A Facile Route Towards the Synthesis of Polystyrene

Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 13, 694–697, 2013

A Facile Route Towards the Synthesis of Polystyrene/Zinc Oxide Nanocomposites Long Giang Bach1 , Md. Rafiqul Islam1 , Yong Hun Kim1 , Sung Deok Seo1 , Chan Park2 , Hyun Gyu Kim3 , and Kwon Taek Lim1 ∗

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1

Department of Imaging System Engineering, Pukyong National University, Busan, 608-737, Korea 2 Division of Meterial Engineering, Pukyong National University, Busan, 608-737, Korea 3 Busan Center, Korea Basic Science Institute, Busan 609-735, Korea

ZnO nanoparticles were covalently wrapped by polystyrene (PS) through surface thiol-lactam initiated radical polymerization using the grafting from approach. The surface of ZnO nanoparticles was initially modified by 3-mercapto propyltrimethoxysilane to afford thiol functionalized ZnO nanoparticles (ZnO-SH). The controlled radical polymerization of styrene (St) was subsequently accomplished by using an initiating system of ZnO-SH and butyrolactam. FT-IR, XPS, XRD, TGA, DSC, TEM and DLS were employed to investigate the chemical structure, morphology, thermal properties, the size and size distribution of nanocomposites. The dispersibility of ZnO nanoparticles was observed to be significantly improved upon functionalization by PS brushes. The controlled nature of the surface initiated thiol-lactam aided polymerization of St from the ZnO nanoparticles surface was confirmed by GPC analysis. ZnO Nanoparticles, Polymer PS-g-ZnO, Surface Initiated&Polymerization. Keywords: Delivered by Publishing Technology to: KoreaBrush, Advanced Institute of Science Technology (KAIST) IP: 143.248.125.165 On: Sat, 06 Apr 2013 08:02:27 Copyright American Scientific Publishers

1. INTRODUCTION Organic/inorganic hybrid materials offer the possibility of delivering new generation smart materials with excellent optical, electrical, thermal, mechanical, magnetic and catalytic properties.1 Nanosized particles such as zinc oxide (ZnO) exhibits broad-band UV absorption and has been widely used as photo-catalysts, ultraviolet filters, and UV reflectors.2 3 The ZnO/polymer nanocomposites provide the stable dispersion of ZnO nanoparticles in a polymer, which can be used for coatings, plastics, sealants and fibers. ZnO nanoparticles could be modified via noncovalent or covalent approaches. Covalent functionalization via control radical polymerization such as ATRP, NMP, RAFT are strategic methods for tailoring the surface properties of nanoparticles.4 5 However, most of the techniques require many reaction steps and/or it is difficult to remove a metallic catalyst. Thiols are widely used as a chain-transfer agent in free radical polymerization to regulate the molecular weight of polymers. An initiating system of n-dodecylmercaptan with -caprolactam in the polymerization of styrene was recently reported.6 A facile surface polymer grafting strategy from the thiol ∗

Author to whom correspondence should be addressed.

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functionalized inorganic with the help of butyrolactam (BL) has also been disclosed.7–9 In the present work, the covalent grafting of PS with ZnO nanoparticles (NPs) via the “grafting from” protocol was discussed. The ZnO surface was reacted first with 3-mercaptopropyl trimethoxysilane (MPTMS) to prepare thiol functionalized ZnO nanoparticles (ZnO-SH). Subsequently, the surface initiated radical polymerization of styrene was undertaken in the combined role of ZnO-SH and BL.

2. EXPERIMENTAL DETAILS 2.1. Materials Styrene (St) was dried over CaH2 and distilled under reduced pressure prior use. ZnO NPs, MPTMS, BL and solvents were used as received. 2.2. Grafting of MPTMS Onto ZnO NPs ZnO (2 g) were mixed with methanol (50 ml), and then MPTMS (5 g) was added to the system. The mixture was first dispersed for 20 min using an ultrasonic instrument and was stirred at reflux temperature for 8 h. The mixture 1533-4880/2013/13/694/004

doi:10.1166/jnn.2013.6929

Bach et al.

A Facile Route Towards the Synthesis of Polystyrene/Zinc Oxide Nanocomposites

was then cooled and diluted with n-propanol. After filtration, ZnO-SH nanoparticles were dried under vacuum for 24 h.

3. RESULTS AND DISCUSSION Figure 1 shows the typical FT-IR spectra of ZnO nanoparticles, the MPTMS modified ZnO and PS-g-ZnO

Fig. 1.

FT-IR spectra of (A) ZnO, (B) ZnO-SH and (C) PS-g-ZnO.

J. Nanosci. Nanotechnol. 13, 694–697, 2013

Fig. 2. XPS wide-scan spectra of (A) ZnO and (B) ZnO-SH, (C) S2p core-level spectra of ZnO-SH, and (D) wide-scan of the PS-g-ZnO.

