Transparent photo-curable co-polyacrylate/silica nanocomposites prepared by sol-gel process

Nanocomposites Prepared by Sol-Gel Process

Yen-Chun Chou, Yu-Young Wang, T.-E. Hsieh

Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu 30010, Taiwan, Republic of China

Received 7 July 2005; accepted 3 November 2006 DOI 10.1002/app.26228

Published online 3 May 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: This work prepared the highly transparent

photo-curable co-polyacrylate/silica nanocomposites by using sol-gel process. The FTIR and13C NMR analyses indi-cated that during the sol-gel process, the hybrid precursors transform into composites containing nanometer-scale silica particles and crosslinked esters/anhydrides. Trans-mission electron microscopy (TEM) revealed that the silica particles within the average size of 11.5 nm uniformly dis-tributed in the nanocomposite specimen containing about 10 wt % of Si. The nanocomposite specimens exhibited sat-isfactory thermal stability that they had 5% weight loss decomposition temperatures higher than 1508C and coeffi-cient of thermal expansion (CTE) less than 35 ppm/8C. Analysis via derivative thermogravimetry (DTG) indicated that the crosslinked esters/anhydrides might influence the thermal stability of nanocomposite samples. The UV-visi-ble spectroscopy indicated that the nanocomposite resins

possess transmittance higher than 80% in visible light region. Permeability test revealed a higher moisture per-meation resistance for nanocomposite samples, which indicated that the implantation of nano-scale silica par-ticles in polymer matrix forms effective barrier to moisture penetration. Adhesion test of nanocomposite samples on glass substrate showed at least twofold improvement of adhesion strength compared with oligomer. This evi-denced that the silica and the hydrophilic segments in nanocomposite resins might form interchains hydrogen bonds with the

OH groups on the surface of glass so the substantial enhancement of adhesion strength could be achieved. Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 105: 2073–2082, 2007

Key words: nanocomposites; sol-gel process; silica; photo-curable; OLED encapsulation

INTRODUCTION

Because of the advantages such as low power con-sumption, high efficiency, wide viewing angle, short response time, and compact/lightweight nature of devices, organic light-emitting devices (OLEDs) have become one of the major flat-panel display technolo-gies in recent years. However, the realization of OLEDs must overcome several difficulties such as the lifetime of light-emitting materials,1 device struc-tures,2and the need of encapsulation with extremely low H2O/O2 permeability.3,4 To achieve a reliable

OLED packaging by direct encapsulation, the resin with low curing temperature, short curing time, and low moisture/oxygen permeability is required. Photo-curable nanocomposites hence become one of feasible materials for such an application.

Organic–inorganic composites based on polymers have been widely investigated and they have many practical applications due to their unique physical and

chemical properties. For examples, adding the nano-sized inorganic components into polymer matrices could improve the mechanical properties5–8and per-meability9,10of polymers. One of the processing meth-ods to add the inorganic fillers into polymers, the in situ sol-gel process, has been widely studied due to its high chemical uniformity, high purity, and good reproducibility. Starting from a homogeneous solu-tion, sol-gel process provides better control over the microstructure and size of the inorganic fillers and has been adopted to prepare nano-particles11,12 and organically modified silica13,14 in polymers. For instance, Landry et al. had systematically reported the morphology, dynamic mechanical properties, and chemical analyses of PMMA/silica nanocomposite prepared by in situ sol-gel process.15,16However, most nanocomposites prepared via sol-gel process were thermally cured; relatively few reports were related to the curable nanocomposites. Recently, photo-curable nanocomposite polymers have attracted a lot of research interests due to their versatile applica-tions.17,18

This work reported the preparation and characteri-zation of photo-curable co-polyacrylate composite res-ins containing nano-scale silica particles via sol-gel process. In contrast to the conventional sol-gel process Correspondence to: T.-E. Hsieh ([email protected]).

