Facile Fabrication of Polymer/Palygorskite Microcapsules via Pickering Emulsion Photopolymerization

Facile Fabrication of Polymer/Palygorskite Microcapsules via Pickering Emulsion Photopolymerization | 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 Facile Fabrication of Polymer/Palygorskite Microcapsules via Pickering Emulsion Photopolymerization Jin Li, Xinyang Wang, Pengying Zhang, Quan Chen, Dandan Min, Xiaowu Jiang, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4407913/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 Feb, 2025 Read the published version in Journal of Polymer Research → Version 1 posted 5 You are reading this latest preprint version Abstract A facile photocatalytic Pickering emulsion polymerization process was developed to fabricate polymer/clay composite microcapsules. Photolatent Pickering emulsions of oil (monomer, crosslinker, Irgacure 819 and octane) in water were prepared using palygorskite fibers (PAL) as particulate emulsifier. The effect of PAL concentration on the emulsion stabilities was well studied. 3 wt% of PAL fibers were required in order to obtain a stable Pickering emulsion. Moreover, serials of factors on the formation of microcapsules have been investigated, such as PAL concentration, photoinitiator percentage, light intensity, crosslinker/monomer radio and monomer type. Under the appropriate conditions, polymer/PAL microcapsules with spherical morphology can be produced easily. Consequently, a formation mechanism of the microcapsules has been proposed. Suspension photopolymerization Pickering emulsion Palygorskite Microcapsules Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Introduction The polymer microcapsules are of great interest due to the wide range of available polymers and functionalities [ 1 ], which have been extensively developed in various industries, such as agrochemicals [ 2 , 3 ], pharmaceuticals [ 4 ], catalysis [ 5 ], energy storage [ 6 ], cosmetics [ 7 ], self-lubricating coatings [ 8 , 9 ], and so on. Up to date, a variety of preparation techniques have been developed range from colloidal templates [ 10 ], layer-by-layer assembly [ 11 ], spray-drying [ 12 ], as well as heterophase polymerization [ 13 ]. However, most of the approaches have to use "templates" in order to build the capsule-like structure for the microcapsules. Where silica or polymer nanoparticles used as hard templates, it often requires an extra procedure and hazardous materials for the removal of the templates. In comparison to the hard-templating technique, soft templates are normally using amphiphilic surfactants as emulsifiers to form emulsion droplets. The microcapsules can be formed by subsequently polymerization of the emulsion droplets. It simplifies the preparation procedure of microcapsules. However, the surfactants are often toxic to human bodies and difficult to remove completely from the microcapsules [ 9 ]. Therefore, a facile preparation method with great technical applicability to access polymer microcapsules is remaining highly required. Pickering emulsion is an emulsion that stabilized by solid particles instead of amphiphilic organic surfactants [ 14 ]. Compared to traditional emulsions stabilized by surfactants, Pickering emulsions have the advantages of convenient preparation, excellent stabilization, low toxicity and high sustainability [ 15 ]. Polymerization of Pickering emulsion to fabricate microcapsules has attracted extensive attention [ 16 – 19 ]. The mechanical and thermal properties of polymer shells of the formed microcapsules can be further enhanced in comparison to surfactants [ 20 ]. Besides, the functionality, structure and size for the microcapsules can be regulated by the design of Pickering emulsion formulations [ 17 , 21 ]. Thus, a variety of microcapsules, such as TiO 2 /PS [ 22 ], SiO 2 /Polymer [ 8 , 23 ], Clay/Polymer [ 24 ], Chitosan/PLGA [ 25 ], MOF/PS [ 26 ], GO/polymer [ 27 , 28 ], etc., have been prepared via Pickering emulsion templated polymerization. However, most of the polymerization process is induced by thermal treatment, which required strict reaction conditions and time-consuming operations. Photopolymerization is well-known for its sustainable, energy efficient, high polymerization rates and temporal controllable [ 29 ], which has been widely used in polymer-based coating industry [ 30 ]. In our previous work, a Pickering emulsion templated photopolymerization procedure was developed [ 31 ]. PMMA/palygorskite microspheres with mesoporous structure were prepared successfully via such a facile photopolymerization process, but not microcapsules. In comparison to microspheres, the formation of microcapsules is much more difficult [ 32 ]. As the formed capsule wall is very thin in most cases, it leads to the easy broken of the microcapsule. Besides, to the best of our knowledge, access microcapsules using a Pickering emulsion template photopolymerization process have not been reported in the literature. Therefore, the photo chemical factors (such as photoinitiator concentration, light intensity, etc.) and formulation compositions to the pickering emulsion template photopolymerization kinetics and the morphology of microcapsules are still unclear. Herein, an effective and practical Pickering emulsion photopolymerization method was developed to prepare polymer/PAL microcapsules. Photolatent pickering emulsions of oil (monomer, crosslinker, Irgacure 819 and octane) in water were prepared using palygorskite fibers (PAL) as particulate emulsifier. The effect of PAL concentration on the emulsion stabilities as well as the microcapsule morphology was well studied. Furthermore, the photopolymerizaiton factors, such as photoinitiator content, light intensity, etc., on the formation of the microcapsules have been investigated. The factors of crosslinker/monomer ratio and monomer type on the microcapsule morphology were also studied. Consequently, a formation mechanism of the microcapsules has been proposed. Experimental part Materials Palygorskite (PAL, > 90 wt %) was obtained from Jiangsu Zhongyuan Minerals Co. Ltd. (Huaian, China). Methyl methacrylate (MMA), Styrene (ST), Diethylaminoethyl methacrylate (DEAEMA), poly(ethylene glycol) diacrylate (EGDMA), octane (AR) and aluminum oxide (AR) were purchased from Aladdin Chemical Co. Ltd. Ethanol (AR) was supplied by Shanghai Jiuyi Chemical Reagent Co. Ltd. Methyl acrylate (MA) was obtained from Sinopharm Chemical Reagent Co. Ltd. n -Butyl acrylate (BA) was purchased J&K Scientific Ltd. n -Butyl methacrylate (BMA) was purchased from Tokyo Chemical Industry. Irgacure 