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nanocomposites. The ZnO nanoparticles exhibited broad absorption bands at 3446 cm−1 , which confirms the presence of OH groups (Fig. 1(A)). The ZnO-SH shows bands in the region 2800–3000 cm−1 corresponding to the CH2 and CH3 group of MPTMS and reveals a weak but visi2.3. Synthesis of PS-g-ZnO Nanocomposites ble band at 2571 cm−1 assigning for S–H stretching. The 1 g of St, 0.2 g of ZnO-SH, 0.1 g of BL, 4 mL of toluene characteristic bands at 3051 and 3024 cm−1 are assigned and a Teflon-coated stir bar were placed in a 25 mL round to the C–H aromatic stretching, and at 1442, 1491, and flask equipped with a reflux condenser. The flask was 1605 cm−1 are ascribed to the phenyl ring stretching of PS purged with N2 , heated to 90 C and kept stirring. After in PS-g-ZnO (Fig. 1(C)). Above results strongly suggest polymerization, the flask was cooled to room temperature that PS was covalently attached onto the surface of ZnO and the reaction mixture was precipitated in methanol. The nanoparticles. crude product was diluted in toluene and filtered to colThe XPS analyses of ZnO, ZnO-SH and PS-g-ZnO lect the PS-g-ZnO and dried under vacuum for 24 h. The were conducted as shown in Figure 2. The immobilizagrafted polymer on ZnO nanoparticles was cleaved using tion of the initiator on the surface of ZnO is suggested HCl. The cleaved PS in the organic layer was precipitated by the characteristic signals of Zn2p3 at 1022.8, O1s at in methanol. 531.5 eV, C1s at 286.1 eV, S2p at 163.6 eV and Si2p at 102.4 eV (Fig. 2(B)). The binding energy (BE) of S2p at 163.6 eV clearly demonstrates the existence of S–H 2.4. Characterization groups. The XPS scan of PS-g-ZnO shows that the C1s Fourier Transformed Infrared (FT-IR) spectra were peak with high intensity slightly shifted to the higher BE recorded on BOMEM Hartman and Braun. Surface indicating that the polymeric chains were anchored to the composition was investigated using X-ray Photoelectron surface of ZnO nanoparticles. Spectroscopy (XPS, Thermo VG Multilab). The crystalIn order to investigate the molecular weight of grafted lographic state was determined by a Philips X’pert-MPD. PS and the control nature of the polymerization, the PS-gThe differential scanning calorimetry (DSC) measureZnO nanocomposites were dispersed in toluene and then ments were accomplished using a Perkin Elmer calorimePS brushes were cleaved using HCl. The PS brushes DeliveredGPC by Publishing Technology to: Korea Advanced Institute of Science & Technology (KAIST) ter (DSC6200). was performed using an Agilent 1200 were from the solution and subjected to the GPC IP: 143.248.125.165 On: Sat, 06 isolated Apr 2013 08:02:27 Series equipped with the tetrahydrofuran solvent and PS analysis. Figure 3 demonstrates the plots of the number Copyright American Scientific Publishers as standard. Thermogravimetric analysis (TGA) was conaverage molecular weight (Mn and molecular weight disducted with Perkin-Elmer Pyris 1. Transmission Electron tributions (PDI = Mw /Mn of the grafted PS against the Microscopy (TEM) images were captured using a Joel overall monomer conversions. It was observed that the Mn JEM 2010. The hydrodynamic diameter was determined of PS increased (26.2, 53.5, 77.6 kg/mol) with increasby the Brookhaven BI-200SM. ing monomer conversion (19.7, 31.1, 61.2%) along with increasing polymerization time (4, 8, 12 h), suggesting the

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Fig. 3. Mn , Mw /Mn of the cleaved PS (from PS-g-ZnO nanocomposites) against monomer conversion.

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Fig. 5. DSC curves of (A) PS, PS-g-ZnO after polymerization time of (B) 4 h, (C) 8 h, and (D) 12 h, respectively.

