arises from the proton of HO-CH\\u003csub\\u003e2\\u003c/sub\\u003e-. In the \\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectrum of KH-550 (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eb), signals at 3.93\\u0026ndash;3.66, 2.64\\u0026ndash;2.41, 1.52\\u0026ndash;1.29, and 0.64\\u0026thinsp;\\u0026minus;\\u0026thinsp;0.45 ppm (labeled as 1, 2, 4, and 3, respectively) correspond to the protons of CH\\u003csub\\u003e3\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-, NH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-, -CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-, and -CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-Si-, respectively. The protons of methyl groups and amino groups are observed at 1.24\\u0026ndash;0.97 ppm.\\u003c/p\\u003e\\n\\u003cp\\u003eIn the \\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectrum of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ec), signals at 4.05\\u0026ndash;3.93, 3.83\\u0026ndash;3.70, 3.51\\u0026ndash;3.36,1.94\\u0026ndash;1.70, 1.68\\u0026ndash;1.60, 1.61\\u0026ndash;1.50, 1.48\\u0026ndash;1.31, and 0.64\\u0026thinsp;\\u0026minus;\\u0026thinsp;0.45 ppm (labeled as 9, 7, 2, 5, 8, 6, 3, and 4, respectively) correspond to the protons of methylene groups. The -NH\\u003csub\\u003e2\\u003c/sub\\u003e protons are observed at 1.09\\u0026ndash;1.01 ppm. Notably, the signals at 3.83\\u0026ndash;3.7 ppm are noticeably weaker compared to those observed in KH-550. This observation suggests that the oxyethyl groups of KH-550 are not fully consumed by the excessive hydroxyl groups of PDO, likely due to the increasing steric hindrance effect during the reaction [\\u003cspan class=\\\"CitationRef\\\"\\u003e28\\u003c/span\\u003e]. The \\u003csup\\u003e1\\u003c/sup\\u003eH NMR spectra of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e and the raw materials provide compelling evidence confirming the successful synthesis of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e. Furthermore, the GPC spectrum of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, depicted in Figure S3 and Table \\u003cspan class=\\\"InternalRef\\\"\\u003eS1\\u003c/span\\u003e, supports this conclusion. Additionally, the hydroxyl value of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e was determined using titration methods, and the results are presented in Table S2.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003cdiv id=\\\"Sec8\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e3.2. Cross-linking density and mechanical properties of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites\\u003c/h2\\u003e\\n\\u003cp\\u003eThe presence of abundant hydroxyl and amino groups in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e enables chemical cross-linking with isocyanate groups, facilitating the formation of covalent bonds between HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e and TDI. This cross-linking mechanism enhances the overall cross-linking density of the composites, resulting in a significant improvement in their strength.\\u003c/p\\u003e\\n\\u003cp\\u003eThe relaxation time-dependent variation of NMR signal intensity in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites was analyzed to characterize their properties (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (a)). Additionally, the cross-linking density and the average molecular weight between cross-linking points were determined using low-field nuclear magnetic resonance (NMR) (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (b)). As depicted in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (a), the decrease in NMR signal intensity becomes more pronounced with increasing HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e content, indicating an enhanced binding effect and cross-linking density in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites [\\u003cspan class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e]. This observation is also supported by Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (b). Comparing HTPB-0 with HTPB-6, the cross-linking density increased by 26.3% (3.65 \\u0026times; 10\\u003csup\\u003e\\u0026minus;\\u0026thinsp;4\\u003c/sup\\u003e mol\\u0026middot;mL\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e), while the average molecular weight between cross-linking points decreased by 25.97% (2.58 kg\\u0026middot;mol\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e). These changes can be attributed to the incorporation of a significant number of active amino and hydroxyl groups from HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e into HTPB. These functional groups react with the isocyanate, actively participating in the curing process of HTPB and resulting in an improved cross-linking density in the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites.