PMMA Hybrids with Improved

Research Article www.acsami.org

Preparation of Transparent Bulk TiO2/PMMA Hybrids with Improved Refractive Indices via an in Situ Polymerization Process Using TiO2 Nanoparticles Bearing PMMA Chains Grown by Surface-Initiated Atom Transfer Radical Polymerization Satoshi Maeda,† Masato Fujita,† Naokazu Idota,‡ Kimihiro Matsukawa,§ and Yoshiyuki Sugahara*,†,‡ †

Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan § Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *

ABSTRACT: Transparent TiO2/PMMA hybrids with a thickness of 5 mm and improved refractive indices were prepared by in situ polymerization of methyl methacrylate (MMA) in the presence of TiO2 nanoparticles bearing poly(methyl methacrylate) (PMMA) chains grown using surface-initiated atom transfer radical polymerization (SI-ATRP), and the effect of the chain length of modified PMMA on the dispersibility of modified TiO2 nanoparticles in the bulk hybrids was investigated. The surfaces of TiO2 nanoparticles were modified with both m-(chloromethyl)phenylmethanoyloxymethylphosphonic acid bearing a terminal ATRP initiator and isodecyl phosphate with a high affinity for common organic solvents, leading to sufficient dispersibility of the surface-modified particles in toluene. Subsequently, SI-ATRP of MMA was achieved from the modified surfaces of the TiO2 nanoparticles without aggregation of the nanoparticles in toluene. The molecular weights of the PMMA chains cleaved from the modified TiO2 nanoparticles increased with increases in the prolonging of the polymerization period, and these exhibited a narrow distribution, indicating chain growth controlled by SI-ATRP. The nanoparticles bearing PMMA chains were welldispersed in MMA regardless of the polymerization period. Bulk PMMA hybrids containing modified TiO2 nanoparticles with a thickness of 5 mm were prepared by in situ polymerization of the MMA dispersion. The transparency of the hybrids depended significantly on the chain length of the modified PMMA on the nanoparticles, because the modified PMMA of low molecular weight induced aggregation of the TiO2 nanoparticles during the in situ polymerization process. The refractive indices of the bulk hybrids could be controlled by adjusting the TiO2 content and could be increased up to 1.566 for 6.3 vol % TiO2 content (1.492 for pristine PMMA). KEYWORDS: TiO2/PMMA hybrids, surface-initiated atom transfer radical polymerization, dispersibility, transparency

■

nanoparticles with high refractive indices (TiO2 nanoparticles (refractive indices of 2.5−2.7), for example) have therefore attracted attention for their ability to overcome the limitations on the refractive indices of common polymers.2−24 Although optical

INTRODUCTION Although polymers have been widely used as optical materials because of their transparency, light weight, excellent formability, and low cost, their applications in optical materials requiring high refractive indices have been limited because common optical polymers such as poly(methyl methacrylate) (PMMA) and epoxy resin have a restricted range of refractive indices of from 1.3 to 1.7.1 Organic−inorganic hybrids bearing inorganic © XXXX American Chemical Society

Received: September 27, 2016 Accepted: November 23, 2016 Published: November 23, 2016 A

DOI: 10.1021/acsami.6b10427 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

to prevent aggregation and phase separation of the nanoparticles. Because initiator-modified nanoparticles are not highly dispersed in organic solvents, however, this method has not been used in the preparation of transparent bulk hybrids requiring high dispersibility of nanoparticles in previous studies.13 We report here the preparation of bulk TiO2/PMMA hybrids with improved refractive indices achieved by combining the “grafting from” method and the in situ polymerization process. PMMA chains were grafted from the surfaces of TiO2 nanoparticles using a surface-initiated (SI)-ATRP technique. A co-modification technique was used to control the graft density of the PMMA chains: nanoparticles were co-modified with m-(chloromethyl)phenylmethanoyloxymethyl phosphonic acid (CPMP) as an ATRP initiator and isodecyl phosphate ester (IDP), which exhibits a better affinity with organic solvents. In addition, the chain length of the modified PMMA on the nanoparticles was controlled by SI-ATRP, because chain length is an important factor for controlling the affinity to polymer matrices and the excluded volume effect. Nanoparticles bearing PMMA chains were then dispersed in an MMA monomer, and 5 mm thick bulk hybrids were prepared by in situ polymerization of the monomer dispersion. In this study, emphasis was placed on the effect of the chain length of the modified polymers on the dispersibility of the bulk hybrids in PMMA matrices and their optical properties as functions of the TiO2 content and the chain length of the modified polymers.