control nature of the surface thiol-lactam initiated radical nanocomposites was observed to be superior to that of polymerization. the pure PS. It suggests that the incorporation of ZnO The XRD patterns of ZnO nanoparticles before and after improves the thermal stability of PS. PS grafting were recorded as shown in Figure 4. The TEM micrographs of ZnO nanoparticles and PS-g-ZnO characteristic peaks of PS-g-ZnO nanocomposites almost are shown in Figure 7. In the TEM image of ZnO nanoparresembled with those of ZnO nanoparticles, suggesting ticles, a huge number of agglomerated particles are visuthat the grafting polymerization did not alter the crystalline alized (Fig. 7(A)). The reason behind the phenomenon is structure of ZnO nanoparticles. that the high surface energy of ZnO nanoparticles may Figure 5 suggests that the glass transition temperacause the aggregation of particles. To prevent the agglomture (Tg of the PS grafted ZnO is higher than that of Delivered by Publishing Technology to: Korea Advanced Institute of Science & Technology (KAIST) eration, surface of ZnO nanoparticles should be well the corresponding neat PS, by about 5–20 C. This pheIP: 143.248.125.165 On: Sat, 06 Aprthe 2013 08:02:27 modifiedPublishers to ensure its perfect dispersion. In the covanomenon can be explained such a wayCopyright that ZnO American nanopar- Scientific lently modified ZnO nanoparticles, it is clearly observed ticles are able to block the movement of PS chain during that the nanocomposites are well dispersed (Fig. 6(B)), DSC heating. it might be because of the reduced surface energy origThe weight loss of ZnO-SH between 50 C and 700 C inated from steric hindrance between ZnO nanoparticles. was estimated to be 6.5% by TGA analysis (Fig. 6(B)). Moreover, DLS was employed to determine the size and The PS content was increased from 40.1% to 71.2% size distribution of nanocomposites. It is explicitly seen upon polymerization from 4 h to 12 h. The TGA results in Figures 7(C)–(D) that the mean diameter of PS-g-ZnO demonstrate a moderate degree of functionalization of nanocomposites is smaller (ca. 58 nm) than that of ZnO ZnO particles by PS. The thermal stability of PS-g-ZnO nanoparticles (ca. 218 nm). These DLS results further

Fig. 4. XRD patterns of (A) ZnO, (B) ZnO-SH, and PS-g-ZnO nanocomposites after polymerization time of (C) 4 h, (D) 8 h and (E) 12 h, respectively.

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Fig. 6. TGA spectra of (A) ZnO, (B) ZnO-SH, PS-g-ZnO after polymerization time of (C) 4 h, (D) 8 h, (E) 12 h, respectively, and (F) cleaved PS from PS-g-ZnO.


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crystalline behavior of ZnO nanoparticles. The DSC scan demonstrated that the PS-g-ZnO exhibited higher Tg compared with pure PS. The thermal stability of the grafted PS was significantly improved in comparison to that of pure PS as measured by TGA. The covalent grafting of ZnO by PS dramatically improved the dispersion of ZnO nanoparticles in the polymeric matrix as studied by TEM and DLS, which suggests its potential applications in diverse areas. Acknowledgments: This work was financially supported by the grant from the Industrial Source Technology Development Program (Project No. 10035163) of the Ministry of Knowledge Economy (MKE) of Korea

1. S. Park, P. S. K. Murthy, S. Park, Y. M. Mohan, and W. G. Koh, J. Ind. Eng. Chem. 17, 293 (2011). 2. H. M. Xiong, D. P. Liu, Y. Y. Xia, and J. S. Chen, Chem. Mater. 17, 3062 (2005). 3. B. Kulyk, V. Kapustianyk, V. Tsybulskyy, O. Krupka, and suggest that upon covalent functionalization of ZnO by PS, B. Sahraoui, J. Alloys Compd. 502, 24 (2010). 4. M. Sato, A. Kawata, S. Morito, Y. Sato, and I. Yamaguchi, Eur. Polym. the dispersion was improved remarkably. J. 44, 3430 (2008). 5. S. Edmondson, V. L. Osborne and W. T. S. Huck, Chem. Soc. Rev. 33, 14 (2004). 4. CONCLUSION 6. Y. H. Hu, C. Y. Chen, C. C. Wang, and S. P. Wang, J. Polym. Sci. Polym. Chem. 42, 4976 (2004). A facile and efficient surface initiated radical polymerPublishing Technology Korea Advanced Technology (KAIST) 7. H. S.Institute Hwang, J.ofH.Science Bae, H. G.&Kim, and K. T. Lim, Eur. Polym. J. izationDelivered method tobyprepare chemically bondedto:PS-g-ZnO IP: 143.248.125.165 On: Sat, 06 46, Apr 1654 2013 (2010). 08:02:27 nanocomposites has been demonstrated in this report. The Copyright American Scientific Publishers 8. M. H. Rashid, J. H. Bae, C. Park, and K. T. Lim, Mol. Cryst. Liq. grafting from thiol-lactam initiated polymerization was Cryst. 532, 514 (2010). found to be controlled in nature. XRD results suggest 9. L. G. Bach, M. R. Islam, J. T. Kim, S. Y. Seo, and K. T. Lim, Appl. that the chemical immobilization of PS did not alter the Sur. Sci. 258, 2959 (2012). Fig. 7. TEM micrographs of (A) ZnO nanoparticles, (B) PS-g-ZnO; particle size distribution of (C) ZnO nanoparticles, (D) PS-g-ZnO.

Received: 30 November 2011. Accepted: 21 May 2012.


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References and Notes