Transparent organic–inorganic hybrid thin films prepared from acrylic polymer and aqueous monodispersed colloidal silica

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Transparent organic–inorganic hybrid thin films prepared from

acrylic polymer and aqueous monodispersed colloidal silica

Yang-Yen Yu

a

, Wen-Chang Chen

a,b,∗

a_{Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan} b_{Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan} Received 31 December 2002; received in revised form 17 April 2003; accepted 28 April 2003

Abstract

Highly transparent hybrid films containing nano-sized silica domain were synthesized from acrylic polymer and aqueous monodispersed colloidal silica (CS) with a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA). The silica content in the hybrid thin films was varied from 0 to 50 wt.%. The experimental results showed that the silica particle size in the precursor solution and the hybrid films was varied from 20 to 40 nm. It could be controlled by the mole ratio of MSMA to silica. The results of scanning electron microscope (SEM), transmission electron microscope (TEM), and elemental analysis support the above conclusion. The prepared hybrid films showed high film uniformity and optical transparence. The thermal stability of the prepared hybrid films increased with the increasing silica content. The refractive index decreased linearly with the increasing silica fraction in the hybrid films. The experimental results suggest that the hybrid thin films have potential applications as passive films for optical devices.

Keywords: Aqueous colloidal silica; Acrylic/silica hybrid; Microstructure; Optical properties

1. Introduction

Organic–inorganic hybrid materials have been regarded as a new class of optoelectronic materials[1–3]. They could be applied to various areas of optoelectronics, including pro-tective optical coating[4], high refractive index films[5–8], contact lenses [9], thin film transistor [10], light-emitting diodes[11–13], solar cell[14], optical waveguides materials [15,16], and photochromic materials[17]. The properties of the hybrid materials could be tuned through the functionality or segment size of each component, including thermal, me-chanical, electronic, optical, and optoelectronic properties. The flexibility of material design, synthesis, and properties have stimulated extensive research interests.

The hybrid materials of poly(methyl methacrylate) (PMMA)/inorganic oxide has been widely studied[18–29]. The silica network in the hybrid materials was generally prepared from alkoxysilanes [18–23] or colloidal silica (CS)[24–29]. The approach of the colloidal silica provides the advantage of precise control on the size distribution in the hybrid materials. Ford and his coworkers prepared

∗_{Corresponding author. Tel.:}_{+886-2-23628398;} fax:+886-2-23623040.

E-mail address: [email protected] (W.-C. Chen).

PMMA/silica composites for the application of narrow bandwidth optical filters[24–27]. In these studies, colloidal silica with the size larger than 100 nm was required to diffract the light. However, such hybrid materials were not suitable for highly transparent optical applications. Besides, the thermal stability of PMMA limits the curing tempera-ture of the hybrid materials to be below 100◦C. Hence, the incomplete condensation of Si–OH might exist and affect the optical properties. Hence, nanoscale colloidal silica and highly corsslinked acrylic polymers might be necessary for the preparation of the acrylic/silica hybrid optical films. In a previous study, we have successfully prepared the hybrid film from acrylic polymer/nanoscale colloidal silica from acrylic polymer and solvent based colloidal silica[29]. Al-though the solvent based colloidal silica could be used to prepare hybrid films, the environmental pollution problem and the cost limit their applications. Therefore, the prepara-tion of hybrid films from crosslinkable acrylic polymer and water based colloidal silica-poly(acrylic) hybrid materials is important for their applications.

In this study, poly(acrylic)–silica hybrid thin films were prepared from various acrylic monomers, water based monodispersed colloidal silica and a coupling agent. A mixed solvent of ethylene glycol ethyl ether was used to modify the reaction medium for preventing the phase

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condensation H2C C CH3 CO2 (CH2)3 Si (OCH3)3 CO2 (CH2)3 Si CH3 C H2C (OH)3 CH3 C CO2 H2C (CH2)3 Si OH OH cellosolve, 60˚C, 2hr Si (CH2)3 OH OH OH OH OH OH OH OH O H+,H2O (MSMA) association CO2 H2C C CH3 (SiO2)x (SiO2)x O O CH3OH

-MSMA-SiO

2 H Si Si O (a) + _CH 2 C CO2 CH3 CH3 + CH2 CH CO2 CH2 CC2H5 (MMA) (TMPTA)

MSMA-SiO

2

BPO

spin coating

3

curing

60˚C

( )