819 (I819) was obtained from BASF. Pickering emulsion preparation Firstly, PAL particles (0.15 ~ 0.40 g) were dispersed into 10 mL deionized water by ultra-sonification procedure (PS-40A, Jeken Ultrasonic) for 5 min, forming homogeneous PAL/water dispersion. Then, MMA (0.5 mL, 4.67 mmol), EGDMA (0.5 mL, 2.65 mmol) and I819 (0.06 g) was dissolved into octane (4 mL), forming a homogeneous oil phase. Consequently, the formation oil phase and the PAL/water dispersion were mixed via a lab dissolver (NSR-I, Shanghai Nengu) operating at 12,000 rpm for 5 min. An O/W type pickering emulsion was formed. Pickering emulsion photopolymerization The photopolymerization of pickering emulsion was carried out via a special homemade quartz micro-reactor setup (5 ± 0.1 mL, internal channel diameter 1000 ± 5 µm) [ 31 ]. The emulsion was loaded into the micro-reactor via a peristaltic pump with a flow rate of 5 mL h − 1 under 395 nm LED irradiation. Then the photopolymerized product was filtered and washed for three times using ethanol. Finally, the solid product was dried at 50 o C under vacuum for 12h. Characterization Polarizing optical photographs of the PAL stabilized emulsions were obtained from Leica DMLP polarized optical microscope. Morphology of the photopolymerized products was characterized by scanning electron microscopy (SEM, Hitachi S-3000N, working at 25 kV). Before analysis, the samples were coated with a 15 nm thick layer of gold to reduce the charging effect on the surface. FT-IR spectra of the PAL and polymerized samples were recorded via a Nicolet 5700 spectrophotometer with a resolution of 4 cm − 1 in the range of 4000 − 600 cm − 1 for 32 scans. For all of the sample powders, they were characterized by using KBr pellet technique. The monomer conversions were determined from the mass ratio of added monomer to obtained polymer. Results and discussion PAL-stabilized pickering emulsion PAL is a kind of natural fibrous phyllosilicate mineral [ 33 ]. As shown in Fig. 1 , the size of the PAL fibrous crystal used in this research is typically with 0.5-2 µm in length. It is a very effective emulsifier for O/W type pickering emulsion because of its high aspect ratios, suitable hydrophilicity and large surface area. Because of the plenty of silanol groups on their surfaces, the PAL fibers can be well dispersed in water, forming very stable suspensions. By adding an amount of octane solution that containing MMA, EGDMA and I819 as oil phase in such PAL water suspension, an oil in water (O/W) type Pickering emulsion can be formed after emulsification. Figure 2 a provides digital photographs of Pickering emulsions prepared with different percentage of PAL fibers. As can be seen, the emulsion prepared with a low percentage of 1.5 wt% PAL fibers is not stable. Clearly, a phase separation occurred. In comparison to octane and monomer, the density of water is higher. Thus, the formed transparent liquid at the bottom of the bottle is separated water from the emulsion. Along with the increasing of PAL concentration, the fraction of water separated from the emulsion is decreasing (Fig. 2 b). When the PAL percentage increased to 3 wt%, there is almost no water separated from the emulsion. Direct evidence was provided by the polarizing microscopy characterization. Figure 2 c illustrates the polarizing micrographs of pickering emulsion stabilized by 3 wt% of PAL fibers. As can be seen, there are plenty of spherical rings. Each ring is composed by a light halo surrounding a dark and round spot. As the PAL crystal has birefringent properties, such phenomenon only can be observed during the PAL fibers absorbed on the surface of the emulsion droplets and nearly wrapped up the droplet surface. It suggests that the PAL fibers formed a close packing structure on the emulsion droplet surfaces, which is crucial for the solid particles serving as particulate emulsifier in literature [ 34 ]. Preparation and characterization of Polymer/PAL composite microcapsules The Pickering emulsion photopolymerization can be carried out conveniently. Different experimental conditions were further investigated. Figure 3 displays the morphology of the PMMA/PAL microcapsules prepared with different concentration of PAL fibers. Some spherical microcapsules can be observed only with 1.5 wt% PAL fibers (Fig. 3 a). Many fragments of microcapsules also can be seen. Interestingly, the number of formed microcapsules is increased along with the increasing of PAL concentration (Fig. 3 b-d). When the PAL concentration reached to 3 wt%, plenty of of PMMA/PAL microcapsules were formed (Fig. 3 d). The fragments of microcapsules almost disappeared. However, only broken microcapsules were observed by further increasing the PAL concentration to 3.5 wt% (Fig. 3 e) or even higher. It can be expected that the formation of microcapsules became better along with the increasing of PAL concentration as the emulsion became more and more stable. When the PAL concentration reached to 3.5 wt%, the mechanical properties of the formed PAL/PMMA composite shell is not enough to support the microcapsules maintain their structure. The high PAL concentration partially impeded the photopolymerization and probably it leads to such result. SEM image of PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers (corresponding to Fig. 3 d) at higher magnifications is displayed in Fig. 4 a. The PAL fibers can be seen clearly that dispersed randomly on the surface of microcapsule. Such result is quite in accordance with the observation of the polarizing micrographs of the emulsion (Fig. 2 c). It further proves the PAL fibers was absorbed on the surface of the emulsion droplets and played as the role of particulate emulsifier. Figure 4 b shows the SEM image of PMMA/PAL microcapsules after grinding. It demonstrates that the hollow structure of the microcapsule is formed successfully via such pickering emulsion templated photopolymerization process. The PMMA/PAL shell of the microcapsule is very thin, only with few hundred nanometers. Figure 5 shows the FTIR spectra of PAL fibers and PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers. The absorption bands located at 3,800-3,395 cm − 1 in Fig. 5 a can be attributed to the hydroxyl groups of coordinated water in the tunnels of PAL crystals [ 35 ]. The absorption bands at 1,030 cm − 1 and 982 cm − 1 can be ascribed to the stretching and the bending vibrations of Si-O-Si bonds [ 36 ]. Such absorption bands also can be seen in Fig. 5 b, which is the FTIR spectrum of PMMA/PAL microcapsules. Furthermore, the presence of absorption