Contract grant sponsor: Ministry of Education of the Republic of China; contract grant number: 91-E-FA04-2-4. Journal of Applied Polymer Science, Vol. 105, 2073–2082 (2007)

V

utilizing liquid water, ambient moisture was adopted in this work to hydrolyze precursors so as to confine the sizes of silica particles to nanometer scale and to avoid water molecules residing in composites. To comprehend the influences of moisture on other two ingredients in sol-gel reaction, the effects of acrylic acid/tetraethyl orthosilicate (TEOS) ratio on material properties were investigated in detail. Interactive rela-tionships between TEOS/acrylic acid/moisture were explored to interpret the experimental results. Nano-composite resins developed in this work were also applied to the direct encapsulation of OLEDs.19 Related study indicated that higher adhesion strength and lower moisture permeability provided by the nanocomposite resins contribute to the better device performance and lifetime of encapsulated OLEDs.

EXPERIMENTAL Materials

The oligomer, bisphenol A epoxy diacrylate (viscosity at 258C ¼ 27.1M g/(cm-s), Mw ¼ 484.18), was

pur-chased from Sartomer Co. (Exton, PA) and its chemi-cal structure is shown in Figure 1. The radichemi-cal photo-polymerization initiator, 1-hydroxycyclohexylphenyl ketone, was obtained from Chembridge International Co. (Taipei, Taiwan) Acrylic acid, tetraethyl orthosili-cate (TEOS), anhydrous ethanol, isopropyl alcohol, and methanol to wash away the contaminants on glass substrates were purchased from Aldrich Chemicals (St. Louis, MO).

Synthesis of photo-curable polyacrylate/ silica nanocomposites

Oligomer resins and TEOS were first mixed in a three-necked flask heated at 808C. Appropriate amount of acrylic acid was then added dropwise into the oligomer/TEOS mixture. Instead of adding liquid water, the mixture was exposed to the atmospheric ambient at the relative humidity about 60% to acquire the moisture in ambient to hydrolyze the TEOS during the addition of acrylic acid. Previous process lasted about 2 h; a relatively small amount of water mole-cules entered into the system so that nano-scale silica particles could form in the resin samples. After that, the whole mixture was isolated from the ambient and continuously stirred for 24 h at 808C to complete the sol-gel process. During the reaction, acrylic acid not only provided an acidic environment for the in situ

sol-gel reaction, but also served as a reactive monomer for subsequent radical photo-polymerization. After the 24-h stirring, about 5.0 wt % photo-initiators were added into the mixtures and another 2-h stirring was carried to complete the preparation of nanocomposite resin samples. The content of photo-initiator in each of the samples was calculated as follows: (weight of ini-tiators) 7 [(weight of oligomer) þ (weight of acrylic acid)] 100%. The thin-film specimens were prepared by spin-coating the composite resins onto 76  26  1.3 mm3_{glass substrates followed by curing in an}

UV oven (CL-1000, UVP) in which the UV irradiation comes from an array of 8 W dual bipin discharge tubes emitting UV light in the wavelengths ranging from 350 to 400 nm (peak wavelength at about 365 to 370 nm) to induce the photo-polymerization. After UV curing, the films were all transparent and then post-cured at 808C for 60 min.

Characterizations IR and13C NMR analyses

Chemical structures of oligomer, acrylic acid, and resin samples were characterized by FTIR and NMR. The 13C NMR spectra were obtained from a Varian UnityInova 500 NMR and CDCl3was used as the

sam-ple solvent. The FTIR spectra were obtained from a Nicolet Prote´ge´ 460 IR spectrometer.

Thermal analysis

DuPont 2950 thermogravimetric analyzer (TGA) was used to analyze the thermal properties and inorganic contents of composite samples. For the identifications of residual weight of inorganic content and 5.0% weight decomposition temperature, the samples were heated at a rate of 208C/min from 30 to 9008C and then isothermally soaked for 5 min in air and N2

atmospheres, respectively. The TGA data were also transformed to the derivative thermogravimetry (DTG) curves. In-plane thermomechanical analysis (TMA) was carried out in a DuPont 2940 TMA with micro-expansion probe at a heating rate of 58C/min from 30 to 1308C to identify the coefficient of thermal expansion (CTE).

Microstructure analysis

The transmission electron microscopy (TEM) sample was prepared by spin-coating the composite resins onto KBr disc substrate. After the completion of cur-Figure 1 Chemical structure of bisphenol A epoxy diacrylate.

ing, the KBr was then dissolved away in an acetone/ DI water solution with the assistance of ultrasonic vibration to obtain the nanocomposite thin films. The thin films about 100 nm in thickness were then mounted on the carbon-coated copper (Cu) mesh and sent to a JEOL 2000FX TEM operating at 200 kV for microstructure characterization.