polymers with refractive indices enhanced by molecular design have been recently developed,25 demand remains for improvement of optical properties using commercially available polymers by incorporating nanoparticles for various industrial applications, including optical waveguides,2 lenses,3 antireflective coatings,6,10 and LED encapsulations.14,19 The key to preparation of practical inorganic nanoparticle/ polymer hybrids is preventing aggregation or phase separation of the nanoparticles in the polymer matrices. Aggregation and phase separation of nanoparticles can cause a decline in the transparency of hybrids due to Rayleigh scattering at the interfaces between the inorganic components and the polymer matrices. In general, it is necessary to disperse nanoparticles whose sizes are below 40 nm in polymer matrices to obtain transparent hybrids.26 Several approaches to producing transparent hybrids with high refractive indices involving the in situ particle formation method,3,7,11,16−18 ex situ blending method,9,13,15,19,21−23 and in situ polymerization method5,8,12 have been reported. Using these methods, transparent polymer-based hybrids containing inorganic nanoparticles were successfully prepared for use in thin hybrid films. Because the scattering intensity increases as a hybrid thickens, most of the previous studies on organic−inorganic hybrids have focused on hybrid films with thicknesses of less than several micrometers.27 Although bulk hybrids with high refractive indices have been desired for various applications, such as photovoltaic devices, ophthalmic lenses, and other domeshaped LED encapsulants, the bulk hybrids with thicknesses >1 mm in previous studies are still of lower quality in terms of optical properties than thin hybrid films.8,12,20,24 Because the effect of the dynamic changes in the compositions of polymer solutions containing nanoparticles during the formation of bulk hybrids through solvent evaporation is significant,13 in situ polymerization of nanoparticle/monomer dispersion involving no solvent evaporation is an appropriate method for the preparation of bulk hybrids. Bulk polymer-based hybrids have thus been prepared through in situ polymerization using nanoparticles modified with monomer residues.12,20 The affinities of monomers are, however, commonly different from those of polymer matrices, and aggregation of the monomer-modified nanoparticles consequently occurred during the in situ polymerization. Thus, formulating a new strategy for the preparation of bulk hybrids with high refractive indices remains a challenge. Grafting polymer chains that exhibit compositions identical to polymer matrices in the hybrids is an ideal strategy for improving dispersion stability in bulk hybrids, because the grafted polymer chains covering the surfaces of the nanoparticles enhance the affinity between the nanoparticle surfaces and the polymer matrices. In addition, the excluded volume effect of the modified polymer chains is expected to suppress nanoparticle aggregation during in situ polymerization.28 In general, two methods of grafting polymer chains onto nanoparticles have been used, the “grafting to” and “grafting from” methods.29 In the “grafting from” method, the polymer chains are grown from initiators anchored on the nanoparticles, and control of the chain length of the modified polymers has been achieved by surface-initiated living polymerization techniques, such as atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition−fragmentation chain transfer (RAFT).29 The density of the polymer chains in the “grafting from” method can be also controlled by adjusting the reactive sites on the nanoparticle surfaces.30 The “grafting from” method with surface-initiated living polymerization is therefore more suitable for designing nanoparticles with grafted polymer chains

■

EXPERIMENTAL SECTION

Materials. Copper(I) chloride was washed with acetic acid to remove any soluble oxidized species before use. Purification of 4-dimethylaminopyridine was achieved by recrystallization in toluene. MMA was purified by passing it through a basic activated alumina column. Benzoyl peroxide (BPO), bromotrimethylsilane (TMSBr), 3-(chloromethyl)benzoyl chloride, copper(II) chloride, diethyl(hydroxymethyl)phosphonate, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), and trimethylamine were used without further purification. A methanol dispersion of rutile-type TiO2 nanoparticles (TiO2/methanol dispersion (15.5 mass%)) and isodecyl phosphate (IDP; mixture of monoester and diester, average molecular weight = 309) were thoughtfully provided by Sakai Chemical Industry Co., Ltd. (Osaka, Japan). A phosphonic acid bearing a terminal ATRP initiator, CPMP, was synthesized in accordance with the method in previous papers.31,32 (See the Supporting Information for details.) Characterization. The BET surface area of dried bare TiO2 nanoparticles was measured with a MicrotracBEL BELSORP-mini II instrument. Liquid-state 1H nuclear magnetic resonance (NMR) spectroscopy was performed with a JEOL JNM-Lambda500 spectrometer, and (CD3)2SO was used as the solvent. Solid-state31P magic angle spinning (MAS) NMR spectra were recorded with a JEOL CMX400 spectrometer (spinning rate = 8.0 kHz, frequency = 160.26 MHz, and pulse delay = 30 s). Fourier transform infrared (FT-IR) spectra were recorded with a JASCO FT-IR-460 Plus spectrometer using the KBr disk technique. Thermogravimetry (TG) was performed with a PerkinElmer TGA7 thermobalance in the temperature range from 30 to 800 °C at a heating rate of 10 °C/min under an air flow. Inductively coupled plasma (ICP) emission spectrometry was performed with a VISTA-MPX CCD Simultaneous ICP-OES instrument after the samples had been dissolved in a mixture of HNO3, H2SO4, and HF at 200 °C for 2 h. The particle size distributions based on dynamic light scattering (DLS) were measured with a Nikkiso Nanotrac Wave-UT151 instrument. Transmission electron microscopy (TEM) images of modified nanoparticles were obtained with a JEOL JEM-1011 microscope operating at 100 kV. TEM images of bulk hybrids were obtained using a JEOL JEM2010 at 200 kV, and samples were prepared by cutting the hybrids into approximately 50 nm slices with an RMC PowerTome. The molecular weight and polydispersity of grafted polymers cleaved from TiO2 B