\\u003c/p\\u003e\\n\\u003cp\\u003eBased on the aforementioned analysis, it is evident that the incorporation of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e enhances the cross-linking density of the composites, which may potentially compromise their mechanical properties. The results of the tensile tests are presented in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e. As depicted in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e, the addition of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e leads to simultaneous improvements in the tensile strength and elongation at break of HTPB. The tensile strength increases with increasing HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e content, with HTPB-6 exhibiting the highest tensile strength of 2.08 MPa, representing a remarkable 181.08% increase compared to HTPB-0 (0.74 MPa). Meanwhile, the elongation at break gradually reaches its peak at 625.24% when the loading of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e reaches 2.0wt%, demonstrating a 72.79% increase compared to HTPB-0 (361.86%). In the cross-linked network of HTPB, the hyperbranched structure of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e acts as the soft component, while the benzene ring structure of TDI serves as the hard component, significantly improving the tensile properties of HTPB through a synergistic effect of flexibility and rigidity. The abundant functional groups in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e facilitate the formation of multiple hydrogen bonds within the composites and increase the number of covalent bonds, thereby enhancing the cross-linking density. Furthermore, the unique spherical structural network of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e possesses a significant amount of free volume, enabling it to absorb energy from external sources and generate new voids when subjected to external forces [\\u003cspan class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e]. All of these factors contribute to the absorption and dissipation of a considerable amount of failure energy when HTPB is subjected to external stress, resulting in an improvement in the tensile strength. Typically, an increase in cross-linking density and tensile strength leads to a reduction in the elongation at break [\\u003cspan class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e]. However, in the case of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites, an increase in the elongation at break is observed. This can be attributed to the presence of abundant Si-O-C flexible chain segments in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, which enhance the flexibility of the molecular chains by facilitating the rotation of chemical bonds and chain segments. However, as the amount of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e increases, the cross-linking density becomes excessively high, impeding the movement of molecular chain segments and gradually reducing the improvement in elongation at break.\\u003c/p\\u003e\\n\\u003cp\\u003eIn order to validate the underlying mechanism of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e enhancement, the tensile fracture surfaces of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites were examined using SEM imaging, as illustrated in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e The SEM image depicted demonstrates a gradual transition from a flat and smooth surface to a rough and heterogeneous morphology upon the addition of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e. In the case of HTPB-0 (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e(a)), the fracture surface appears flat and smooth, devoid of any noticeable phase separation. As the quantity of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e increases, the fracture morphology of the composites progressively becomes rough, displaying the presence of rough filaments known as \\\"dimples,\\\" which are indicative of toughness fracture. In this condition, stress can be effectively distributed, thereby showcasing relatively outstanding mechanical properties. Nonetheless, if the dosage of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e becomes excessively high (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e(d)), it can lead to the aggregation in HTPB, resulting in a more heterogeneous and irregular fracture surface. This, in turn, leads to the concentration of stress in specific regions, potentially affecting the mechanical properties of the composites.\\u003c/p\\u003e\\n\\u003cp\\u003eTo explore the behavior of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites under low temperature conditions, a comparative analysis of their mechanical properties was conducted at both 25\\u0026deg;C and \\u0026minus;\\u0026thinsp;40\\u0026deg;C. The results, as presented in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e, reveal that the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites exhibit enhanced tensile strength and elongation at break when subjected to a -40\\u0026deg;C environment. Specifically, HTPB-4 demonstrates a tensile strength of 9.7 MPa and an elongation at break of 455.54%, representing a remarkable increase of 448.02% and 26.41%, respectively, compared to HTPB-0. It is noteworthy that the modification effect of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e on the tensile strength of HTPB is particularly pronounced at low temperatures, surpassing