Other properties

The transmittance of nano-composite films was char-acterized by Hewlett–Packard UV-Visible 8453 spec-trometer with scanning wavelengths ranging from 190 to 1200 nm. The moisture permeability was measured at 408C and 90% RH using a permeation detection ap-paratus (PERMATRAN-W 3/60, MOCON) and the di-mension of resin specimens is about 80 80  0.1 mm3. The size distributions of silica in composite resins were evaluated by a Honeywell UPA150 particle ana-lyzer. Adhesion strengths of resin samples on glass substrates were measured in accordance with ASTM D-3528 standard.

RESULTS AND DISCUSSION

Synthesis of composite resins via sol-gel process and subsequent radical photo-polymerization

To characterize the influence of the moisture amount in sol-gel reaction, we defined the weight ratio of acrylic acid to TEOS as the F factor. Table I presents the composition of samples prepared in this work.

Figure 2(a) shows the FTIR spectra of samples contain-ing different weight percentages of Si in nanocompo-site resins with F factor¼ 1 and Figure 2(b) shows the spectra difference of neighboring spectra in Figure 2(a). The

CH2peak at about 2950 cm
1was assigned

as the reference peak for comparison and differences higher than that were identified as the chemical varia-tions. In Figure 2(a), the presence of

Si

O

Si

peak near 1090 cm
1 represents that the silica was indeed derived from TEOS via sol-gel reaction as reported by Landry et al.15 Further, the spectra broaden as the content of Si increases from 5 to 10 wt %. However, as shown in Figure 2(b), there is no appa-rent broadening of

Si

O

Si

peak when Si con-tent exceeds 20 wt % and similar phenomenon was also observed in the

Si

O

C

stretching band between 798 to 793 cm
1.20,21It was also observed that TABLE I

Samples Prepared in this Work and Their Composition

Sample designation Sia(wt %) F factorb Oligomer OA1Si5c 5.0 (19.81)d 1.0 (57.26/19.81)e OA1Si10c 10.0 (22.35)d 1.0 (64.69/22.35)e OA1Si20c 20.0 (23.80)d 1.0 (69.18/23.80)e OA2Si10c 10.0 (19.80)d 2.0 (57.26/19.80)e OA4Si10c 10.0 (12.59)d 4.0 (72.82/12.59)e OA0.25Si10c 10.0 (43.45)d 0.25 (31.40/43.45)e OA0.25Si20c 20.0 (49.70)d 0.25 (35.92/49.70)e a_{The weight percentage of Si added per 100g of}

oligomer is measured according to molecular weight of Si molecular weight of TEOS

weight of TEOS

weight of oligomers 100%.

b

The ratio of acrylic acid to TEOS is defined as the F factor¼weight of acrylic acid

weight of TEOS .

c

‘‘O’’, ‘‘Si,’’ and ‘‘A,’’ respectively, represents the oligomer resin, TEOS, and acrylic acid. The subscript of Si represents the Si content while the subscript of A represents the value of F factor.

d

The molar fraction of TEOS within oligomer/acrylic acid/TEOS mixture.

e_{The molar fraction of acrylic acid/TEOS within previous}

mixture.

Figure 2 (a) FTIR spectra of the nanocomposite resin sam-ples containing different Si wt % with fixed F factor (¼ 1); (b) difference of neighboring spectra shown in (a). The signs ‘‘þ’’/‘‘
’’ represent the increase/decrease of absorbance of each functional group deduced from subtraction.

although the calculated molar fraction of acrylic acid increases from 57.26% to 69.18% with the increase of Si content, the broad hydroxyl group peak in alcohol, acid and

Si

OH20,21in between 3750 to 3300 cm
1, gradually shrinks with the increase of Si content. These results evidence that the entire alkoxide groups of TEOS were not completely hydrolyzed into

OH groups and the silica formation was suppressed dur-ing the polycondensation stage. It also indicates that the organic molecular species containing

OH groups might undergo some other chemical reactions during sol-gel reaction.