Research Article

ACS Applied Materials & Interfaces nanoparticles by sequential treatment with HF and HCl were determined by gel permeation chromatography (GPC) at 40 °C with a Showa Denko Shodex KF-804L column connected to a JASCO RI-4030 refractive index detector. Tetrahydrofuran (THF) with a stabilizer was used as an eluent at a flow rate of 1.0 mL/min, and a calibration curve was prepared using Shodex PMMA standards. The volume fractions of TiO2 nanoparticles in the bulk hybrids were calculated using a Shimadzu AUX320 with an SMK-401 density meter (the densities of rutile TiO2 and PMMA were 4.23 and 1.18 g/cm3, respectively). UV−vis spectra were recorded with a JASCO V-630 spectrometer in the transmittance mode. The refractive indices of neat PMMA and TiO2/PMMA hybrids were measured on an ATAGO DR-A1 Abbe’s refractometer at 589 nm. Surface Modification of TiO2 Nanoparticles with Phosphorus Coupling Reagents. ATRP initiator moieties and alkyl groups were bound to TiO2 nanoparticles by phosphorus coupling reagents, CPMP and IDP.33,34 A dispersion of TiO2 nanoparticles in methanol (6 mL) was diluted with methanol (18 mL) in a 100 mL round-bottom flask. CPMP (0.0648 g) and IDP (0.380 g) were dissolved in methanol (24 mL), and the solution was added dropwise to the flask (molar ratio in feed; CPMP:IDP = 1:5). The dispersion was stirred for 18 h at room temperature. After reaction, the modified TiO2 nanoparticles were spontaneously precipitated and separated by centrifugation (4800 rpm, 30 min). The precipitate was washed twice with methanol (10 mL) by the dispersion−precipitation method using centrifugation at 4800 rpm for 1 h to remove the unreacted CPMP and IDP. CPMP_IDP_TiO2 was obtained as a white solid with an approximately 100% yield. Grafting of PMMA Chains from TiO2 Nanoparticles via SIATRP. The as-synthesized CPMP_IDP_TiO2 was redispersed in toluene (17 mL), and the dispersion was sonicated for 15 min. The toluene dispersion of CPMP_IDP_TiO2 in a reaction flask and a monomer solution containing MMA (2.37 mL), PMDETA (78.09 μL), and toluene (3 mL) in another flask were carefully degassed in three freeze−pump−thaw cycles. After degassing, CuCl (6.7 mg) and CuCl2 (1.0 mg) were added to the monomer solution, and the mixture was stirred until the CuCl and CuCl2 were completely dissolved to form Cu complexes. The monomer solution containing the Cu complexes was then added to the toluene dispersion of CPMP_IDP_TiO2 after N2 purging, and polymerization was allowed to proceed at 60 °C for 6, 12, or 18 h. The molar ratio in feed was adjusted to [CPMP]:[MMA]: [PMDETA]:[CuCl]:[CuCl2] = 1:300:5:0.9:0.1, and the monomer concentration was 1.00 mol/L. After quenching of the polymerization by exposure to air, the reaction solution was added dropwise to methanol (30 mL) with stirring to remove the Cu complexes. PMMAmodified TiO2 nanoparticles (PMMA_TiO2_xh, x is the polymerization period) were obtained as a white solid after centrifugation (1000 rpm, 3 min). Preparation of Bulk TiO2/PMMA Hybrids. PMMA_TiO2_xh was redispersed in CH2Cl2 (10 mL) by stirring for 24 h and ultrasonication. A prescribed amount of PMMA_TiO2_xh/CH2Cl2 dispersion was added to MMA (3 mL) in a 50 mL Schlenk flask to adjust the volume ratio of the TiO2 content in the range of 0−6.3 vol %. After BPO (2.0 mg) was added to the dispersion, CH2Cl2 was completely removed by evaporation. The series of dispersions were degassed in three freeze− pump−thaw cycles, and in situ polymerization proceeded for 24 h at 60 °C without stirring. After polymerization, bulk hybrids abbreviated as TiO2/PMMA_xh with 5 mm thicknesses were prepared by cutting and surface grinding.

Figure 1. IR spectrum of CPMP_IDP_TiO2.

ν(OH) absorption bands observed in the spectrum of CPMP at around 3500 cm−1 also disappeared in the spectrum of CPMP_IDP_TiO2. These results suggest that the CPMP and IDP moieties were successfully bound to the TiO2 nanoparticles. The 31P NMR spectrum of IDP and the solid-state 31P MAS NMR spectra of CPMP and CPMP_IDP_TiO2 are shown in Figure 2. In the solid-state 31P MAS NMR spectrum of

Figure 2. 31P NMR spectrum of (a) IDP and 31P MAS NMR spectra of (b) CPMP and (c) CPMP_IDP_TiO2.