Acrylic polymer-silica hybrid film

(b)

Fig. 1. Reaction scheme for preparing (a) colloidal MSMA–SiO2 and (b) acrylic polymer–silica hybrid thin films, ST10-ST62.

separation of the prepared hybrid materials. It is because wa-ter based colloidal silica was used for the inorganic moiety and acrylic polymer is not soluble in water. The example of preparing the acrylic polymer–silica hybrid films is shown inFig. 1. 3-(Trimethoxysilyl)propyl methacrylate (MSMA) was hydrolyzed and reacted onto the silica surface to form the MSMA–SiO2(MS). Then, it was mixed with the acrylic

monomer of methyl methacrylate (MMA) or trimethylol-propane trimethacrylate (TMPTA) and polymerized to form a precursor solution. The precursor solution was then spun coated on the substrate and followed by curing to obtain the hybrid film. The mixture of MMA/TMPTA was used to prevent the fast polymerization if only TMPTA was used for the hybrid materials. The chemical structures, morphology, thermal, mechanical and optical properties of the prepared hybrid thin films were examined. The effects of the MSMA and silica content on the structure and properties of the hybrid thin films were also discussed.

2. Experimental 2.1. Materials

Methyl methacrylate (99%, Aldrich), trimethylolpropane trimethacrylate (90%, Aldrich), 3-(trimethoxysilyl)propyl

methacrylate (MSMA, 98% Aldrich), aqueous colloidal silica (Bayer Taiwan Co., 15 nm, 30 wt.%, pH = 3.8), ethylene glycol ethyl ether (Cellosolve, Acros), and ben-zoyl peroxide (BPO, Acros) were used for the synthesis of hybrid thin films.

2.2. Preparation of colloidal MSMA–SiO2

The scheme for preparing colloidal MSMA–SiO2solution

is shown inFig. 1(a). 3-(Trimethoxysilyl)propyl methacry-late (MSMA), 15 nm aqueous colloidal silica and ethylene glycol ethyl ether (cellosolve, as a solvent) were mixed at the various compositions as shown in Table 1. Then, the reaction mixture was poured into a three-necked reactor to proceed the hydrolysis/condensation reaction. The reaction temperature was maintained at 60◦C and the solution was stirred under a nitrogen flow for 2 h to obtain the colloidal MSMA–SiO2solution.

2.3. Preparation of hybrid acrylic polymer–silica films

The scheme for preparing hybrid acrylic polymer–silica films is shown inFig. 1(b). The precursor solution for prepar-ing the hybrid films was obtained accordprepar-ing to the compo-sition shown in Table 1. The experimental procedures are

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Table 1

Monomer compositions (wt.%) for preparing the hybrid thin films HS0-HS50a _{and ST0-ST62}a

Sample Colloidal silica MSMA MMA TMPTA

HS0 0.00 45.00 55.00 – HS10 10.00 41.00 49.00 – HS20b _20.00 _36.00 _44.00 _– HS33 33.00 30.00 37.00 – HS40 40.00 27.00 33.00 – HS50 50.00 23.00 27.00 – ST0b _20.00 _36.00 _44.00 _0.00 ST10 20.00 33.00 37.00 10.00 ST20 20.00 30.00 30.00 20.00 ST32 20.00 27.00 21.00 32.00 ST42 20.00 24.00 14.00 42.00 ST52 20.00 20.00 8.00 52.00 ST62 20.00 18.00 0.00 62.00

a_{All of polymerization mixtures had the following fixed} composi-tion: [MSMA]/(acrylic monomer + [MSMA]) = 25 mol%. The acrylic monomer is MMA for the case of HS0-HS50 and MMA and TMPTA for the case of ST0-ST62; [BPO]/([MMA] + [MSMA]) = 3.75 mol%.

b_{The compositions of HS20 and ST0 are the same.}

described below. The colloidal MSMA–SiO2 obtained in

the procedure ofSection 2.2was subsequently mixed with a homogeneous cellosolve solution of the acrylate monomer (MMA or MMA/TMPTA) and the initiator, BPO, to proceed the polymerization reaction under nitrogen purging. The re-action temperature for preparing HS0-HS50 was maintained at 60◦C for 3 h. For the case of ST0-ST62, the reaction tem-perature was maintained at 60◦C and the reaction time was ranged from 3 h (ST0) to 80 min (ST62), depending on the TMPTA content. Then, the reaction solution was spin coated on a 0.1524 m silicon wafer for 20 s at the speed of 0.319 m/s. The coated thin film was then cured on a hot plate at 60◦C for 20 min, 80◦C for 30 min and 150◦C for 1.5 h, respectively. For the case of thick films, the coating liquid was concen-trated under room temperature to remove the solvent cello-solve using a rotary vacuum evaporator before spin-coating. The curing process was the same as for that of thin films.

