Constructing polystyrene brushes on tio2 nanoparticles by in situ reversible addition fragmentation chain transfer polymerization - Long Giang Bach

Journal of Science and Technology 54 (1A) (2016) 292-299 CONSTRUCTING POLYSTYRENE BRUSHES ON TIO2 NANOPARTICLES BY IN SITU REVERSIBLE ADDITION FRAGMENTATION CHAIN TRANSFER POLYMERIZATION Long Giang Bach 1* , Bui Thi Phuong Quynh 1 , Kwon Taek Lim 2 1 NTT Institute of Hi-Technology, Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, Ho Chi Minh City, Vietnam 2 Pukyong National University, Busan, 608-737, Republic of Korea * Email: blgiangntt@gmail.com Received: 19 September 2015; Accepted for publication: 26 October 2015 ABSTRACT This paper presents the preparation of polystyrene functionalized TiO2 nanoparticles using the reversible addition fragmentation chain transfer (RAFT) polymerization. The surface of TiO2 NPs with an average particle size of about 5 nm was modified by S-benzyl S’- trimethoxysilylpropyltrithiocarbonate in order to obtain the RAFT agent functionalized TiO2 NPs (TiO2-RAFT). Subsequently, styrene was radically polymerized through the immobilized RAFT agent on the silica surface, in the presence of 2,2’-azobisisobutylnitrile (AIBN) as an initiator, to achieve the TiO2-g-PS nanocomposite. The characteristics of the as-synthesized nanocomposite were determined using FT-IR, EDX, XPS, TGA, XRD, TEM and SEM analyses. Keywords: TiO2 nanoparticles, TiO2-g-PS, RAFT polymerization. 1. INTRODUCTION The development of multifunctional nanomaterials has attracted huge attention for applications in various areas including electronics, optics, biomedicine, and renewable energy generation [1 - 3]. However, because of their extremely large surface-area/volume ratio, nanoparticles are vulnerable to agglomeration thus often resulting in decreased desired properties. Recent research has shown that the encapsulation of inorganic particles by polymer can create advanced materials possessing not only better resistance against aggregation but also improved optical properties and surface chemistry [4 - 5]. As an important nanomaterial, TiO2 nanoparticles (TiO2 NPs) have found their extensive applications such as photocatalysts, electro- catalysts, pigments, fillers, or whiteners. Furthermore, the incorporation of TiO2 NPs into polymer matrice or functionalization of titanium surface with polymeric moieties has been explored to offer great enhancement in stability and also a significant improvement in photovoltaic, photocatalytic, electrical, and chemical properties [6 - 9]. Among a variety of approaches for designing polymeric nanocomposites, tethering polymers directly from or onto the surface of nanoparticles via reversible addition-fragmentation Constructing Polystyren brushes on TiO2 nanoparticles 293 chain transfer (RAFT) polymerization is a facile, but efficient technique [10, 11]. This often results in a core-shell structure in which protective polymer brushes cover the inorganic core. The main advantages of RAFT polymerization are its versatility in monomer choice and polymerization conditions. For instance, a wide variety of vinyl monomers with various functional groups such as (meth)acrylates, (meth)acrylamides, acrylonitrile, styrene derivatives, butadiene, vinyl acetate, N-vinylpyrrolidone, and N-vinylcarbazole have been used for the RAFT process, in the presence of a chain transfer agent. The resulting polymer brushes offer important advantages such as well-control of molecular weight, narrow molecular weight distribution and advanced architectures [12 - 14]. This work aims at functionalizing TiO2 NPs with polystyrene (PS-g-TiO2) via reversible addition-fragmentation chain transfer (RAFT) polymerization. The grafting of polystyrene from TiO2 NPs following a two-step synthetic procedure: one-step direct anchoring of the RAFT agent onto the TiO2 NPs surface and subsequent grafting polymerization from TiO2 NPs based on chain transfer to RAFT agent on TiO2 NPs surfaces. The surface modification and the modulated properties of surface-modified TiO2 NPs were characterized by FT-IR, XPS, TGA, TEM and SEM. 2. MATERIALS AND METHODS 2.1. Materials The synthesis of S-benzyl S-trimethoxysilylpropyltrithiocarbonate (BTPT) followed the procedure as described elsewhere [15]. 