■

RESULTS AND DISCUSSION Surface Modification of TiO2 Nanoparticles. The IR spectrum of CPMP_IDP_TiO2 is shown in Figure 1. The following absorption bands in the spectrum are assignable to an organic group of CPMP and the isodecyl group of IDP: ν(CH) at around 2900 cm−1, ν(CO) at 1713 cm−1, δ(CH2) at 1462 cm−1, and ν(COC) at 1197 cm−1.35,36 Although ν(PO) absorption bands were observed in the spectra of CPMP and IDP at 1110 and 1232 cm−1, the corresponding bands disappeared in the spectrum of CPMP_IDP_TiO2. In addition,

CPMP_IDP_TiO2, two broad signals attributed to CPMP and IDP moieties were observed at 10−28 and −15−10 ppm, respectively. Both signals shifted upfield compared to the chemical shifts of CPMP (25 ppm) and IDP (4.4 ppm for monoester and 3.2 ppm for diester). In previous studies,23,37 when an organic phosphonic acid or a phosphate ester was modified on TiO2 surfaces in a bidentate or tridentate environment through a reaction between the PO and Lewis acid sites on the TiO2 surfaces and dehydration condensation between the POH and C


Research Article

ACS Applied Materials & Interfaces TiOH, the phosphorus signals of 31P MAS NMR spectra shifted upfield. This result therefore suggested that most of the CPMP and IDP moieties were present in the bidentate or tridentate environment.37 The ratio of the integral values of their signals assigned to modified CPMP and IDP was 1:5, which corresponds to their molar ratio in feed, and the selective surface modification thus did not occur depending on the type of phosphorus coupling reagent. The modification density of phosphorus coupling agents on the nanoparticles is calculated as 2.2 × 10−6 mol/m2 on the basis of the ICP results of CPMP_IDP_TiO2, BET analysis of bare TiO2 nanoparticles (283 m2/g), and the PO4 group head area (24 Å2),38 and the density corresponds to a coverage of 31%. The modification density of CPMP is calculated as 3.7 × 10−7 mol/m2. Although TiO2 nanoparticles modified with CPMP alone did not show dispersibility in any organic solvents, CPMP_IDP_TiO2 was clearly dispersed with high transparency in toluene, THF, CH2Cl2, and CHCl3, as shown in Figure S4. This is likely due to an improved affinity between the TiO2 surfaces and the organic solvents due to IDP modification. By contrast, CPMP_IDP_TiO2 was not dispersed in MMA. The median size of CPMP_IDP_TiO2 in toluene (obtained by DLS measurement as shown in Figure S5) was changed from 4.9 nm for bare TiO2 nanoparticles to 8.6 nm while maintaining a narrow distribution. Considering the molecular sizes of CPMP and IDP, the diameter of the particles modified as monolayers of CPMP and IDP moieties can be estimated as 7.9 nm, which corresponds to their median size considering that DLS measurement provides hydrodynamic radii for particle sizes. It is therefore suggested that CPMP and IDP moieties are present as monolayers on the surface and that the nanoparticles are monodispersed in toluene. This result is consistent with the TEM image of CPMP_IDP_TiO2 (Figure 3). The modified TiO2 nanoparticles were individually observed as a large number of dark spots with gaps of a few nanometers, which are probably CPMP and IDP moieties present as monolayers.

Figure 4. TG curves of PMMA_TiO2.

PMMA_TiO2_xh are shown in Figure 4. The mass loss between 30 and 200 °C is assignable to the evaporation of absorbed H2O and/or solvent from the TiO2 nanoparticle surfaces. The mass loss of CPMP_IDP_TiO2 between 200 and 600 °C was 13.9%. In the IR spectrum of calcined CPMP_IDP_TiO2 at 800 °C (not shown), only ν(TiOTi) and broad ν(PO) absorption bands were observed, suggesting that the phosphorus was present in the calcined residues as a PO4 environment created through TiOP bonds.19 Considering the molar ratio of modified CPMP and IDP based on the 31P MAS NMR results, moreover, the theoretical TG mass loss of organic groups in the CPMP and IDP moieties was 10.2%. The TG mass loss between 200 and 600 °C therefore seems to include decomposition of organic groups in the CPMP and IDP moieties. The mass losses of organic components in PMMA_TiO2_6h, PMMA_TiO2_12h, and PMMA_TiO2_18h were 48.2, 58.2, and 70.0%, respectively. These results indicate that the chain growth of MMA monomers increased with increases in the SI-ATRP period. The GPC traces for PMMA chains cleaved from PMMAmodified TiO2 nanoparticles are shown in Figure 5. The number-

Figure 5. GPC traces for PMMA chains cleaved from modified TiO2 nanoparticles. Figure 3. TEM image of CPMP_IDP_TiO2.

average molecular weights, Mn, of cleaved PMMA chains for ATRP periods of 6, 12, and 18 h were 7.77 × 103, 1.31 × 104, and 2.36 × 104, respectively. These results suggest that the molecular weight depended on the polymerization period, that the modified CPMP initiated uniform PMMA chain growth, and that active chain growth was maintained during the SI-ATRP process. In addition, the polydispersity indices, Mw/Mn, are calculated as 1.28, 1.27, and 1.09 for PMMA_TiO2_6h, PMMA_TiO2_12h, and PMMA_TiO2 _18h, respectively,

Grafting of PMMA Chains from TiO2 Nanoparticles. As shown in Figure S6, absorption bands attributable to PMMA are observed in the IR spectra of PMMA_TiO2_18h, including ν(C O) at 1713 cm−1, ν(CO) at 1262 cm−1, and ν(COC) at 1150 cm−1.39 Similar results were obtained for the IR spectra of PMMA_TiO2_6h and PMMA_TiO2_12h, suggesting progress of the polymerization. The TG curves of CPMP_IDP_TiO2 and D