2.4. Characterization

IR spectra of the prepared thin films were obtained on a KBr pellet using a Jasco Model FTIR 410 spectropho-tometer. The 13_{C and}29_{Si NMR spectra of the solid-state}

hybrids were determined (Bruker, DSX-400 WB) with cross-polarization combined with the magic angle spinning (CP/MAS) technique. The measured condition of the 29Si NMR spectra at 79.4 MHz were as follows: 200–300 mg;

1_{H 40}◦_{pulse width: 2.5}_{␮s; spinning frequency: 7 kHz; and}

recycle time: 15 s. The fracture surfaces of hybrid thin films were examined on the Hitachi H-2400, scanning electron microscope (SEM). The particle sizes of the colloidal par-ticle were measured by the Hitachi H-7100 transmission electron microscope (TEM).

The contents of C, H and N in the prepared materials were measured by element analysis using a Neraeus VarioEL-III Element Analyzer. The relative shell thickness of the MSMA and MMA layers on the surface of colloidal silica was esti-mated from the elemental analysis result, as reported previ-ously[29].

Thermal analyses, thermogravimetric analysis (TGA) and differential scanning calormetry (DSC) were performed under a nitrogen flow using a DuPont Model 951 thermo-gravimetric analysis and a DuPont Model 910S differen-tial scanning calorimeter at a heating rate of 20◦C/min and 10◦C/min, respectively. The TGA and DSC samples were prepared by spin-coating the precursor solution on a glass substrate, followed by curing at different temperature steps as described in the film preparation. The thermal– mechanical properties of the prepared films were tested by a TA 2980 dynamic mechanical analysis (DMA) and a TA 2940 thermo mechanical analysis (TMA). The heat-ing rate was 3 and 10◦C/min and the temperature ranged from room temperature to 250◦C for DMA and TMA, respectively.

The transmittance of the prepared films was measured by using the UV-Vis/NIR spectrophotometer Jasco V-570. A n&k analyzer (Model 1200, n&k Technology Inc.) was used to measure the refractive index (n) and the extinction coef-ficient (k) of the prepared films in the wavelength range of 190–900 nm. The thickness (h) of the prepared films was also determined simultaneously. An atomic force micro-scope (Digital Instrument Inc., Model DI 5000 AFM) was used to probe the surface morphology of the coated films. The hardness was measured by using a pencil test.

3. Results and discussion 3.1. Analysis of chemical structure

Fig. 2illustrates the FTIR spectra of (a) colloidal silica, (b) MSMA–SiO2, (c) HS10, (d) HS50 and (e) ST62,

respec-tively. There are two characteristics from the comparison of the spectra. The first comes from the comparison of the Si–OH absorption band in the spectra. The Si–OH of the pure colloidal silica and MSMA/silica is observed at 963 (Fig. 2(a)) and 916 cm−1(Fig. 2(b)), respectively, which are similar to those reported in the literature[20,29]. However, the Si–OH peak is completely disappeared in the spectra of (c) and (e) but shown in the spectrum of (d). This suggests that the complete condensation of the Si–OH bond on the colloidal silica or MSMA–SiO2in the cases of (c) and (e).