bands at 2970 cm − 1 can be ascribed to C-H stretching vibration of methyl group. The sharp and strong absorption bands at 1732 cm − 1 is the characteristic carbonyl stretching vibration of ester group. These results indicated that the PMMA/PAL composite has been fabricated successfully. Figure 6 further illustrates the monomer conversion value of emulsions with different percentage of PAL fibers after the photopolymerization process. The monomer conversion is decreased along with the increasing of PAL concentration. When the PAL concentration is 1.5 wt%, the monomer conversion is 83.6%. By increasing the PAL concentration to 3.5 wt%, the monomer conversion decreased to 70.2%. Such result is probably because of the light scattering induced by the PAL fibers located at the surface of emulsion droplets (Fig. 2 c and Fig. 4 a). It hinders the photolysis of photoinitiator to initiate the radical polymerization of monomers. The higher amount of PAL concentration will scatter more light, which lead to the lower monomer conversion. After the investigation of the PAL concentration effect, further studies on the influence of photoinitiator concentration were carried out. Figure 7 displays the SEM images of PMMA/PAL microcapsules prepared with different concentration of photoinitiator. No microcapsule or microsphere is formed only with 2 wt% of I 819 photoinitiator (Fig. 7 a). The number of formed microcapsules is increasing along with the growth of the photoinitiator concentration (Fig. 7 b-c). As can be seen from Fig. 7 c, there are plenty of microcapsules when the concentration of I 819 reached to 6 wt%. As it is well-known, 2 wt% of photoinitiator is sufficient enough in most cases for the conventional radical photopolymerization [ 37 ]. However, here it required a higher photoiniatiator concentration due to the light scattering of the solid emulsifier in Pickering emulsion photopolymerization. Figure 8 shows the SEM images of PMMA/PAL microcapsules prepared under different light irradiation intensity. As can be seen from Fig. 8 a, plenty of PMMA/PAL microcapsules were formed with 20 mW/cm 2 irradiation intensity. The morphology of the most microcapsules was not spherical. Along with the decreasing of light irradiation intensity, the spherical microcapsules became more and more. Besides, the number of broken microcapsules was increasing. As it is well-known, the photoinitiator decomposed into free radical fragments will be faster with higher light irradiation intensity for photopolymeriztion process, leading to higher polymerization rate and lower molecular polymer chain [ 38 ]. In this case, the light reached to the photoinitiator that inside of the emulsion droplets is decreased dramatically due to the light shielding of PAL fibers. It leads to the insufficient polymerization and the large amount of broken microcapsules (Fig. 8 c). Combining the removal of octane, the shell composed by low molecular weight polymer (high light irradiation intensity) and PAL fibers couldn’t afford the shrinkage, inducing the non-spherical morphology of the formed microcapsules. After optimal the light intensity, further studies were carried out on the crosslinker and monomer radios. Figure 9 displays the SEM images of PMMA/PAL-3 microcapsules prepared at different volume ratios of crosslinker (EGDMA) and monomer (MMA). When the EGDMA/MMA ratio was 6/10, many cracked microcapsules were formed (Fig. 9 a). Obviously, spherical shape microcapsules were formed and their number was increased along with the increasing of the EGDMA/MMA ratio (Fig. 9 b and Fig. 9 c). However, only fragments of microcapsules could be observed by further increasing the EGDMA/MMA ratio to 12/10 (Fig. 9 d). However, it is still unclear whether a wide range of radically polymerizable monomers can be used for the preparation of microcapsules via Pickering emulsion templated photopolymerization process. Figure 10 shows the SEM images of Polymer/PAL-3 microcapsules prepared with different monomers (MA, MMA, BA, BMA, St and DEAEMA). Interestingly, the microcapsules can be formed for all of the monomers. However, the best monomer for the formation of spherical microcapsules is DEAEMA (Fig. 10 f). In comparison to MA monomer (Fig. 10 a), the formation of microcapsules is better by using BA as monomer (Fig. 10 c). It is probably because the monomer of BA has a butyl tail, which endowed the formed poly( n -butyl acrylate) with better toughness [ 39 ]. Such phenomenon is really interesting, but it still requires further investigation in order to find out the proof. The formation mechanism of PMMA/PAL microcapsules was schematically displayed in Fig. 11 . Firstly, the homogeneous oil phase of octane solution that containing monomers and photoinitiators was dispersed in PAL water gel. The oil droplets were surrounded by PAL fibers and forming stable oil in water type Pickering emulsion, as it has been proved by polarizing microscopy observation in Fig. 2 c. In the second step, the formed Pickering emulsion was further polymerized via light irradiation in a homemade quartz reactor. The formation of PMMA polymers that induced them separated from the octane in each PAL wrapped droplet, as the octane is a poor solvent for the polymer. Along with the light irradiation, the droplets became PMMA/PAL microcapsules which encapsulated octane inside. Finally, PMMA/PAL microcapsules were achieved by removing the octane. Conclusions Polymer/PAL microcapsules were fabricated successfully via Pickering emulsion photopolymerization. The effect of PAL concentration, photoinitiator percentage, light intensity, crosslinker/monomer radio and monomer type on the formation of microcapsules was investigated. Although the emulsion stability was along with the PAL concentration, it was found that in the presence of 3 wt% of PAL emulsifier, the emulsion droplets gave the best transformation of microcapsules. Furthermore, 6 wt% of photoinitiator with 2 mW/cm 2 irradiation intensity provided the most suitable photopolymerization conditions for the formation of polymer/PAL microcapsules. DEAEMA as monomer with crosslinker of EGDMA in 10/10 volume ratio offered the best formulation to form spherical microcapsules. Such microcapsule probably formed because of the polymer separated from the octane in each PAL wrapped droplet during photopolymerization. Declarations Acknowledgements The authors would like to thank the financial support from the Natural Science Foundation of Jiangsu Province (No. BK20191484) and Six Talent Peaks Project in Jiangsu Province (No. 2019KTHY007). Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. References Wichaita W, Polpanich D, Tangboriboonrat P (2019) Ind Eng Chem Res 58(46):20880-20901. Yu B, Cheng J, Fang Y, Xie Z, Xiong Q, Zhang H, Shang W, Wurm FR, Liang W, Wei F, Zhao J (2024) ACS Nano 18:10031-10044. Yang C, Li J, Zhang Y, Wu C, Li D (2023) J Appl Polym Sci 140(15):e53716. Yan C, Kim SR (2024) ACS Appl Bio Mater 7(2):692-710. Tian D, Zhang X, Shi H, Liang L, Xue N, Wang JH, Yang H (2021) J Am Chem Soc 143(40):16641-16652. Sun S, Gao Y, Han N, Zhang X, Li W (2021) Energy 219:119630. Patravale VB, Mandawgade SD (2008) Int J Cosmetic Sci 30:19-33. 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Cite Share Download PDF Status: Published Journal Publication published 11 Feb, 2025 Read the published version in Journal of Polymer Research → Version 1 posted Reviewers agreed at journal 29 May, 2024 Reviewers invited by journal 28 May, 2024 Editor invited by journal 20 May, 2024 Editor assigned by journal 13 May, 2024 First submitted to journal 12 May, 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-4407913","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":307518160,"identity":"0a7589ed-4ff0-4a28-a560-6c566c9ef7a7","order_by":0,"name":"Jin Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jin","middleName":"","lastName":"Li","suffix":""},{"id":307518161,"identity":"29638e1e-3c3f-43a9-b12c-6906e3abb804","order_by":1,"name":"Xinyang 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09:18:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4407913/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4407913/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10965-025-04296-1","type":"published","date":"2025-02-11T15:57:39+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":58027230,"identity":"6bc29587-0b4d-4431-9a6a-42dbccc4226e","added_by":"auto","created_at":"2024-06-10 06:58:24","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":90112,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of PAL fibers\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/e24c32613e33628f6093f6d3.png"},{"id":58027674,"identity":"4565d23e-a670-4b46-85f9-1df65bb66389","added_by":"auto","created_at":"2024-06-10 07:06:24","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":204345,"visible":true,"origin":"","legend":"\u003cp\u003e(a)\u003cstrong\u003e \u003c/strong\u003eDigital photographs of pickering emulsions prepared with different percentage of PAL fibers after 30 days, (b) Fraction of water separated from the Pickering emulsion with corresponding samples, (c) Polarizing micrographs of pickering emulsion stabilized by 3wt% of PAL fibers.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/2c965bac13e0a5cc98dc71af.png"},{"id":58028324,"identity":"ad35cbee-7cb9-434f-8c58-6197a68c1adb","added_by":"auto","created_at":"2024-06-10 07:14:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":325142,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of PMMA/PAL microcapsules prepared with different concentration of PAL fibers: (a) 1.5 wt%, (b) 2 wt%, (c) 2.5 wt%, (d) 3 wt%, (e) 3.5 wt%, (f) 4 wt%.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/1971e2f1a0aaf4473115c2b4.png"},{"id":58027233,"identity":"83df51f2-ab2a-4341-8ede-77f3e3e0fc17","added_by":"auto","created_at":"2024-06-10 06:58:24","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":99200,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of PMMA/PAL microcapsules prepared with 3wt% of PAL fibers at highermagnifications: (a) before grinding and (b) after grinding\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/ffbec6a7f239acce86450107.png"},{"id":58027240,"identity":"d66842ef-b8b0-4821-84ed-5cbb9d58b27e","added_by":"auto","created_at":"2024-06-10 06:58:24","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":533137,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of PAL fibers (a) and PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers (b)\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/fba17d46a4dd9ed578085535.png"},{"id":58028325,"identity":"cfc75f4b-fa6e-471b-a66c-50b7ca674565","added_by":"auto","created_at":"2024-06-10 07:14:24","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":11240,"visible":true,"origin":"","legend":"\u003cp\u003eMonomer conversion of PAL stabilized pickering emulsions with different concentration of PAL fibers\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/a6b3c673be003a97fcb78dbc.png"},{"id":58027676,"identity":"59c2f7ab-7c9d-47e9-a6b7-dff011dd1198","added_by":"auto","created_at":"2024-06-10 07:06:24","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":236343,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of PMMA/PAL microcapsules prepared with different concentration of photoinitiator: (a) 2wt%, (b) 4 wt%, (c) 6 wt%, (d) 8wt%\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/c8e26af82462dd691dc6aeab.png"},{"id":58027235,"identity":"6f184f8f-d2f3-450e-9151-d7ecec3aeb40","added_by":"auto","created_at":"2024-06-10 06:58:24","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":224282,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of PMMA/PAL microcapsules prepared under different light irradiation intensity: (a) 20 mW/cm\u003csup\u003e2\u003c/sup\u003e, (b) 2 mW/cm\u003csup\u003e2\u003c/sup\u003e, (c) 0.5 mW/cm\u003csup\u003e2\u003c/sup\u003e, (d) 0.1 mW/cm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/c77b02f8eeb5ffd0fd9d735e.png"},{"id":58027678,"identity":"967c3d48-39d4-4c50-8d27-a272fe5f6cdc","added_by":"auto","created_at":"2024-06-10 07:06:24","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":223795,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of PMMA/PAL microcapsules prepared at different EGDMA/MMA ratios (\u003cem\u003ev/v\u003c/em\u003e): (a) 6/10, (b) 8/10, (c) 10/10, (d) 12/10\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/c6028eb969c0d58ca3a2e148.png"},{"id":58027237,"identity":"c09c12fd-c383-4ab4-bcee-0e36aa007279","added_by":"auto","created_at":"2024-06-10 06:58:24","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":363613,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of Polymer/PAL-3 microcapsules prepared using different monomer: (a) MA, (b) MMA, (c) BA, (d) BMA, (e) St, (f) DEAEMA\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/6567ca0075d7a9541e006a83.png"},{"id":58027239,"identity":"2913a9d6-4636-4790-bdb7-69ef94925a32","added_by":"auto","created_at":"2024-06-10 06:58:24","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":55203,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the formation mechanism of PMMA/PAL