Research Article

ACS Applied Materials & Interfaces indicating progress of well-defined polymerization by ATRP. Thus, these results clearly indicate that the surface modification of PMMA on TiO2 nanoparticles via SI-ATRP was successful and that the chain length could be controlled by adjusting the polymerization period. Preparation of Bulk TiO2/PMMA Hybrids. PMMAmodified TiO2 nanoparticles exhibited high dispersibility in MMA with transparency, although CPMP_IDP_TiO2 was aggregated in MMA, as mentioned above. We then prepared bulk hybrids comprising PMMA_TiO2_xh nanoparticles and PMMA matrices through in situ polymerization. Photographs of the TiO2/PMMA_xh hybrids are shown in Figure 6a. Although

these TEM images strongly suggest that the decline in transparency is due to the aggregation and phase separation of nanoparticles during in situ polymerization. In addition, the size of these aggregates was smaller in the PMMA-modified nanoparticles undergoing a longer ATRP period, leading to higher transparency of the hybrids. The relationship between the ATRP period and the degree of aggregation and phase separation of TiO2 nanoparticles can be explained on the bases of the following three factors: (1) affinity at the interfaces between TiO2 nanoparticles and PMMA matrices generated during in situ polymerization; (2) distance between nanoparticles; and (3) entanglement of surface-grafted and matrix PMMA chains. For ideal conformation of a polymer chain, the radius of the polymer gyrations, S, in a good solvent and with no solvent can be described by the equations40 Ssol =

nl 2 6

Snon‐sol =

(in good solvent) n2/3l 2 6

(1)

(with no solvent)

(2)

where n is the degree of polymerization and l is the distance between contiguous elements (PMMA: l = 1.5 nm). The radius of the gyration of a PMMA chain before and after in situ polymerization can therefore be determined by substituting n from the GPC results in eqs 1 and (2). From these radii of gyrations, the A, B and A/B values shown below are obtained: A: πS2(cross-section area of a polymer chain) × 16.3 (number of polymer chains per TiO2 nanoparticle (4.9 nm)) B: 4π(8.6/2 + S)2 (surface area of a sphere whose radius is assumed to equal the radius of a modified nanoparticle + the radius of gyration) A/B: the degree of nanoparticle surface coverage by PMMA chains (A/B ≥ 1 indicates that all surfaces of the nanoparticles are covered with PMMA chains) A, B, and A/B before and after in situ polymerization for PMMA_TiO2_6h, PMMA_TiO2_12h, and PMMA_TiO2_18h are summarized in Tables 1 and 2. The A/B values show that the surfaces of the TiO2 nanoparticles of all the PMMA_TiO2_xh in a good solvent are covered with PMMA chains before in situ polymerization. This is consistent with the results showing that PMMA_TiO2_6h, PMMA_TiO2_12h, and PMMA_TiO2_18h were all well-dispersed in MMA due to the excellent affinity of PMMA-grafted nanoparticles with MMA. By contrast, the A/B values for PMMA_TiO2_6h, PMMA_TiO2_12h, and PMMA_TiO2_18h after polymerization are decreased to 58, 72, and 90%, respectively. An increase in the surface coverage with PMMA chains can provide a high affinity between TiO2 nanoparticle surfaces and PMMA matrices and inhibit the approach of nanoparticles due to the excluded volume effect of PMMA chains grafted onto nanoparticles. Therefore, a longer polymerization period contributes to preventing the aggregation and phase separation of nanoparticles during in situ polymerization. It is also

Figure 6. (a) Photographs of bulk TiO2/PMMA hybrids (5 mm thickness) and TEM images of (b) TiO2/PMMA_6h (6.3 vol %), (c) TiO2/PMMA_12h (6.2 vol %), and (d) TiO2/PMMA_18h (6.3 vol %) hybrids.

the modified TiO2 nanoparticles dispersed in MMA before in situ polymerization, TiO2/PMMA_6h and TiO2/PMMA_12h became turbid. By contrast, the TiO2/PMMA _18h hybrid maintained transparency after in situ polymerization. TEM images of a 50 nm slice of bulk hybrids are shown in Figure 6b, with PMMA-modified TiO2 nanoparticles observed as a large number of dark spots just a few nanometers in diameter. In the TEM images, the area occupied by dark spots is much higher than the TiO2 content in the hybrids (6.3 vol %). The TEM provides two-dimensional projection images of TiO2 nanoparticles in the slices of the hybrids, and all of the nanoparticles inside the hybrids are consequently observed in the images, as reported in previous papers.8,12,20 In the TEM images, aggregates of ca. 200 nm for TiO2/PMMA_6h and ca. 100 nm for TiO2/ PMMA_12h are observed. On the other hand, the nanoparticles are uniformly dispersed in TiO2/PMMA_18h. Because the aggregation of nanoparticles whose sizes are above 40 nm in polymer matrices induces Rayleigh scattering at the interfaces,26

Table 1. Calculation Results of Total PMMA Section Area per a TiO2 Nanoparticle, Surface Areas of Nanoparticles, and Their Ratios before in Situ Polymerization

PMMA_TiO2_6h PMMA_TiO2_12h PMMA_TiO2_18h

Ssol (nm)

A = πSsol2 × 16.3 (nm2)

B = 4π(8.6/2 + Ssol)2 (nm2)