Note that the mole ratio of MSMA to silica was 0.98, 0.11, and 0.22 for the case of (c–e), respectively. The FTIR spec-tra of the studied HS and ST hybrids show that the Si–OH residue could be observed if the mole ratio of the MSMA to silica is smaller than 0.22. Thus, the incomplete condensa-tion of the Si–OH bond in the case of (d) is probably resulted from a high silica content and thus the MSMA could not

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4000 3000 2000 1000 (e) (d) (c) (b) (a) 964 916 963 Transmittance( % ) Wavenumber(cm-1)

Fig. 2. FT-IR spectra of (a) pure colloidal silica; (b) MSMA–SiO2; (c) HS10; (d) HS50; and (e) ST62 hybrid polymer films.

react with all of the Si–OH bonds on the silica surface. The complete condensation of the Si–OH bond on the colloidal silica is important in controlling the size of silica particle for the prepared hybrid film. It is because the residual silanols on the surface of colloidal silica particles increase the prob-ability of further particle growth. The second feature in the comparison of the spectra of (b)–(e) is the absorption band of C=C at 1650 cm−1. This band is shown in the spectrum of (b) but disappeared at those of (c)–(e), which suggests the complete polymerization of the acrylic monomers. The ab-sorption bands of C–O–C or Si–O–Si, Si–C, C=O, C–H, and O–H bands are observed at 1040–1121, 1265, 1729–1749, 2951, and 3491–3623 cm−1, respectively. The positions of the absorption bands are similar to those reported in the lit-eratures[19–21,29]. The Si–O–Si band at 1100 cm−1 grad-ually increases its intensity with increasing the silica con-tent, which indicates the successful incorporation of silica content in the hybrid films.

Fig. 3 illustrates13C CP/MAS spectrum of the prepared hybrid films: (a) pure colloidal silica, (b) HS10, (c) HS50, (d) ST42 and (e) ST62. The spectrum of the pure colloidal silica shows a strong adsorption peak at 106 ppm. The band might be due to the carbon atom of the Si–OCH3group, as

suggested in the literature[28,29]. The result indicates that the Si–OCH3residue exists in the pure colloidal silica. For

the case ofFig. 3(b), the carbon atoms on the methyl and

200 100 0 -100 55 7 (e) 52 45 17 42 ppm (a) (b) (c) (d)

Fig. 3.13_{C CPMAS NMR spectra of (a) pure colloidal silica; (b) HS10;} (c) HS50; (d) ST42 and (e) ST62.

methylene groups shown are observed at 9, 17, 22, 45, 52, 55, 67, 106 and 177 ppm, which are similar to those described in our previous study[29]. The absorption peaks at 17, 45, 52 and 55 ppm are assigned to be the poly(methyl methacrylate) segment[23]while those at 9, 22 and 67 ppm are attributed to the MSMA segment [28]. The peak at 106 ppm is as-signed to be the Si–OCH3residue as suggested byFig. 3(a).

The carbon absorption peaks due to the PMMA segment de-creases fromFig. 3(b) and (c)and completely disappeared in the case ofFig. 3(e). For the cases ofFig. 3(d)–(e), the char-acteristic absorption peaks at 7 and 42 ppm are contributed by the trifunctional acrylate (TMPTA) moiety. The absorp-tion band of the C=C bond at 168 ppm is not observed in Fig. 3, which suggests the successful polymerization of the acrylic moiety in the prepared HS and ST hybrid materials. The results indicate that the chemical structure of the pre-pared hybrid thin films consists with the original design.

The chemical shifts from the29Si CP/MAS NMR spectra of the prepared hybrid thin films, HS0-HS50, are shown in Table 2. The six chemical shifts at−49.2 to −50.3, −58.5 to

−59.5, −66.1 to −66.9, −95.1 to −97.8, −100.0 to −103.8,

and−111.2 to −112.2 ppm, are assigned to T1_{, T}2_{, T}3_{, Q}2_,

Q3, and Q4, respectively. The nomenclatures Ti and Qiare taken from the literature[30], where i refers to the number of Si–O–Si groups bonded to the silicon atom of interest.

Tiand Qi denote species that have one and no organic side groups, respectively. The positions of these peaks are similar to those reported in the literatures[23,28]. The proportion of the Ti, Qi, and Dcin the hybrid materials was determined

by a quantitative analysis based on the peak areas of species, as listed inTable 2. The degree of condensation (Dc) of the

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Table 2

Chemical shift and relative proportions of Tiand Qispecies in the prepared hybrids, HS0-HS50 obtained from the29_{Si CP/MAS NMR spectra}

No. Chemical shifts (ppm) Proportion (%) Dc(%)