that observed at room temperature, as illustrated in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e. However, the effect on elongation at break is slightly diminished due to the higher cross-linking density. These results suggest that the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites maintain elevated mechanical strength under low temperature conditions. Nevertheless, as the temperature decreases, the mobility of material molecules is restricted, leading to reduced activity and flexibility of the molecular chains. Consequently, the malleability and elongation at break of the composites decrease. Overall, the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites exhibit commendable mechanical strength at low temperatures, showcasing exceptional adaptability in low-temperature environments.\\u003c/p\\u003e\\n\\u003cp\\u003eBased on the extensive investigation conducted on the mechanical properties of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites, we have elucidated the mechanism behind their enhancement, as depicted in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003e. Firstly, the active -OH and -NH\\u003csub\\u003e2\\u003c/sub\\u003e groups present in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e play a crucial role in forming multiple hydrogen bonds within the composites, thereby augmenting the intermolecular forces. Additionally, these groups can undergo reactions with isocyanates, leading to the formation of covalent bonds and a subsequent increase in the cross-linking density of the composites. This substantial enhancement in the cross-linking density significantly reinforces the strength of HTPB.\\u003c/p\\u003e\\n\\u003cp\\u003eFurthermore, the abundant presence of flexible Si-O-C segments within HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e contributes to the improved mobility of molecular chains, particularly at low temperatures. This enhanced mobility imparts greater toughness to HTPB, rendering it more resilient in low-temperature environments. In summary, the remarkable mechanical properties and adaptability of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites can be attributed to the synergistic effects of multiple factors, as described above.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003cdiv id=\\\"Sec9\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e3.3. Dynamic thermomechanical analyses of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites\\u003c/h2\\u003e\\n\\u003cp\\u003eDMA was carried out to investigate the effect of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e on the storage modulus and T\\u003csub\\u003eg\\u003c/sub\\u003e of HTPB composites, as shown in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003e. Figure\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003e(a) illustrates the storage modulus curves. In comparison to HTPB-0, HTPB-4 demonstrates a notable increase in its storage modulus. This enhancement signifies that the inclusion of HBPSi-NH2 substantially augments the rigidity of HTPB, consequently improving the material's stability and load-bearing capacity in practical applications. The principal factor contributing to this effect is the presence of abundant active functional groups in the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e. These functional groups actively participate in the curing process of HTPB, leading to an increase in the composites' crosslink density and rigidity. Figure\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003e(b) illustrates the tan\\u0026delta; curves, which exhibit a single peak over the entire temperature testing range, with the peak temperature corresponding to the glass transition temperature (T\\u003csub\\u003eg\\u003c/sub\\u003e). The symmetrical peak shape indicates that HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites possess a homogeneous structure, and there is strong compatibility observed between HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e and HTPB. Furthermore, it is evident that the T\\u003csub\\u003eg\\u003c/sub\\u003e of HTPB-4 has decreased to -75.62\\u0026deg;C, which is 9.67\\u0026deg;C lower than that of HTPB-0. This reduction can be primarily attributed to the presence of flexible Si-O-C segments within HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e. These segments diminish the intermolecular attractive forces, facilitating molecular chain movement at lower temperatures and resulting in a lower T\\u003csub\\u003eg\\u003c/sub\\u003e. Additionally, the highly branched structure of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e may introduce additional gaps between HTPB molecules, reducing their close packing and further contributing to the lowered T\\u003csub\\u003eg\\u003c/sub\\u003e. These factors collectively benefit HTPB by enabling it to maintain relatively stable performance at low temperatures while preserving its elasticity and toughness.