A/B

5.4 7.0 9.4

1493 2509 4525

1182 1605 2359

1.26 1.56 1.92

E


Research Article


Table 2. Calculation Results of Total PMMA Section Area per a TiO2 Nanoparticle, Surface Areas of Nanoparticles, and Their Ratios after in Situ Polymerization PMMA_TiO2_6h PMMA_TiO2_12h PMMA_TiO2_18h

Snonsol (nm)

A = πSnonsol2 × 16.3 (nm2)

B = 4π(8.6/2 + Snonsol)2 (nm2)

A/B

2.6 3.1 3.8

346 492 739

598 688 824

0.58 0.72 0.90

probable, moreover, that longer polymer chains are characterized by a stronger entanglement between PMMA chains grafted onto TiO2 nanoparticles and matrix PMMA chains, and that the mobility of the nanoparticles is reduced as a result. Photographs of TiO2/PMMA_18h hybrids with different TiO2 contents and UV−vis spectra of the hybrids in the range from 300 to 800 nm are shown in Figure 7. Even though the

Figure 8. Refractive indices of TiO2/PMMA_18h as a function of their TiO2 content.

nanoparticle content was achieved (1.9−6.3 vol % TiO2 content), and the bulk hybrids exhibited reasonable or high levels of transparency. Thus, it was demonstrated that polymer chains grafted from nanoparticles by SI-ATRP were effective for preparing transparent bulk hybrids with improved refractive indices. With this procedure, phosphorus coupling agents (CPMP and IDP) can form stable M−O−P bonds with various inorganic oxides,42 and ATRP can be applicable to a variety of polymers. The present strategy is therefore applicable to many inorganic−organic hybrids.

Figure 7. (a) Photographs of TiO2/PMMA_18h with different TiO2 contents (5 mm thickness) and (b) UV−vis spectra of the bulk hybrids.

■

CONCLUSIONS We prepared transparent TiO2/PMMA hybrids with a thickness of 5 mm and improved refractive indices by in situ polymerization of MMA from the surfaces of TiO2 nanoparticles bearing PMMA chains via SI-ATRP. TiO2 nanoparticles co-modified with CPMP and IDP were well-dispersed in various organic solvents. PMMA chains with controlled molecular weights were then grafted from the TiO2 nanoparticle surfaces. Bulk hybrids were prepared by in situ polymerizations of MMA in the presence of well-dispersed TiO2 nanoparticles bearing PMMA chains. The transparency of the hybrids increased with increases in the ATRP period. Three factors were thought to contribute to this result: affinity between the surfaces of the TiO2 nanoparticles and the PMMA matrices generated by in situ polymerization, the approach distance of the nanoparticles, and the entanglement of polymer chains involving PMMA chains grafted onto TiO2 nanoparticles and matrix PMMA chains. The bulk hybrids with a 5 mm thickness prepared over an 18 h ATRP period exhibited refractive indices up to 1.566 for 6.3 vol % TiO2 loading (n = 1.492 for neat PMMA) with a reasonable transparency at 633 nm of 76.1%. These results strongly suggest that the present strategy involving co-modification of phosphorus coupling reagents, SI-ATRP for the “grafting from” process, and in situ

thicknesses of the bulk hybrids were 5 mm, their light transmittance values were >76.1% at 633 nm. TiO2 nanoparticles often induce polymer-based hybrids to turn yellowish due to their photocatalytic effect. No significant discoloring of the bulk hybrids was observed when the samples were stored under ambient conditions. Rutile-type TiO2 nanoparticles were used as nanofillers in the PMMA-based hybrids in this study, and it is well-known that the rutile type exhibits lower photocatalytic activity than the anatase type because of its lower bandgap energy. In addition, it was reported that surface modification of organophosphonic acids on photocatalytic particles led to a decrease in the generation of oxidizing species from the surfaces.41 Thus, the photocatalytic effects of rutile-type TiO2 nanoparticles modified with PMMA chains through Ti−O−P bonds should not be significant. The refractive indices of the TiO2/PMMA_18h hybrids at 633 nm were plotted as a function of TiO2 content as shown in Figure 8. The refractive index of neat PMMA film is 1.492, and the indices of the bulk hybrids increased with increases in the TiO2 content. At 6.3 vol % TiO2 content, the refractive index increased to 1.566. Although the bulk hybrids prepared in this study were thicker than the bulk hybrids in previous studies (1−4 mm thick),8,12,20,24 a similar F