T1 T2 T3 Q2 Q3 Q4 T1 T2 T3 Q2 Q3 Q4 HS0 −49.8 −58.5 −66.9 – – – 12.94 66.28 20.78 – – – 69.28 HS10 −50.3 −59.1 −66.1 −97.8 −103.8 −112.2 10.48 40.32 20.16 4.74 3.98 20.32 76.21 HS20 −48.4 −59.2 −66.7 −95.8 −100.9 −111.2 5.94 28.63 14.00 2.14 4.32 44.92 84.33 HS33 −49.2 −59.5 −66.9 −95.1 −100.0 −111.5 5.03 18.88 11.79 1.80 4.72 57.78 88.27 HS50 −50.3 −58.7 −67.1 −96.2 −103.1 −111.8 2.49 10.03 8.95 1.31 9.76 67.47 91.91

and Qiaccording toEq. (1) [23]:

Dc(%) = T1_{+ 2T}2_{+ 3T}3 3 + Q1_{+ 2Q}2_{+ 3Q}3_{+ 4Q}4 4 ×100 (1)

The proportion of the Ti species decreases with increasing the silica content of the prepared hybrid films but those of the

Qiand Dcspecies show the opposite trend fromTable 2. The

Q3and Q4depend on the Si–OH condensation of the silica and thus they increase with increasing the silica content. Similar explanation can be used for the cases of Tiand Dc.

3.2. Microstructure analysis

Fig. 4 illustrates the TEM diagram of the colloidal MSMA/silica solution for the case of HS10. The image of TEM shows that the particle size of colloidal silica after hydrolysis/condensation reaction, MSMA–SiO2, is about

Fig. 4. TEM diagram of colloidal MSMA–SiO2 for HS10 (silica size: 15–20 nm).

15–20 nm, which is approximately similar to the original size (15 nm) of the pure colloidal silica solution. Similar particle size of the MSMA–SiO2was obtained for the cases

of the HS20 and HS33. The result suggests no further growth of silica particles if the mole ratio of MSMA to colloidal silica is higher than 0.22. However, the coagula-tion of silica particles was observed for the case of HS40 and HS50, which had the particle size of 25–30 nm. Note that the mole ratios of MSMA/silica were 0.16 and 0.11 for HS40 and HS50, respectively. It might not be enough for the encapsulation of the Si–OH on the silica surface as described in the FTIR results and thus the growth of silica particles by the Si–OH residue was possible. The result indicates that the particle size of silica in the hybrid materi-als could be effectively controlled for the optimum ratio of MSMA to colloidal silica.

For the cases of the HS10-HS33 and HS40-HS50 hybrid films, the silica domain estimated the SEM results were 20–25 and 30–40 nm, respectively. They could be related to

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Table 3

Element analysis of pure silica and the prepared hybrid materials, HS10-HS50, and their corresponding layer thicknesses determined from the carbon content of elemental analysis

Sample C (%) H (%) N (%) hMSMAa (nm) hMMAa(nm)

SiO2 1.80 1.55 0.05 – – HS10 46.64 6.42 0.03 7.53 3.89 HS20 39.37 5.64 0.01 4.38 2.65 HS33 31.15 4.34 0.01 2.58 1.98 HS40 27.09 3.70 0.01 1.99 1.69 HS50 20.92 3.07 0.02 1.33 1.32 a_h

MSMA and hMMA are the thickness of the MSMA and MMA layers on the silica surface estimated from the element analysis results.

the size of MSMA–SiO2. As suggested by the TEM result,

the large MSMA–SiO2 size was observed for the cases of

HS40 and HS50 due to the incomplete coverage of silica particle surface. This explains the large silica domain size of HS40 and HS50. The silica domain size of the ST0–ST62 films are 20–25 nm because the mole ratio of the MSMA to silica is larger than 0.22.

The shell thicknesses of the MSMA and MMA layers on the silica surface of the prepared hybrid films, HS0-HS50, are shown in Table 3. They were estimated from the ele-mental analysis result. The average shell thickness decreases from 7.53 to 1.33 nm for the MSMA layer and 3.89–1.32 nm for the MMA layer, with the increasing silica content, re-spectively. The thin shell thickness of the HS40 and HS50 supports the insufficient coverage of MSMA on silica parti-cle suggested from the results of FTIR, TEM and SEM.

