Highly porous organic–inorganic hybrid fiber from copolymers of styrene and polyhedral oligomeric silsesquioxane-derived methacrylate: Syntheses, fiber formation and potential modification

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<ul><li><p>Accepted Manuscript</p><p>Highly Porous Organic-Inorganic Hybrid Fiber from Copolymers of Styrene</p><p>and Polyhedral Oligomeric Silsesquioxane-Derived Methacrylate: Syntheses,</p><p>Fiber Formation and Potential Modification</p><p>Thanarath Pisuchpen, Varol Intasanta, Voravee P. Hoven</p><p>PII: S0014-3057(14)00293-6</p><p>DOI: http://dx.doi.org/10.1016/j.eurpolymj.2014.08.017</p><p>Reference: EPJ 6545</p><p>To appear in: European Polymer Journal</p><p>Received Date: 4 May 2014</p><p>Revised Date: 6 August 2014</p><p>Accepted Date: 13 August 2014</p><p>Please cite this article as: Pisuchpen, T., Intasanta, V., Hoven, V.P., Highly Porous Organic-Inorganic Hybrid Fiber</p><p>from Copolymers of Styrene and Polyhedral Oligomeric Silsesquioxane-Derived Methacrylate: Syntheses, Fiber</p><p>Formation and Potential Modification, European Polymer Journal (2014), doi: http://dx.doi.org/10.1016/</p><p>j.eurpolymj.2014.08.017</p><p>This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers</p><p>we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and</p><p>review of the resulting proof before it is published in its final form. Please note that during the production process</p><p>errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.</p><p>http://dx.doi.org/10.1016/j.eurpolymj.2014.08.017http://dx.doi.org/http://dx.doi.org/10.1016/j.eurpolymj.2014.08.017http://dx.doi.org/http://dx.doi.org/10.1016/j.eurpolymj.2014.08.017</p></li><li><p>1 </p><p>Highly Porous Organic-Inorganic Hybrid Fiber from Copolymers of </p><p>Styrene and Polyhedral Oligomeric Silsesquioxane-Derived </p><p>Methacrylate: Syntheses, Fiber Formation and Potential </p><p>Modification </p><p>Thanarath Pisuchpena,b</p><p>, Varol Intasantac, Voravee P. Hoven</p><p>a* </p><p>aOrganic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn </p><p>University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand </p><p>bCenter of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, </p><p>Phayathai Road, Pathumwan, Bangkok 10330, Thailand </p><p>cNational Nanotechnology Center, National Science and Technology Development Agency, </p><p>Thailand Science Park, Phahonyothin Road, Klong Luang, Pathumthani, 12120, Thailand </p><p>Correspondence to: Voravee P. Hoven (E-mail: vipavee.p@chula.ac.th) </p><p>Abstract </p><p>Copolymers comprising styrene and polyhedral oligomeric silsesquioxane-derived methacrylate </p><p>(PS-co-PMAPOSS) of various MAPOSS:styrene ratios with Mn over 100 kDa were prepared by </p><p>concurrent ARGET ATRP-RAFT. Fibrous mats of the synthesized copolymers are then </p><p>fabricated through electrospinning process using various conditions (solvents and POSS </p><p>content). The results from SEM and EDS analysis unveiled the fibers physical and chemical </p><p>characteristics as a clear footprint of the influence of solvent selection (tetrahydrofuran (THF) </p><p>and dimethylformamide (DMF)). It is evident that liquid-liquid phase separation followed by </p><p>phase segregation of PS and POSS constitute to the varying degree of porosity in the electrospun </p><p>fibers. Finally, preliminary tests suggest that highly porous PS-co-PMAPOSS fiber can be </p><p>modified with high temperature, plasma and silanization for further novel applications. </p><p>Keywords: porous fibers; electrospinning; polystyrene; POSS; phase separation </p></li><li><p>2 </p><p>1. Introduction </p><p>Porous materials represent a class of functional constituents in catalysis, filtration, storage, </p><p>control release, sensing, insulation and absorption. Fabrications of such structures can be </p><p>accomplished by a number of approaches. Phase separation among incompatible components at </p><p>small length scale, in particular, epitomizes one of the effective ways that help creating porosity. </p><p>Recent advances in molecular design and polymer syntheses