Research Article


Content via γ-Ray Irradiation Initiated Bulk Polymerization. Adv. Mater. 2006, 18, 1188−1192. (9) Nakayama, N.; Hayashi, T. Preparation and Characterization of TiO2 and Polymer Nanocomposite Films with High Refractive Index. J. Appl. Polym. Sci. 2007, 105, 3662−3672. (10) Chen, C.-C.; Lin, D.-J.; Don, T.-M.; Huang, F.-H.; Cheng, L.-P. Preparation of Organic−Inorganic Nano-Composites for Antireflection Coatings. J. Non-Cryst. Solids 2008, 354, 3828−3835. (11) Antonello, A.; Brusatin, G.; Guglielmi, M.; Bello, V.; Mattei, G.; Zacco, G.; Martucci, A. Nanocomposites of Titania and Hybrid Matrix with High Refractive Index. J. Nanopart. Res. 2011, 13, 1697−1708. (12) Lin, Z.; Cheng, Y.; Lü, H.; Zhang, L.; Yang, B. Preparation and Characterization of Novel ZnS/Sulfur-Containing Polymer Nanocomposite Optical Materials with High Refractive Index and High Nanophase Contents. Polymer 2010, 51, 5424−5431. (13) Tao, P.; Li, Y.; Rungta, A.; Viswanath, A.; Gao, J.; Benicewicz, B. C.; Siegel, R. W.; Schadler, L. S. TiO2 Nanocomposites with High Refractive Index and Transparency. J. Mater. Chem. 2011, 21, 18623− 18629. (14) Chung, P. T.; Yang, C. T.; Wang, S. H.; Chen, C. W.; Chiang, A. S. T.; Liu, C.-Y. ZrO2/Epoxy Nanocomposite for LED Encapsulation. Mater. Chem. Phys. 2012, 136, 868−876. (15) Kaneko, T.; Kamochi, Y.; Yamamoto, H.; Matsukawa, K.; Sugahara, Y. Preparation of Epoxy-Based Hybrid Films from an Aqueous TiO2 Dispersion via Solvent Exchange and Surface Modification with nOctylphosphonic Acid. Compos. Interfaces 2012, 19, 593−601. (16) Asai, T.; Sakamoto, W.; Yogo, T. In Situ Synthesis of Transparent TiO2 Nanoparticle/Polymer Hybrid. J. Mater. Sci. 2013, 48, 7503−7509. (17) Yu, Y.-Y.; Yu, H.-H. High Refractive Index Organic−Inorganic Composites with TiO2 Nanocrystal. Thin Solid Films 2013, 529, 195− 199. (18) Chaudhuri, T. K.; Patel, M. G. High Refractive Index Films of ZnS/PVP Nanocomposite by In Situ Thermolysis. J. Exp. Nanosci. 2015, 10, 135−147. (19) Kobayashi, M.; Saito, H.; Boury, B.; Matsukawa, K.; Sugahara, Y. Epoxy-Based Hybrids Using TiO2 Nanoparticles Prepared via a NonHydrolytic Sol-Gel Route. Appl. Organomet. Chem. 2013, 27, 673−677. (20) Tao, P.; Li, Y.; Siegel, R. W.; Schadler, L. S. Transparent Dispensible High-Refractive Index ZrO2/Epoxy Nanocomposites for LED Encapsulation. J. Appl. Polym. Sci. 2013, 130, 3785−3793. (21) Liu, B.-T.; Li, P.-S.; Chen, W.-C.; Yu, Y.-Y. Ex Situ Synthesis of High-Refractive-Index Polyimide Hybrid Films Containing TiO2 Chelated by 4-Aminobenzoic Acid. Eur. Polym. J. 2014, 50, 54−60. (22) Chang, C.-C.; Hong, S.-Y.; Cheng, L.-P.; Yu, Y.-Y. TiO2 Nanoparticles Synthesized in an Aprotic Solvent and Applied to Prepare High-Refractive-Index TiO2-Polyimide Hybrid Thin Films. J. Sol-Gel Sci. Technol. 2014, 71, 129−135. (23) Fujita, M.; Idota, N.; Matsukawa, K.; Sugahara, Y. Preparation of Oleyl Phosphate-Modified TiO2/Poly(methyl methacrylate) Hybrid Thin Films for Investigation of Their Optical Properties. J. Nanomater. 2015, 2015, 297197. (24) Enomoto, K.; Kikuchi, M.; Narumi, A.; Kawaguchi, S. Design of Epoxy/ZrO2 Hybrid Transparent Bulk Materials. Kobunshi Ronbunshu 2015, 72, 82−89. (25) Macdonald, E. K.; Shaver, M. P. Intrinsic high refractive index polymers. Polym. Int. 2015, 64, 6−14. (26) Althues, H.; Henle, J.; Kaskel, S. Functional Inorganic Nanofillers for Transparent Polymers. Chem. Soc. Rev. 2007, 36, 1454−1465. (27) Kango, S.; Kalia, S.; Celli, A.; Njuguna, J.; Habibi, Y.; Kumar, R. Surface Modification of Inorganic Nanoparticles for Development of Organic−Inorganic NanocompositesA Review. Prog. Polym. Sci. 2013, 38, 1232−1261. (28) Huang, X.; Kim, C.; Jiang, P.; Yin, Y.; Li, Z. Influence of aluminum nanoparticle surface treatment on the electrical properties of polyethylene composites. J. Appl. Phys. 2009, 105, 014105. (29) Barbey, R.; Lavanant, L.; Paripovic, D.; Schüwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Polymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications. Chem. Rev. 2009, 109, 5437−5527.

polymerization of monomers is applicable for the preparation of various inorganic−organic hybrids.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10427. Synthesis of CPMP; IR, 1H NMR, and 31P NMR spectra of CPMP; photographs of CPMP_IDP_TiO2 dispersed in toluene, THF, CH2Cl2, CHCl3, and MMA; particle size distributions obtained by DLS measurement of TiO2/ methanol and CPMP_IDP_TiO2/toluene dispersion; IR spectra of CPMP_IDP_TiO2 and PMMA_TiO2_18h (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(Y.S.) E-mail: [email protected]. ORCID