The film thickness and surface roughness of the prepared films are listed in Table 4. The film thickness (h), average roughness (Ra), and mean square roughness (Rq) of HS10

Table 4

Properties of the prepared materials

HS0 HS10 HS20 HS33 HS40 HS50 ST0 ST10 ST20 ST32 ST42 ST52 ST62 Td (◦C) 279 292 293 303 309 314 292 344 347 350 356 358 360 900◦C residuea _17.42 _29.59 _36.02 _49.18 _52.73 _63.22 _36.01 _37.09 _32.97 _35.93 _35.94 _36.24 _37.11 900◦C residueb _9.67 _18.69 _27.73 _39.47 _45.79 _54.83 _27.68 _27.04 _26.40 _25.76 _25.12 _24.27 _23.84 240◦C weight loss (%)c _9.03 _5.21 _4.19 _3.92 _3.62 _3.53 _4.19 _2.05 _1.90 _2.00 _1.61 _1.59 _1.42 260◦C weight loss (%)c _14.77 _8.51 _6.49 _6.24 _5.99 _4.91 _6.49 _3.66 _2.98 _2.95 _2.55 _2.44 _2.07 hd _(Å) ₁₇₉₀ ₁₈₇₀ ₂₄₅₀ ₃₀₅₀ ₂₃₈₀ ₂₃₂₀ ₁₇₉₀ ₁₈₂₀ ₁₈₅₀ ₁₉₀₀ ₂₀₀₀ ₂₀₂₀ ₂₂₆₀ he _(Å) ₁₂₀₄₀ ₂₂₉₆₀ ₅₁₁₈₀ ₁₅₈₅₀ ₃₄₅₈₀ ₁₁₂₇₀ ₂₁₅₁₀ ₂₃₄₀₀ ₂₄₅₁₀ ₂₄₀₀₀ ₂₅₀₁₀ ₂₆₅₇₀ ₃₀₀₁₀ Raf (Å) 2.5 14.3 28.4 31.9 33.9 33.9 28.4 27.4 35.2 27.9 29.2 30.3 33.1 Rqf (Å) 3.1 18.4 35.1 37.5 42.9 43.7 36.1 35.7 36.8 36.5 36.0 40.4 42.1 n633nmg 1.495 1.482 1.475 1.473 1.457 1.440 1.475 1.495 1.502 1.510 1.515 1.543 1.548 Hardnessh _HB _5H _5H _5H _5H _5H _5H _5H _5H _5H _5H _5H _5H Hardnessi 3H 9H 9H 9H 9H 9H 9H 9H 9H 9H 9H 9H 9H

a_{Experimental results from TGA.}

b_{Theoretical values based on the assumption that only inorganic moieties are present at 900}◦_C. c_{Weight loss from TGA at 240 and 260}◦_{C isothermal for 1 h.}

d_{Thickness of the prepared thin film.}

e_{Thickness of the prepared thick films by concentrating solution.} f_R

a and Rq are the average and root mean square roughness of the prepared thin films, respectively. g_{n is the refractive index of the prepared thin films.}

h_{Hardness of the prepared thin film.} i_{Hardness of the prepared thick film.}

are 1870, 14.3, and 18.4 Å, respectively. The comparison of surface roughness with the film thickness is less than 2.0%, which suggests the excellent surface planarity of the prepared hybrid films. Another trend of surface roughness shown in Table 4, it increases with the silica content due to the growing particle size at a high silica content. For the practical application, film thickness with a few micrometers might be necessary. As shown in theTable 4, film thickness of the prepared hybrid films as thick as 5␮m could be ob-tained. This indicates the potential applications of the pre-pared hybrid films. There is no direct correlation between the film thicknesses and the silica content because it depends on the spin speed, solid content, and viscosity of the coating solution. However, the film thicknesses of the hybrid films with a high silica contents, HS40-HS50, are lower than oth-ers. It is probably due to the low acrylic monomer content or low degree of acrylic polymerization on the silica surface.

3.3. Thermal analysis

Fig. 5 shows the TGA curves of PMMA, ST0 (HS20), ST10 and ST62. The order of the thermal decomposition temperature (Td) is ST62 > ST10 (HS20) > ST0 > PMMA.