2,2’-azobisisobutyronitrile (AIBN) was re-crystallized with ethanol before use. Styrene (St) and Tetrahydrofuran (THF) were dried over CaH2 and distilled under reduced pressure before use. Titanium dioxide nanoparticles (TiO2, anatase) and all solvents were used as received. All of the above chemicals were purchased from Aldrich. 2.2. Grafting of BTPT onto TiO2 nanoparticles The grafting procedure of BTPT on the surface of TiO2 is as follows: after dispersing 10 g of TiO2 nanoparticles in 200 mL of toluene, 1g of BTPT was added and the resulting solution was stirred for 24 h under argon atmosphere. Modified TiO2 was isolated by centrifugation and washed repeatedly with toluene. Finally, it was dried at 50 o C under vacuum for 24 h. 2.3. Synthesis of PS-g-TiO2 nanocomposites by RAFT 1 g of styrene, 0.2 g of TiO2-RAFT, 2.04 mg of AIBN, 4 mL of toluene and a Teflon- coated stir bar were placed in a 25 mL round flask equipped with a reflux condenser. The flask was purged with N2, heated to 100 o C and kept stirring. After polymerization, the flask was cooled to room temperature and the reaction mixture was precipitated in methanol. The product was filtered and dried in a vacuum oven. The polymer product was diluted in toluene and centrifuged to collect the PS-g-TiO2 and obtain the polymer-grafted TiO2 free from the unbound polymer. 2.4. Instrumentation Transmission Electron Microscopy (TEM) images were recorded using a Hitachi H-7500 instrument operated at 80 kV. Fourier-transformed infrared spectrophotometry (FT-IR) was Long Giang Bach et al. 294 employed to characterize the change in the surface functionalities of TiO2 using a BOMEM Hartman & Braun FT-IR spectrometer. Thermogravimetric analysis (TGA) was conducted with Perkin-Elmer Pyris 1 analyzer (USA). Surface composition was investigated using X-ray Photoelectron Spectroscopy (XPS) (Thermo VG Multilab 2000) in ultra high vacuum with Al Kα radiation. The morphology and elemental analysis of the hybrids were carried out by using Field Emission Scanning Electron Microscopy (FE-SEM, Hitachi JEOL- JSM-6700F system, Japan). The crystallographic states of the samples were determined by a Philips X’pert-MPD system diffractometer (the Netherlands) with Cu Kα radiation. 3. RESULTS AND DISCUSSION FT-IR was first performed to determine changes in surface chemical bonds of TiO2 NPs upon functionalization with RAFT agent and PS (Figure 1). As seen in the spectrum of the pristine TiO2 nanoparticles (Figure 1A), the characteristic strong absorption bands at 3421 and 1633 cm -1 could be assigned to the -OH stretching and H–O–H bending vibrations, respectively, (Figure 1A) while the absorption bands between 500 and 800 cm -1 are attributable to the vibrations of Ti-O and Ti-O-Ti framework bonds. In order to realize the chemical bond between TiO2 NPs and PS via RAFT, it is necessary to immobilize RAFT groups onto the surface of TiO2 NPs via reaction between the hydroxyl groups available on theTiO2 surface and triethoxysilane groups of BTPT to form Ti–O–Si linkages. The FT-IR spectrum of TiO2-RAFT shows the characteristic absorptions at 2822 and 2928 cm -1 due to the aliphatic C–H stretching of the coupling agent residues (Figure 1B), the stretching vibration bands for Si–O–Si and Si–C bonds at 1194 and 1252 cm -1 originated from the silane groups. After RAFT polymerization, the as- received