Naokazu Idota: 0000-0001-6913-1787 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (24102002) of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We thank Sakai Chemical Co., Ltd. for donating the TiO2/ methanol dispersion and isodecyl phosphate. REFERENCES

(1) Lü, C.; Yang, B. High Refractive Index Organic−Inorganic Nanocomposites: Design, Synthesis and Application. J. Mater. Chem. 2009, 19, 2884−2901. (2) Yoshida, M.; Prasad, P. N. Sol−Gel-Processed SiO2/TiO2/ Poly(vinylpyrrolidone) Composite Materials for Optical Waveguides. Chem. Mater. 1996, 8, 235−241. (3) Lü, C.; Cui, Z.; Guan, C.; Guan, J.; Yang, B.; Shen, J. Research on Preparation, Structure and Properties of TiO2/Polythiourethane Hybrid Optical Films with High Refractive Index. Macromol. Mater. Eng. 2003, 288, 717−723. (4) Lü, C.; Cui, Z.; Wang, Y.; Li, Z.; Guan, C.; Yang, B.; Shen, J. Preparation and Characterization of ZnS−Polymer Nanocomposite Films with High Refractive Index. J. Mater. Chem. 2003, 13, 2189−2195. (5) Lü, C.; Cui, Z.; Li, Z.; Yang, B.; Shen, J. High Refractive Index Thin Films of ZnS/Polythiourethane Nanocomposites. J. Mater. Chem. 2003, 13, 526−530. (6) Krogman, K. C.; Druffel, T.; Sunkara, M. K. Anti-Reflective Optical Coatings Incorporating Nanoparticles. Nanotechnology 2005, 16, S338− S343. (7) Sangermano, M.; Malucelli, G.; Amerio, E.; Bongiovanni, R.; Priola, A.; Di Gianni, A.; Voit, B.; Rizza, G. Preparation and Characterization of Nanostructured TiO2/Epoxy Polymeric Films. Macromol. Mater. Eng. 2006, 291, 517−523. (8) Lü, C.; Cheng, Y.; Liu, Y.; Liu, F.; Yang, B. A Facile Route to ZnS− Polymer Nanocomposite Optical Materials with High Nanophase G


Research Article

ACS Applied Materials & Interfaces (30) Layek, R. K.; Nandi, A. K. A Review on Synthesis and Properties of Polymer Functionalized Graphene. Polymer 2013, 54, 5087−5103. (31) Ma, H.; Wells, M.; Beebe, T. P.; Chilkoti, A. Surface-Initiated Atom Transfer Radical Polymerization of Oligo(ethylene glycol) Methyl Methacrylate from a Mixed Self-Assembled Monolayer on Gold. Adv. Funct. Mater. 2006, 16, 640−648. (32) Edizer, S.; Avci, D. Synthesis and Photo-Polymerization of an Aryl Diphosphonic Acid-Containing Dimethacrylate for Dental Materials. Des. Monomers Polym. 2010, 13, 337−347. (33) Queffélec, C.; Petit, M.; Janvier, P.; Knight, D. A.; Bujoli, B. Surface Modification Using Phosphonic Acids and Esters. Chem. Rev. 2012, 112, 3777−3807. (34) Guerrero, G.; Alauzun, J. G.; Granier, M.; Laurencin, D.; Mutin, P. H. Phosphonate Coupling Molecules for the Control of Surface/ Interface Properties and the Synthesis of Nanomaterials. Dalton Trans. 2013, 42, 12569−12585. (35) Imai, Y.; Terahara, A.; Hakuta, Y.; Matsui, K.; Hayashi, H.; Ueno, N. Transparent Poly(bisphenol A carbonate)-Based Nanocomposites with High Refractive Index Nanoparticles. Eur. Polym. J. 2009, 45, 630− 638. (36) Murugan, E.; Shanmugam, P. Efficient Functionalization of Poly(styrene) Beads Immobilized Metal Nanoparticle Catalysts for the Reduction of Crystal Violet. Bull. Mater. Sci. 2015, 38, 629−637. (37) Guerrero, G.; Mutin, P. H.; Vioux, A. Anchoring of Phosphonate and Phosphinate Coupling Molecules on Titania Particles. Chem. Mater. 2001, 13, 4367−4373. (38) Alberti, G.; Casciola, M.; Costantino, U.; Vivani, R. Layered and Pillared Metal(IV) Phosphates and Phosphonates. Adv. Mater. 1996, 8, 291−303. (39) Gao, Y.; Gao, X.; Zhou, Y.; Yan, D. Preparation of Poly(methyl methacrylate) Grafted Titanate Nanotubes by In Situ Atom Transfer Radical Polymerization. Nanotechnology 2008, 19, 495604. (40) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: 2003. (41) Djafer, L.; Ayral, A.; Boury, B.; Laine, R. M. Surface Modification of Titania Powder P25 with Phosphate and Phosphonic Acids − Effect on Thermal Stability and Photocatalytic Activity. J. Colloid Interface Sci. 2013, 393, 335−339. (42) Bergman, N. A.; Halvarsson, T. Chemical Stability of a Prostacyclin Analog due to the Absence of Intramolecular Catalysis. J. Org. Chem. 1988, 53, 2548−2552.

H


PMMA Hybrids with Improved

Recommend Documents