The thermal decomposition temperature (Td) of the prepared

hybrid materials is shown in Table 4. The Td of the HS

and ST hybrid materials is in the range of 279–314◦C and 292–360◦C, respectively. It increases with increasing silica content or the corsslinking moiety. The thermal stability of the prepared hybrid films are better than the regular used optical polymer, PMMA. The residue at 900◦C increases with increasing silica content, which suggests the successful incorporation of the silica moiety into the hybrid materials. The higher experimental residue than the theoretical value

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0 200 400 600 800 0 20 40 60 80 100 ST62 ST10 ST0 PMMA Weig h t Fraction(% ) Temperature (oC)

Fig. 5. TGA curves of PMMA, ST0, ST10 and ST62 at a heating rate of 20◦C/min under nitrogen flow.

is probably due to the trapping of the polymer moiety in the silica. The black color of the polymer residues after the TGA runs also provides the evidence of the organic moiety has been trapped in the silica matrix. The low weight loss of the prepared hybrid films at 240 and 260◦C for 1 h suggests the films could have potential applications as a high temperature coating layer. The DSC analysis was performed on the PMMA and the prepared hybrid materials. Only the PMMA shows a Tg at 125◦C among the three

studied materials. The Tgdid not exist in all of the prepared

hybrid materials. It suggests the enhancement of thermal

100 200 300 400 500 600 700 800 900 1000 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 HS0~HS50 k n refract ive index, n Wavelength(nm) 0.0 0.2 0.4 0.6 0.8 1.0 extinction coefficient, k

Fig. 6. Variation of refractive index (n) and extinction coefficient (k) of the hybrid films HS0-HS50 in the wavelength range of 190–900 nm.

stability with incorporating the silica moiety. The thermal transition of the prepared hybrid materials was also studied by DMA and TMA and none of them showed a Tg in the

studied materials.

3.4. Hardness analysis

The hardness of the prepared hybrid thin films was mea-sured by a pencil test, as shown in Table 4. The hardness of the HS0 hybrid film is HB and 3H for the cases of thin film and thick film, respectively. It increases to 5H (for the

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thin film case) or 9H (for the thick film case) with increas-ing the silica content for the cases of the HS and ST hybrid materials. It suggests that the enhancing of hardness by in-corporating the silica moiety in the acrylic polymers.

3.5. Optical properties

Fig. 6 illustrates the dispersion of the refractive index (n) and extinction coefficient (k) of HS0-HS50 in the wave-length range of 190–900 nm. The refractive index (n) of the prepared hybrid thin films is listed inTable 4. As shown in Table 4, the n at 633 nm decreases from 1.495 of HS0 to 1.440 of HS50. It is because the smaller refractive index of pure silica than that of the acrylic polymer. The n linearly decreases with increasing the silica content, which are simi-lar to the previous report[29]. The result suggests that the n of the prepared HS hybrid thin films could be tuned through the silica content. On the other hand, the n of the ST hy-brid materials increases with increasing the TMTPA content from 1.475 (ST0) to 1.548 (ST62) as shown inTable 4. It is because the higher refractive index of the TMTPA than that of the silica. The extinction coefficients (k) of the films of the HS0-HS50 are almost zero in the wavelength range of 190–900 nm, as shown in Fig. 6. The result suggests that the prepared hybrid thin films have an excellent opti-cal transparency in the UV and visible region. According to the Rayleigh equation, the silica particle with a larger size (>50 nm) results in a serious light scattering. The particle size of the prepared hybrid films is in the range of 20–40 nm. Therefore, significant scattering loss is avoided. This ex-plains the results of optical transparence shown inFig. 6.

4. Conclusions

Acrylic/silica hybrid thin films containing nano-size sil-ica were successfully prepared from acrylic polymer and aqueous monodispersed colloidal silica with coupling agent. The experimental results showed the silica domain in the hybrid film was varied from 20 to 40 nm through the mole ratio of MSMA to colloidal silica. The prepared hybrid films from the crosslinked acrylic polymer moiety showed excel-lent surface planarity, good thermal stability, and hardness in comparison with the PMMA/silica hybrid films. The pre-pared hybrid films showed a tunable refractive index with the silica fraction in the films. Excellent optical transparence was obtained in the prepared hybrid films. These results show that such hybrid thin films have potential applications as passive films for optical devices.

Acknowledgements

We thank the National Science Council and Industrial Development Bureau of Taiwan for the financial support of this work.

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