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003cdiv id=\\\"Sec10\\\" class=\\\"Section2\\\"\\u003e\\n\\u003ch2\\u003e3.3. Thermal analyses of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e/HTPB composites\\u003c/h2\\u003e\\n\\u003cp\\u003eThe thermal properties of HTPB in the presence of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e were investigated using the TGA method. As depicted in Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e10\\u003c/span\\u003e (a), both HTPB-0 and HTPB-4 exhibit similar thermal decomposition processes, suggesting that the addition of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e does not alter the underlying thermal decomposition mechanism of HTPB.\\u003c/p\\u003e\\n\\u003cp\\u003eAlthough the initial decomposition temperatures(T\\u003csub\\u003e5%\\u003c/sub\\u003e, corresponding to 5wt% loss) of HTPB-4 and HTPB-0 are similar, the maximum decomposition temperature (T\\u003csub\\u003e50%\\u003c/sub\\u003e, corresponding to 50wt% loss) of HTPB-4 is significantly higher. This disparity can be attributed to the presence of raw monomer and residual ethanol in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, leading to a comparatively marginal elevation in the initial decomposition temperature of the composites. However, as the temperature rises, the beneficial impact of chemical bonds such as -OH, -O-, -CH\\u003csub\\u003e2\\u003c/sub\\u003e-CH\\u003csub\\u003e2\\u003c/sub\\u003e-, and others present in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, become conspicuous in augmenting the thermal stability of HTPB. Despite an increase in the decomposition rate, Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e10\\u003c/span\\u003e (b) illustrates that the decomposition peak of HTPB-4 shifts to higher temperature. The TGA results demonstrate that the HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e effectively preserves the basic thermal stability of HTPB composites. The tailed thermal decomposition data are presented in Table S3.\\u003c/p\\u003e\\n\\u003c/div\\u003e\"},{\"header\":\"4. Conclusions\",\"content\":\"\\u003cp\\u003eIn this study, we successfully synthesized a novel hyperbranched modifier called HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e through a straightforward polycondensation reaction. The primary objective was to enhance both the mechanical properties and low-temperature adaptability of HTPB, reducing its glass transition temperature (T\\u003csub\\u003eg\\u003c/sub\\u003e) simultaneously. Remarkably, the addition of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e led to substantial improvements in the tensile strength and elongation at break of HTPB-4, with increases of 125.68% and 62.66% compared to HTPB-0, respectively. Interestingly, even at -40\\u0026deg;C, HTPB-4 still exhibited impressive tensile strength and elongation at break, with values maintained at 447.02% and 26.41% compared to room temperature, respectively. Furthermore, the considerable number of flexible Si-O-C chains in HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e effectively improved the mobility of molecular chains, thus enhancing the flexibility and adaptability of HTPB at low temperatures. Consequently, this results in a reduction of its T\\u003csub\\u003eg\\u003c/sub\\u003e (-65.95\\u0026deg;C to -75.62\\u0026deg;C), while leaving the thermal stability of HTPB unaffected. This comprehensive investigation provides valuable insights into the strengthening mechanism of HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e on HTPB, establishing a solid foundation for future research in this area. Moreover, the proposed approach offers a novel pathway for the preparation of HTPB-PU with outstanding low-temperature adaptability, presenting promising opportunities for various applications.\\u003c/p\\u003e \"},{\"header\":\"Declarations\",\"content\":\"\\u003ch2\\u003eCredit authorship contribution statement\\u003c/h2\\u003e\\n\\u003cp\\u003eJunshan Yuan: Conceptualization, Formal analysis, Writing - original draft. Xiaoying Huang: Investigation. Rui Wang: Review. Wei Tian: Supervision. Weixu Feng: Supervision. Hongxia Yan: Conceptualization, Writing - review \\u0026amp; editing.\\u003c/p\\u003e\\n\\u003ch2\\u003eConflicts of interest\\u003c/h2\\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\\u003ch2\\u003eAcknowledgements\\u003c/h2\\u003e\\n\\u003cp\\u003eThis work is sponsored by the National Natural Science Foundation of China (22175143) and Foundation of Science and Technology on Combustion and Explosion Laboratory (J-JK-JJ-2201/6142603032210). Thanks to the Analytical \\u0026amp; Testing Center of Northwestern Polytechnical University for test assistance.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eYarmohammadi M, Shahidzadeh M, Ramezanzadeh B (2018) Designing an elastomeric polyurethane coating with enhanced mechanical and self-healing properties: The influence of disulfide chain extender. Prog Org Coat 121:45\\u0026ndash;52\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eShahidzadeh M, Varkaneh ZK, Ramezanzadeh B, Pedram MZ, Yarmohammadi M (2020) Self-healing dual cured polyurethane elastomeric coatings prepared by orthogonal reactions. Prog Org Coat 140:105503\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eToosi FS, Shahidzadeh M, Ramezanzadeh B (2015) An investigation of the effects of pre-polymer functionality on the curing behavior and mechanical properties of HTPB-based polyurethane. 