PS-g-TiO2 exhibits new characteristic adsorption bands assigned to the grafted PS, including the C-H aromatic stretching vibrations at 3056 and 3023 cm -1 , the C-H aliphatic stretching vibrations at 2937 and 2833 cm -1 , and the phenyl ring stretching vibrations at 1448, 1493, and 1602 cm -1 (Figure 1C). The successful grafting of PS from the surface of TiO2 nanoparticles was thus obviously confirmed. Figure 1. FT-IR spectra of (A) TiO2 NPs, (B) TiO2-RAFT, (C) TiO2-g-PS nanocomposites. The EDX and XPS data were collected as presented in Figure 2. All the samples, including the pristine TiO2, TiO2-RAFT, TiO2-g-PS, exhibit the characteristic peaks of Ti and O elements Constructing Polystyren brushes on TiO2 nanoparticles 295 in TiO2 nanoparticles. The EDX spectra of both TiO2-RAFT and TiO2-g-PS show small but detectable signals ascribed to sulfur elements (Figure 2B & 2C). On the other hands, the binding energies of species in the TiO2 and functionalized TiO2 were determined via XPS analysis. In particular, the spectrum of TiO2-RAFT is consisted of O1s at 531.5 eV, Ti2p at 460.0 eV, C1s at 286.1 eV, S2p at 163.6 eV and Si2p at 102.4 eV (Fig. 2B). It is noteworthy that the characteristic S2p peak of the RAFT agent is observed at 163.1 eV, which confirms the presence of active RAFT agent on TiO2 surfaces. After grafting of PS from TiO2 nanoparticles, the XPS scan of PS-g-TiO2 shows that the C1s peak with high intensity slightly shifted to a higher BE, indicating that the polymeric chains were directly grafted from the surfaces of TiO2 nanoparticles. Figure 2. EDX spectrum of (A) TiO2 NPs, (B) TiO2-RAFT, (C) PS-g-TiO2; Wide-scan spectra of (D) TiO2 NPs, (E) TiO2-RAFT, (F) PS-g-TiO2. The amount of immobilized BTPT and the PS grafted from the TiO2 NPs were quantitatively measured by TGA analysis (Fig. 3). When heated from room temperature to 800 o C, the pure TiO2 NPs shows a weight loss of 2.1 wt%, possibly due the loss of water molecules adsorbed onto the surface and the release of the structural water resulted from the bonded hydroxyl groups. The BTPT modified TiO2 shows the weight loss of 7.2 w% in the range of 50 - 800 o C, thus, the content of the grafted coupling agent was estimated to be ca. 5.1 wt% (Fig. 3B). Significant reduction in weight of the PS-g-TiO2 nanocomposite occurred in the range of 285 - 420 o C, which can be attributed mainly to the decomposition of the grafted PS. Accordingly, the amount of grafted polymer on the surface of TiO2 was ca. 40 %, suggesting a Long Giang Bach et al. 296 moderate degree of functionalization of TiO2 nanoparticles by PS using the “grafting from” approach. Figure 3. TGA spectra of (A) TiO2 NPs, (B) TiO2-RAFT, (C) PS-g-TiO2, and (D) grafted PS cleaved from PS-g-TiO2 nanocomposites. FE-SEM and TEM were employed to investigate the morphology of TiO2 nanoparticles before and after grafting PS. The TEM images of the pristine TiO2 NPs and PS-g-TiO2 nanocomposites are shown in Figure 4. The pristine TiO2 NPs shows an aggregation of particles and the single electron diffraction pattern (inset Fig. 4A) consisting of rings indicative of a good crystal structure. On the other hand, the PS-g-TiO2 nanocomposites are much better dispersed which might be due to the steric hindrance exerted by the grafted PS, implying a significantly improved dispersibility and stability of PS-functionalized TiO2 nanoparticles. Comparing the SEM images of the pristine TiO2 NPs and PS-g-TiO2, the nanocomposites clearly exhibit a nearly spherical shape with polymer shell; in other words, TiO2 nanoparticles are embedded in the polymer beads. Figure 4. TEM pictures of (A) TiO2 NPs, (B) PS-g-TiO2 nanocomposites; FE-SEM pictures of (C) TiO2 NPs, and (D) PS-g-TiO2 nanocomposites. Constructing Polystyren brushes on TiO2 nanoparticles 297 Surface-initiated RAFT