have made possible hybridization of </p><p>not only organic and inorganic components, but also those with definite incompatibilities, into </p><p>one single polymeric chain. Through which, structural development driven by phase separation </p><p>could be tuned via the degree of incompatibilities among the choices of monomers during </p><p>syntheses and solvent combination in the course of fabrications. </p><p>Polyhedral oligomeric silsesquioxane (POSS) is an organic-inorganic hybrid compound, of </p><p>which the most well-known is that of cubic structure, also known as T8. Due to its chemical </p><p>similarity to silica, POSS is sometimes referred to as the smallest colloidal silica. Thermal and </p><p>chemical stability of POSS, as well as various functionalities can convey great potential in the </p><p>area of polymer nanocomposites. </p><p>Various routes have been developed as means to assimilate POSS functional moieties into </p><p>polymer matrix, including melt blending [1], solution blending [2] and addition of POSS to </p><p>polymer chains either through grafting [3] or copolymerization [4]. In addition to the mechanical </p><p>and thermal integrity, such incorporation of POSS molecules can affect morphological and </p><p>structural feature of the polymer itself. Polymerized POSS molecules are known to form </p><p>segregated nanosized semi-crystalline or crystalline domains, even if the functional groups on </p><p>main chains and POSS subunits are similar [5]. Random copolymers with POSS on their chains </p><p>have revealed lamellar-like POSS aggregates [6], while block copolymers containing POSS have </p><p>exhibited long-range ordered lamellar or cylindrical structures [7]. </p><p>Electrospinning becomes a prevalent and versatile technique to fabricate precursor polymer </p><p>solutions or melt into non-woven fibrous mat under electric-field induced repulsive coulombic </p><p>force. With sufficiently entangled polymeric networks as spinning precursor, various inorganic </p><p>materials can also be fabricated into ultrafine fibers [8-10]. Its simplicity and ability to induce </p><p>unique micro- and nanostructures bring about innumerable possibilities of applications, including </p><p>POSS-derived nanocomposites, in which electrospinning facilitates dispersion of nanoparticles </p><p>[11,12]. As Cozza and co-workers had demonstrated, electrospinning could also be used to </p></li><li><p>3 </p><p>promote dispersion and prevent aggregation of POSS in cellulose acetate matrix [13]. </p><p>Morphological features of electrospun fibers have also been shown to be affected by an addition </p><p>of POSS. In publication of Xue and co-workers, electrospun fibers from copolymers of </p><p>poly(methyl methacrylate) (PMMA) containing POSS exhibited nanofibrillar structures of </p><p>ordered POSS moieties which were also longitudinally aligned in the fiber direction [14]. </p><p>Herein this research, we propose an approach to generate highly porous organic-inorganic </p><p>hybrid fibrous materials via chemical design, polymer syntheses, fiber formation and interplay </p><p>among solubilities and domain separation. Specifically, we would like to explore an impact of </p><p>POSS incorporation on the morphological and structural feature of polystyrene (PS) through the </p><p>dynamics of electrospining as means for fiber formation. Commercially available methacrylate-</p><p>substituted POSS, heptaisobutyl-(1-propylmethacrylate)-POSS (MAPOSS) was copolymerized </p><p>with styrene employing a controlled radical polymerization method denoted as concurrent </p><p>RAFT-ARGET ATRP (Scheme 1). With subsequent fabrication by electrospinning lined ahead, </p><p>the initial challenge was to attain copolymers with molecular weight high enough for sufficient </p><p>molecular entanglement suitable for electrospinning. This polymerization process is a </p><p>combination of Reversible Addition-Fragmentation Chain Transfer (RAFT) and Activator </p><p>Regenerated by Electron Transfer for Atom Transfer Radical Polymerization (ARGET ATRP). </p><p>Unlike conventional RAFT, which