microcapsules\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/9989b39f8372f5dcd4b0cdbf.png"},{"id":76487691,"identity":"1d375853-1568-47b3-a321-99f150756ca8","added_by":"auto","created_at":"2025-02-17 16:11:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3199712,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4407913/v1/b6b369f4-b464-45e4-9ccc-7254c0f5166c.pdf"}],"financialInterests":"","formattedTitle":"Facile Fabrication of Polymer/Palygorskite Microcapsules via Pickering Emulsion Photopolymerization","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe polymer microcapsules are of great interest due to the wide range of available polymers and functionalities [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], which have been extensively developed in various industries, such as agrochemicals [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], pharmaceuticals [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], catalysis [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], energy storage [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], cosmetics [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], self-lubricating coatings [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], and so on. Up to date, a variety of preparation techniques have been developed range from colloidal templates [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], layer-by-layer assembly [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], spray-drying [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], as well as heterophase polymerization [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. However, most of the approaches have to use \"templates\" in order to build the capsule-like structure for the microcapsules. Where silica or polymer nanoparticles used as hard templates, it often requires an extra procedure and hazardous materials for the removal of the templates. In comparison to the hard-templating technique, soft templates are normally using amphiphilic surfactants as emulsifiers to form emulsion droplets. The microcapsules can be formed by subsequently polymerization of the emulsion droplets. It simplifies the preparation procedure of microcapsules. However, the surfactants are often toxic to human bodies and difficult to remove completely from the microcapsules [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Therefore, a facile preparation method with great technical applicability to access polymer microcapsules is remaining highly required.\u003c/p\u003e \u003cp\u003ePickering emulsion is an emulsion that stabilized by solid particles instead of amphiphilic organic surfactants [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Compared to traditional emulsions stabilized by surfactants, Pickering emulsions have the advantages of convenient preparation, excellent stabilization, low toxicity and high sustainability [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Polymerization of Pickering emulsion to fabricate microcapsules has attracted extensive attention [\u003cspan additionalcitationids=\"CR17 CR18\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The mechanical and thermal properties of polymer shells of the formed microcapsules can be further enhanced in comparison to surfactants [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Besides, the functionality, structure and size for the microcapsules can be regulated by the design of Pickering emulsion formulations [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Thus, a variety of microcapsules, such as TiO\u003csub\u003e2\u003c/sub\u003e/PS [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], SiO\u003csub\u003e2\u003c/sub\u003e/Polymer [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], Clay/Polymer [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e], Chitosan/PLGA [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], MOF/PS [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], GO/polymer [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], etc., have been prepared via Pickering emulsion templated polymerization. However, most of the polymerization process is induced by thermal treatment, which required strict reaction conditions and time-consuming operations.\u003c/p\u003e \u003cp\u003ePhotopolymerization is well-known for its sustainable, energy efficient, high polymerization rates and temporal controllable [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], which has been widely used in polymer-based coating industry [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. In our previous work, a Pickering emulsion templated photopolymerization procedure was developed [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. PMMA/palygorskite microspheres with mesoporous structure were prepared successfully via such a facile photopolymerization process, but not microcapsules. In comparison to microspheres, the formation of microcapsules is much more difficult [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. As the formed capsule wall is very thin in most cases, it leads to the easy broken of the microcapsule. Besides, to the best of our knowledge, access microcapsules using a Pickering emulsion template photopolymerization process have not been reported in the literature. Therefore, the photo chemical factors (such as photoinitiator concentration, light intensity, etc.) and formulation compositions to the pickering emulsion template photopolymerization kinetics and the morphology of microcapsules are still unclear.\u003c/p\u003e \u003cp\u003eHerein, an effective and practical Pickering emulsion photopolymerization method was developed to prepare polymer/PAL microcapsules. Photolatent pickering emulsions of oil (monomer, crosslinker, Irgacure 819 and octane) in water were prepared using palygorskite fibers (PAL) as particulate emulsifier. The effect of PAL concentration on the emulsion stabilities as well as the microcapsule morphology was well studied. Furthermore, the photopolymerizaiton factors, such as photoinitiator content, light intensity, etc., on the formation of the microcapsules have been investigated. The factors of crosslinker/monomer ratio and monomer type on the microcapsule morphology were also studied. Consequently, a formation mechanism of the microcapsules has been proposed.