polymerization is well known to create covalent bonding between the polymer chains and inorganic particles. However, the original crystalline states as well as the intrinsic properties of pristine TiO2 should be maintained for desired applications. To evaluate the effect of the functionalization process on the crystallinity of TiO2, the XRD patterns of the pristine TiO2 NPs, TiO2-RAFT, PS-g-TiO2 and the cleaved PS (from PS-g-TiO2 nanocomposites) in the 2θ range 5 - 80o were collected as shown in Figure 5. The pristine TiO2 NPs (Figure 5A) exhibited several sharp peaks centered at 2θ = 25.46, 37.89, 48.18, 54.10, 55.18, 62.77, 69.04, 70.28 and 76.47, which correspond to the (101), (112), (200), (105), (211), (204), (116), (220) and (215) reflections, respectively, suggesting the anatase form of TiO2 nanoparticles. Meanwhile, the main peaks of TiO2-RAFT and PS-g-TiO2 nanocomposites are similar to those of TiO2 NPs. Upon surface modification of TiO2 nanoparticles with PS, the broad amorphous band as resulted from the grafted PS arose at the 2θ value of 15.7o and the diffraction peaks of crystalline TiO2 appeared unchanged. It can be concluded that the nanocomposites possess a more ordered orientation than the neat polymer owing to the inclusion of TiO2 NPs and the covalent attachment of PS on TiO2 nanoparticles do not affect the crystalline of nanoparticles. Figure 5. XRD patterns of (A) TiO2 nanoparticles, (B) TiO2-RAFT (C) PS-g-TiO2 nanocomposites. 4. CONCLUSIONS A facile method to covalently immobilize PS onto TiO2 nanoparticles via SI-RAFT polymerization and “grafting from” approach has been demonstrated. The chemical structure and thermal property of the as-synthesized PS-g-TiO2 nanoparticles were well characterized using FT-IR, XPS, XRD and TGA techniques. The formation of grafted PS shell covering the TiO2 core helped reduce significantly the agglomeration tendency while did not alter its physical structure. In addition, it was found that the thermal stability of the grafted PS was significantly higher than that of the pure PS as attributed to the incorporation of TiO2. The as-received nanocomposites with enhanced properties can be an interesting material for future investigations regarding its potential applications or integration of new functions. Also, the suggested approach has the potential to be widely applied for preparation of a wide range of advanced nanohybrids. Acknowledgements. This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.02-2014.53 Long Giang Bach et al. 298 REFERENCES 1. 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Constructing Polystyren brushes on TiO2 nanoparticles 299 TÓM TẮT NGHIÊN CỨU CẤU TRÚC VẬT LIỆU POLYSTYRENE GẮN HẠT NANO TIO2 BẰNG PHƢƠNG PHÁP HỆ KHƠI MÀO DO CƠ CHẾ CHUYỂN MẠCH THEO ĐỨT RÁP THUẬN NGHỊCH (RAFT) Long Giang Bạch1, *, Bùi Thị Phƣơng Quỳnh1, Kwon Taek Lim2 1 Viện Kỹ thuật Công nghệ cao NTT, Trường ĐH Nguyễn Tất Thành, 300A Nguyễn Tất Thành, TP. Hồ Chí Minh, Việt Nam 2Trường Đại học Quốc gia Pukyong, Busan, 608-737, Hàn Quốc * Email: blgiangntt@gmail.com Trong nghiên cứu này, polystyrene (PS) lai ghép trên bề mặt hạt nano TiO2 đã đƣợc tổng hợp thành công bằng phƣơng pháp RAFT (hệ khơi mào do cơ chế chuyển mạch theo đứt ráp thuận nghịch). Tác nhân RAFT gắn trên bề mặt hạt nano TiO2 (kích thƣớc hạt trung bình 5 nm) bằng phản ứng giữa bề mặt hạt nano với S-benzyl S’-trimethoxysilylpropyltrithiocarbonate. Styrene đƣợc trùng hợp thông qua tác nhân RAFT gắn cố định trên bề mặt hạt nano TiO2 và sử dụng 2,2’-azobisisobutylnitrile (AIBN) nhƣ một chất khơi mào tạo thành vật liệu cấu trúc nano composite TiO2-g-PS. Các tính chất đặc trƣng của của vật liệu nano composite đƣợc xác định bằng các phƣơng pháp nhƣ FT-IR, EDX, XPS, TGA, XRD, TEM và SEM. Từ khóa: hạt nano TiO2, vật liệu TiO2-g-PS, phản ứng trùng hợp RAFT polymerization.