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Compos Part B-Engineering 190:107901\\u003c/span\\u003e\\u003c/li\\u003e\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"polymer-bulletin\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"pobu\",\"sideBox\":\"Learn more about [Polymer Bulletin](http://link.springer.com/journal/289)\",\"snPcode\":\"289\",\"submissionUrl\":\"https://submission.nature.com/new-submission/289/3\",\"title\":\"Polymer Bulletin\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false},\"keywords\":\"\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-3932592/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-3932592/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eThe inadequate mechanical properties and limited low temperature adaptability of Hydroxy-terminated polybutadiene (HTPB) impose constraints on its practical utilization in solid propellant applications. In the present investigation, a pioneering approach involved the synthesis of a novel hyperbranched polysiloxane, denoted as HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e, which encompasses -NH\\u003csub\\u003e2\\u003c/sub\\u003e groups and Si-O-C chains. The HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e with its unique flexible Si-O-C segments, serving as the soft component in the crosslinked network, in conjunction with the curing agent TDI as the hard component, achieves a synergistic balance of rigidity and flexibility. The resulting HTPB composites not only demonstrate enhanced mechanical properties but also exhibit excellent low temperature adaptability. Remarkably, the HTPB composites exhibit excellent mechanical properties at both 25\\u0026deg;C (0.74 MPa\\u0026thinsp;~\\u0026thinsp;2.08 MPa) and \\u0026minus;\\u0026thinsp;40\\u0026deg;C (1.77 MPa\\u0026thinsp;~\\u0026thinsp;12.49 MPa). This enhancement can be ascribed to the abundant presence of functional groups, namely -OH and -NH\\u003csub\\u003e2\\u003c/sub\\u003e. These active groups significantly augment the cross-linking density within the HTPB system, also promote the formation of numerous hydrogen bonds, enhancing the strength of HTPB. Simultaneously, the abundant presence of Si-O-C flexible chain segments within HBPSi-NH\\u003csub\\u003e2\\u003c/sub\\u003e enhances the reactivity of the HTPB molecular chains, not only improving the toughness of HTPB but also significantly reducing its T\\u003csub\\u003eg\\u003c/sub\\u003e (-65.95\\u0026deg;C to -75.62\\u0026deg;C). Furthermore, this study establishes a pivotal direction for the design and synthesis of high-performance HTPB-PU materials.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Remarkable mechanical performance at low temperatures of hydroxy-terminated polybutadiene enhanced by hyperbranched polysiloxane\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2024-02-08 14:04:13\",\"doi\":\"10.21203/rs.3.rs-3932592/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0},{\"type\":\"decision\",\"content\":\"Revision requested\",\"date\":\"2024-05-06T20:10:21+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorInvitedReview\",\"content\":\"\",\"date\":\"2024-04-15T18:09:05+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewerAgreed\",\"content\":\"c9d79d5c-89f2-4e9a-a905-f511b3a6dd74\",\"date\":\"2024-04-01T10:24:36+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"editorInvitedReview\",\"content\":\"\",\"date\":\"2024-02-11T22:44:04+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewerAgreed\",\"content\":\"06e1e56e-8f93-4643-932d-85e670d417e4\",\"date\":\"2024-02-09T03:54:39+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewersInvited\",\"content\":\"\",\"date\":\"2024-02-08T20:31:54+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorAssigned\",\"content\":\"\",\"date\":\"2024-02-06T09:46:29+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"checksComplete\",\"content\":\"\",\"date\":\"2024-02-06T06:38:55+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"submitted\",\"content\":\"Polymer Bulletin\",\"date\":\"2024-02-06T03:03:20+00:00\",\"index\":\"\",\"fulltext\":\"\"}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"polymer-bulletin\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"pobu\",\"sideBox\":\"Learn more about [Polymer Bulletin](http://link.springer.com/journal/289)\",\"snPcode\":\"289\",\"submissionUrl\":\"https://submission.nature.com/new-submission/289/3\",\"title\":\"Polymer Bulletin\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false}}],\"origin\":\"\",\"ownerIdentity\":\"431c1bb3-b66a-413d-8f8e-43b6713bb372\",\"owner\":[],\"postedDate\":\"February 8th, 2024\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"under-review\",\"subjectAreas\":[],\"tags\":[],\"updatedAt\":\"2024-06-19T19:53:17+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2024-02-08 14:04:13\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-3932592\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-3932592\",\"identity\":\"rs-3932592\",\"version\":[\"v1\"]},\"buildId\":\"qtupq5eGEP_6zYnWcrvyt\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}