\u003c/p\u003e"},{"header":"Experimental part","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003ePalygorskite (PAL, \u0026gt; 90 wt %) was obtained from Jiangsu Zhongyuan Minerals Co. Ltd. (Huaian, China). Methyl methacrylate (MMA), Styrene (ST), Diethylaminoethyl methacrylate (DEAEMA), poly(ethylene glycol) diacrylate (EGDMA), octane (AR) and aluminum oxide (AR) were purchased from Aladdin Chemical Co. Ltd. Ethanol (AR) was supplied by Shanghai Jiuyi Chemical Reagent Co. Ltd. Methyl acrylate (MA) was obtained from Sinopharm Chemical Reagent Co. Ltd. \u003cem\u003en\u003c/em\u003e-Butyl acrylate (BA) was purchased J\u0026amp;K Scientific Ltd. \u003cem\u003en\u003c/em\u003e-Butyl methacrylate (BMA) was purchased from Tokyo Chemical Industry. Irgacure 819 (I819) was obtained from BASF.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003ePickering emulsion preparation\u003c/h2\u003e \u003cp\u003eFirstly, PAL particles (0.15\u0026thinsp;~\u0026thinsp;0.40 g) were dispersed into 10 mL deionized water by ultra-sonification procedure (PS-40A, Jeken Ultrasonic) for 5 min, forming homogeneous PAL/water dispersion. Then, MMA (0.5 mL, 4.67 mmol), EGDMA (0.5 mL, 2.65 mmol) and I819 (0.06 g) was dissolved into octane (4 mL), forming a homogeneous oil phase. Consequently, the formation oil phase and the PAL/water dispersion were mixed via a lab dissolver (NSR-I, Shanghai Nengu) operating at 12,000 rpm for 5 min. An O/W type pickering emulsion was formed.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003ePickering emulsion photopolymerization\u003c/h2\u003e \u003cp\u003eThe photopolymerization of pickering emulsion was carried out via a special homemade quartz micro-reactor setup (5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mL, internal channel diameter 1000\u0026thinsp;\u0026plusmn;\u0026thinsp;5 \u0026micro;m) [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The emulsion was loaded into the micro-reactor via a peristaltic pump with a flow rate of 5 mL h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under 395 nm LED irradiation. Then the photopolymerized product was filtered and washed for three times using ethanol. Finally, the solid product was dried at 50 \u003csup\u003eo\u003c/sup\u003eC under vacuum for 12h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization\u003c/h2\u003e \u003cp\u003ePolarizing optical photographs of the PAL stabilized emulsions were obtained from Leica DMLP polarized optical microscope. Morphology of the photopolymerized products was characterized by scanning electron microscopy (SEM, Hitachi S-3000N, working at 25 kV). Before analysis, the samples were coated with a 15 nm thick layer of gold to reduce the charging effect on the surface. FT-IR spectra of the PAL and polymerized samples were recorded via a Nicolet 5700 spectrophotometer with a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the range of 4000\u0026thinsp;\u0026minus;\u0026thinsp;600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 32 scans. For all of the sample powders, they were characterized by using KBr pellet technique. The monomer conversions were determined from the mass ratio of added monomer to obtained polymer.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003ePAL-stabilized pickering emulsion\u003c/h2\u003e \u003cp\u003ePAL is a kind of natural fibrous phyllosilicate mineral [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the size of the PAL fibrous crystal used in this research is typically with 0.5-2 \u0026micro;m in length. It is a very effective emulsifier for O/W type pickering emulsion because of its high aspect ratios, suitable hydrophilicity and large surface area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBecause of the plenty of silanol groups on their surfaces, the PAL fibers can be well dispersed in water, forming very stable suspensions. By adding an amount of octane solution that containing MMA, EGDMA and I819 as oil phase in such PAL water suspension, an oil in water (O/W) type Pickering emulsion can be formed after emulsification. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea provides digital photographs of Pickering emulsions prepared with different percentage of PAL fibers. As can be seen, the emulsion prepared with a low percentage of 1.5 wt% PAL fibers is not stable. Clearly, a phase separation occurred. In comparison to octane and monomer, the density of water is higher. Thus, the formed transparent liquid at the bottom of the bottle is separated water from the emulsion. Along with the increasing of PAL concentration, the fraction of water separated from the emulsion is decreasing (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). When the PAL percentage increased to 3 wt%, there is almost no water separated from the emulsion. Direct evidence was provided by the polarizing microscopy characterization. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec illustrates the polarizing micrographs of pickering emulsion stabilized by 3 wt% of PAL fibers. As can be seen, there are plenty of spherical rings. Each ring is composed by a light halo surrounding a dark and round spot. As the PAL crystal has birefringent properties, such phenomenon only can be observed during the PAL fibers absorbed on the surface of the emulsion droplets and nearly wrapped up the droplet surface. It suggests that the PAL fibers formed a close packing structure on the emulsion droplet surfaces, which is crucial for the solid particles serving as particulate emulsifier in literature [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003ePreparation and characterization of Polymer/PAL composite microcapsules\u003c/h2\u003e \u003cp\u003eThe Pickering emulsion photopolymerization can be carried out conveniently. Different experimental conditions were further investigated. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e displays the morphology of the PMMA/PAL microcapsules prepared with different concentration of PAL fibers. Some spherical microcapsules can be observed only with 1.5 wt% PAL fibers (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Many fragments of microcapsules also can be seen. Interestingly, the number of formed microcapsules is increased along with the increasing of PAL concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb-d). When the PAL concentration reached to 3 wt%, plenty of of PMMA/PAL microcapsules were formed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). The fragments of microcapsules almost disappeared. However, only broken microcapsules were observed by further increasing the PAL concentration to 3.5 wt% (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee) or even higher. It can be expected that the formation of microcapsules became better along with the increasing of PAL concentration as the emulsion became more and more stable. When the PAL concentration reached to 3.5 wt%, the mechanical properties of the formed PAL/PMMA composite shell is not enough to support the microcapsules maintain their structure. The high PAL concentration partially impeded the photopolymerization and probably it leads to such result.