requires external source of radical, atom transfer process </p><p>provides a more consistent level of radical which is under influence of both ATRP and RAFT </p><p>equilibria. This novel approach provides a superior control over both initiation and propagation </p><p>and, as demonstrated by Matyjaszewski and coworkers, allows for narrow PDI for polymeric </p><p>chains with molecular weight even over 100 kDa [15-17]. The use of ARGET ATRP further </p><p>provides an additional simplicity to the reaction process and copolymer purification, especially </p><p>in this case where economical reducing agent like copper wire was employed. Subsequently, </p><p>fibers from the synthesized copolymers were then fabricated by electrospinning, in which several </p><p>parameters such as copolymer concentration and solvent composition were systematically </p><p>investigated. </p></li><li><p>4 </p><p>S</p><p>S CN</p><p>COOH</p><p>4-cyano-4-(thiobenzoylthio)pentanoic acid</p><p>RSiO</p><p>O</p><p>O SiR</p><p>O</p><p>OSiR</p><p>O</p><p>ORSi</p><p>SiRO</p><p>SiRSi</p><p>RSi</p><p>O OOO</p><p>O O</p><p>N</p><p>NNCu(0)</p><p>CuBr2</p><p>CN</p><p>HOOC S</p><p>S</p><p>RSiO</p><p>O</p><p>O SiR</p><p>O</p><p>OSiR</p><p>O</p><p>ORSi</p><p>SiRO</p><p>SiRSi</p><p>RSi</p><p>O OOO</p><p>O O</p><p>PMDETA</p><p>styrene</p><p>MAPOSS</p><p>PS-co-PMAPOSS</p><p>n m</p><p>R =</p><p>Scheme 1. Synthesis of PS-co-PMAPOSS by concurrent RAFT-ARGET ATRP. </p><p>2. Experimental </p><p>2.1. Materials </p><p>Styrene (Sty) and copper (II) bromide (CuBr2) were purchased from Fluka. Heptaisobutyl-(1-</p><p>propylmethacrylate)-POSS (MAPOSS), pentamethyl- diethylenetri-amine (PMEDTA), 4,4-</p><p>azobis(4-cyanovaleric acid) (ACVA), cyanovaleric acid dithiobenzoate (CVADTB) and </p><p>aluminium oxide were purchased from Sigma-Aldrich. Toluene, tetrahydrofuran (THF), N,N-</p><p>dimethylformamide (DMF) and ethanol were purchased from Lab-scan. All reagents were AR </p><p>grade and used as received. </p><p>2.2. Synthesis of PS and PS-co-PMAPOSS using concurrent RAFT-ARGET ATRP </p><p>2 mg of CuBr2, 26 L of PMDETA, 12 mg of CVADTB (molar ratio of 1:140:10), 4.8 g </p><p>copper wire and designated amount of Sty and MAPOSS were added into a 25 mL scintillation </p><p>vial, which was then sealed with rubber septum. Purged with nitrogen gas for 15 min, the </p><p>reaction was set to carry out under nitrogen atmosphere at 90C for 24 h. The crude products </p><p>were first dissolved in THF, and then passed through a basic alumina column to remove residual </p><p>catalysts. The resulting clear and colorless solution was then precipitated in an excessive amount </p><p>of ethanol under vigorous stirring. The solid was then dried at room temperature under vacuum </p><p>overnight to obtain a final copolymer product. </p></li><li><p>5 </p><p>2.3. Electrospinning </p><p>In the preparation of spinning precursor solutions, the synthesized (co)polymers were </p><p>dissolved in various solvents including toluene, THF and mixture of THF-DMF. Each of the as-</p><p>prepared solution was loaded into a syringe equipped with a syringe pump and attached to a 1.5 </p><p>cm-long blunt tip needle. A high voltage supply connected the needle with a positive voltage </p><p>cable and a receiving aluminum foil with a ground one. A polymer solution was electrospun </p><p>onto the ground aluminum foil under voltage of 20 kV, 15 cm collecting distance and flow rates </p><p>of 6.5-8.5 mL/h. The resulting electrospun fiber mats were left to dry in ambience for 24 h prior </p><p>to characterization. </p><p>2.4. Chemical modification </p><p>To test the tolerance to modification of the electrospun fibers, several procedures were carried </p><p>out. First, tetraethoxysilane (TEOS) (5% of polymer weight) was added to polymer solution prior </p><p>to electrospinning. Next, silanol groups were introduced to surface of the electrospun fiber mats </p><p>through 2 methods, calcination at 400oC for 1 h and oxygen plasma treatment at 100 W for 10 </p><p>min. Finally, electrospun fiber mats with silanol groups were treated with vapor phase of </p><p>trichloromethylsilane (MeSiCl3) following a published procedure [18]. Treated fiber mats were </p><p>characterized by a contact angle goniometer (Ram-Hart, Model 200-F1, USA) for water contact </p><p>angle. </p><p>2.5. Nuclear magnetic resonance spectroscopy (NMR) </p><p>1H-NMR spectra was recorded in CDCl3 using Varian, model Mercury-400 nuclear magnetic </p><p>resonance spectrometer operating at 400 MHz. Chemical shifts () were reported in part per </p><p>million (ppm) relative to the reference signals of tetramethylsilane (TMS) or the residual </p><p>protonated solvent. </p><p>2.6. Gel permeation chromatography (GPC) </p><p>Molecular weight and molecular weight distributions of the synthesized (co)polymers were </p><p>determined by gel permeation chromatography (GPC) using Water 600 controller and pump, </p><p>Waters E600 column connected to Waters 2140 refractive index detector, and THF as eluent. </p><p>The flow rate was 1 mL/min. PS standards were employed to construct a calibration curve. </p></li><li><p>6 </p><p>2.7. Thermogravimetric analysis (TGA) </p><p>Thermal degradation behavior of all (co)polymer samples and percent ash were investigated </p><p>by thermogravimetric analysis (TGA) (Mettler Toledo, model TGA/SDTA 851, USA) over a </p><p>temperature range of 30-600 C at a heating rate of 10 C/min under ambient condition. The </p><p>data were analyzed with STARe SW program version 9.30. </p><p>2.8. Scanning electron microscopy (SEM) and energy dispersive x-Ray spectrometry (SEM-EDS) </p><p>The morphological appearances of the as-spun fibers were investigated using a scanning </p><p>electron microscope (SEM, JEOL, Model JSM-6480LV, Japan). Each sample was placed on the </p><p>holder with an adhesive tape and coated with a thin sputtered layer of gold. The scanning </p><p>electron images were obtained by using an acceleration voltage of 15 kV. The average fiber </p><p>diameter of the electrospun fibers was measured by Semafore software directly from SEM </p><p>images. Elemental analyses were performed under SEM model JSM-5800LV (JEOL, Japan) </p><p>under an EDS mode. </p><p>2.9. X-ray diffractometry (XRD) </p><p>The crystallinity of the synthesized (co)polymers and their respective as-spun fibers were </p><p>investigated using an X-ray diffractometer (model Rigaku TTRAX III, 18 kW, Japan). Each </p><p>sample was grounded into powder prior to analysis. </p><p>3. Results and discussion </p><p>3.1. Synthesis of PS and copolymers by concurrent RAFT - ARGET ATRP </p><p>For a polymer to be able to form fibers by itself via electrospinning, molecular weight of the </p><p>polymer has to be high enough to promote sufficient chain entanglement. To ensure </p><p>electrospinnability, this research aimed to synthesize PS-co-PMAPOSS with molecular weight </p><p>well above 100 kDa. A successful synthetic protocol of the copolymer with Mn (131.7 kDa) </p><p>reaching the expected target (130 kDa) employed 0.96 g of copper wire. As can be seen from </p><p>1H-NMR analysis in Fig.1, the increases in intensity of peaks at 0.9-1.0, assigned to methyl </p><p>protons on isopropyl groups of MAPOSS, and at 0.6 ppm, assigned to methylene protons on </p><p>isopropyl groups of MAPOSS (b and c, respectively in Fig 1A-C) as a function of MAPOSS </p><p>content suggested that the MAPOSS compositions in the copolymers were strongly correlated </p></li><li><p>7 </p><p>with and proportional to those in the feed. Furthermore, there were no peaks in 5.5-6.0 ppm, a </p><p>range assigned to methylene protons of the monomers, indicating the absence of residual </p><p>unreacted monomers after purification. The weight percent of MAPOSS (%MAPOSS) in </p><p>copolymers was calculated from 1H NMR data using relative intensity of the peak at 0.6 ppm, </p><p>assigned to methylene protons on isopropyl groups of MAPOSS (b, Fig 1A-C), and that of the </p><p>peak at 6.2-7.2, assigned to phenyl protons on phenyl group of styrene (a, Fig 1A- C) (data </p><p>shown in Table 1). The %MAPOSS calculated from 1H-NMR, molecular weights of the...</p></li></ul>

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