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSEM image of PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers (corresponding to Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) at higher magnifications is displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. The PAL fibers can be seen clearly that dispersed randomly on the surface of microcapsule. Such result is quite in accordance with the observation of the polarizing micrographs of the emulsion (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). It further proves the PAL fibers was absorbed on the surface of the emulsion droplets and played as the role of particulate emulsifier. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb shows the SEM image of PMMA/PAL microcapsules after grinding. It demonstrates that the hollow structure of the microcapsule is formed successfully via such pickering emulsion templated photopolymerization process. The PMMA/PAL shell of the microcapsule is very thin, only with few hundred nanometers.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the FTIR spectra of PAL fibers and PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers. The absorption bands located at 3,800-3,395 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea can be attributed to the hydroxyl groups of coordinated water in the tunnels of PAL crystals [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. The absorption bands at 1,030 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 982 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be ascribed to the stretching and the bending vibrations of Si-O-Si bonds [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Such absorption bands also can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, which is the FTIR spectrum of PMMA/PAL microcapsules. Furthermore, the presence of absorption bands at 2970 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be ascribed to C-H stretching vibration of methyl group. The sharp and strong absorption bands at 1732 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is the characteristic carbonyl stretching vibration of ester group. These results indicated that the PMMA/PAL composite has been fabricated successfully.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e further illustrates the monomer conversion value of emulsions with different percentage of PAL fibers after the photopolymerization process. The monomer conversion is decreased along with the increasing of PAL concentration. When the PAL concentration is 1.5 wt%, the monomer conversion is 83.6%. By increasing the PAL concentration to 3.5 wt%, the monomer conversion decreased to 70.2%. Such result is probably because of the light scattering induced by the PAL fibers located at the surface of emulsion droplets (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). It hinders the photolysis of photoinitiator to initiate the radical polymerization of monomers. The higher amount of PAL concentration will scatter more light, which lead to the lower monomer conversion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter the investigation of the PAL concentration effect, further studies on the influence of photoinitiator concentration were carried out. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e displays the SEM images of PMMA/PAL microcapsules prepared with different concentration of photoinitiator. No microcapsule or microsphere is formed only with 2 wt% of I 819 photoinitiator (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). The number of formed microcapsules is increasing along with the growth of the photoinitiator concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb-c). As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec, there are plenty of microcapsules when the concentration of I 819 reached to 6 wt%. As it is well-known, 2 wt% of photoinitiator is sufficient enough in most cases for the conventional radical photopolymerization [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. However, here it required a higher photoiniatiator concentration due to the light scattering of the solid emulsifier in Pickering emulsion photopolymerization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the SEM images of PMMA/PAL microcapsules prepared under different light irradiation intensity. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea, plenty of PMMA/PAL microcapsules were formed with 20 mW/cm\u003csup\u003e2\u003c/sup\u003e irradiation intensity. The morphology of the most microcapsules was not spherical. Along with the decreasing of light irradiation intensity, the spherical microcapsules became more and more. Besides, the number of broken microcapsules was increasing. As it is well-known, the photoinitiator decomposed into free radical fragments will be faster with higher light irradiation intensity for photopolymeriztion process, leading to higher polymerization rate and lower molecular polymer chain [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. In this case, the light reached to the photoinitiator that inside of the emulsion droplets is decreased dramatically due to the light shielding of PAL fibers. It leads to the insufficient polymerization and the large amount of broken microcapsules (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec). Combining the removal of octane, the shell composed by low molecular weight polymer (high light irradiation intensity) and PAL fibers couldn\u0026rsquo;t afford the shrinkage, inducing the non-spherical morphology of the formed microcapsules.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter optimal the light intensity, further studies were carried out on the crosslinker and monomer radios. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e displays the SEM images of PMMA/PAL-3 microcapsules prepared at different volume ratios of crosslinker (EGDMA) and monomer (MMA). When the EGDMA/MMA ratio was 6/10, many cracked microcapsules were formed (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea). Obviously, spherical shape microcapsules were formed and their number was increased along with the increasing of the EGDMA/MMA ratio (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec). However, only fragments of microcapsules could be observed by further increasing the EGDMA/MMA ratio to 12/10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHowever, it is still unclear whether a wide range of radically polymerizable monomers can be used for the preparation of microcapsules via Pickering emulsion templated photopolymerization process. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the SEM images of Polymer/PAL-3 microcapsules prepared with different monomers (MA, MMA, BA, BMA, St and DEAEMA). Interestingly, the microcapsules can be formed for all of the monomers. However, the best monomer for the formation of spherical microcapsules is DEAEMA (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ef). In comparison to MA monomer (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea), the formation of microcapsules is better by using BA as monomer (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ec). It is probably because the monomer of BA has a butyl tail, which endowed the formed poly(\u003cem\u003en\u003c/em\u003e-butyl acrylate) with better toughness [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Such phenomenon is really interesting, but it still requires further investigation in order to find out the proof.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe formation mechanism of PMMA/PAL microcapsules was schematically displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. Firstly, the homogeneous oil phase of octane solution that containing monomers and photoinitiators was dispersed in PAL water gel. The oil droplets were surrounded by PAL fibers and forming stable oil in water type Pickering emulsion, as it has been proved by polarizing microscopy observation in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. In the second step, the formed Pickering emulsion was further polymerized via light irradiation in a homemade quartz reactor. The formation of PMMA polymers that induced them separated from the octane in each PAL wrapped droplet, as the octane is a poor solvent for the polymer. Along with the light irradiation, the droplets became PMMA/PAL microcapsules which encapsulated octane inside. Finally, PMMA/PAL microcapsules were achieved by removing the octane.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003ePolymer/PAL microcapsules were fabricated successfully via Pickering emulsion photopolymerization. The effect of PAL concentration, photoinitiator percentage, light intensity, crosslinker/monomer radio and monomer type on the formation of microcapsules was investigated. Although the emulsion stability was along with the PAL concentration, it was found that in the presence of 3 wt% of PAL emulsifier, the emulsion droplets gave the best transformation of microcapsules. Furthermore, 6 wt% of photoinitiator with 2 mW/cm\u003csup\u003e2\u003c/sup\u003e irradiation intensity provided the most suitable photopolymerization conditions for the formation of polymer/PAL microcapsules. DEAEMA as monomer with crosslinker of EGDMA in 10/10 volume ratio offered the best formulation to form spherical microcapsules. Such microcapsule probably formed because of the polymer separated from the octane in each PAL wrapped droplet during photopolymerization.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank the financial support from the Natural Science Foundation of Jiangsu Province (No. BK20191484) and Six Talent Peaks Project in Jiangsu Province (No. 2019KTHY007).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eWichaita W, Polpanich D, Tangboriboonrat P (2019) Ind Eng Chem Res 58(46):20880-20901.\u003c/li\u003e\n \u003cli\u003eYu B, Cheng J, Fang Y, Xie Z, Xiong Q, Zhang H, 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Andrew, New York.\u003c/li\u003e\n \u003cli\u003eFouassier JP (2006) Photochemistry and UV Curing: New Trends. Research Signpost, India.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eWu Y, Cao R, Wu G, Huang W, Chen Z, Yang X, Tu Y (2016) Compos Part A 88:156-164.\u003c/li\u003e\n\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Suspension photopolymerization, Pickering emulsion, Palygorskite, Microcapsules","lastPublishedDoi":"10.21203/rs.3.rs-4407913/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4407913/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA facile photocatalytic Pickering emulsion polymerization process was developed to fabricate polymer/clay composite microcapsules. Photolatent Pickering emulsions of oil (monomer, crosslinker, Irgacure 819 and octane) in water were prepared using palygorskite fibers (PAL) as particulate emulsifier. The effect of PAL concentration on the emulsion stabilities was well studied. 3 wt% of PAL fibers were required in order to obtain a stable Pickering emulsion. Moreover, serials of factors on the formation of microcapsules have been investigated, such as PAL concentration, photoinitiator percentage, light intensity, crosslinker/monomer radio and monomer type. Under the appropriate conditions, polymer/PAL microcapsules with spherical morphology can be produced easily. Consequently, a formation mechanism of the microcapsules has been proposed.\u003c/p\u003e","manuscriptTitle":"Facile Fabrication of Polymer/Palygorskite Microcapsules via Pickering Emulsion Photopolymerization","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-10 06:58:19","doi":"10.21203/rs.3.rs-4407913/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-05-29T13:46:55+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-28T05:15:17+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Journal of Polymer Research","date":"2024-05-20T18:39:21+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-13T07:17:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Polymer Research","date":"2024-05-12T05:17:56+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5cc23d2c-88d7-4e41-94ec-f9969e3d7925","owner":[],"postedDate":"June 10th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-02-17T16:06:10+00:00","versionOfRecord":{"articleIdentity":"rs-4407913","link":"https://doi.org/10.1007/s10965-025-04296-1","journal":{"identity":"journal-of-polymer-research","isVorOnly":false,"title":"Journal of Polymer Research"},"publishedOn":"2025-02-11 15:57:39","publishedOnDateReadable":"February 11th, 2025"},"versionCreatedAt":"2024-06-10 06:58:19","video":"","vorDoi":"10.1007/s10965-025-04296-1","vorDoiUrl":"https://doi.org/10.1007/s10965-025-04296-1","workflowStages":[]},"version":"v1","identity":"rs-4407913","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4407913","identity":"rs-4407913","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}