Figure 3(a) shows the FTIR spectra of nanocompo-site resins containing constant 10 wt % Si with F factor ¼ 0.25, 1, 2, and 2, respectively, and Figure 3(b) shows the differential of neighboring spectra deduced from Figure 3(a). It was found that the sample with F ¼ 4 exhibits an obvious decrease of

Si

O

Si

peak height in comparison with the spectra of the other three samples. Furthermore, no obvious

OH

vibra-tion band was observed in the spectra with F factors  2. However, an apparent positive variation of the shoulders among

C(¼¼O)

peak between 1730 to 1710 cm
1, generally caused by the existence of hydro-gen bonds,15 appeared in the differential spectra of OA2Si10

OA1Si10in comparison with that of OA1Si

10-OA0.25Si10. As indicated by Figure 3(b), the sample

with F ¼ 0.25 comprises a larger

Si

O

Si

(OA0.25Si10  OA2Si10 > OA1Si10  OA4Si10) and a

smaller

OH signals (OA4Si10 > OA2Si10 > OA1Si10

> OA0.25Si10). In typical silica sol-gel process, acid

usually serves as the catalysts that hydrolyze the alk-oxide to hydroxyl groups and the number of

Si

OH groups with respect to entire

Si

OEt groups was interrelated to subsequent polycondensa-tion process. Hence, appropriate amount of water has to be added in proportion to the number of

Si

OEt groups to complete the hydrolysis. When the amount of water in the sol-gel process was limited, the suppression of

Si

O

Si

bonds was expected.

Figure 3 (a) FTIR spectra of the composite resin samples containing 10.0 wt % Si with different F factors; (b) differ-ence of neighboring spectra shown in (a).

Figure 4 13C NMR spectra of composite resin samples con-taining different Si wt % with fixed F factor (¼ 1).

Typically, the

OH signals in the FTIR spectra could be linked with the molar fraction (mf %) of acrylic acid/oligomer in the samples. Although OA1Si10

con-tains 64.69 mf % of acrylic acid and 12.96 mf % of oligomer (calculated values), there is no clear informa-tion in whole OA1Si10spectra indicating the existence

of

OH groups; hence the acrylic acid not only played as a catalyst, but also reacted with other ingre-dients at the condition of F factor  1 in the sol-gel process carried out in this study.

The13C NMR spectra of nanocomposite resins con-taining different weight percentages of Si with fixed F factor (¼ 1) are shown in Figure 4, while the NMR spectra of nanocomposite resins containing 10 wt % Si with different F factors are given in Figure 5. In all spectra, the gradual decline of peaks corresponding to oligomer evidences the decrease of molar fraction of oligomer in nanocomposite samples. In addition, there are new peaks observed in the chemical shift ranging from d ¼ 10 to 65 ppm for all nanocomposite resin samples in comparison with the spectra of oligomer

and acrylic acid. The new peaks localized at 17.6/57.0

ppm are originated from the two singlets

O

C

C*<sub>/

O

C</sub>*

_{C of ethanol,}22 _{and those at}

13.5/58.5 ppm are associated with the

Si

O

C

C*<sub>/

Si

O

C</sub>*

_{C among ethoxy}

groups of TEOS. The

Si

O

C/

Si

O

C

C

peaks correspond to the partial hydrolysis of TEOS inferred from previous FTIR analysis.

A comparison of chemical shifts for

O

C*

C groups in ethanol and

C(¼¼O)

groups in acrylic acid are listed in Table II. The standard position of CDCl3is at 77.36 ppm while in our sample, it appears

at 77.01 ppm. The chemical shift differences for spe-cific carbon larger than 6 0.35 were hence attributed to the formation of new bonds. In Table II, the differ-ences of chemical shift for

O

C*

C exceeds 6 0.35 were observed only in OA1Si5and OA0.25Si10and that

for

C(¼¼O)

was found in all resin samples. Gener-ally, a downfield shift of C-1 next to a hydroxyl group indicated that the carbon within an electron-with-drawn group such as

C(¼¼O)

is bonded to oxygen in

O

C

groups; moreover, the upfield shift of

C(¼¼O)

indicated that an alkoxide or ester group is substituted for a hydroxyl group originally bonded to

C(¼¼O)

groups. Therefore, an esterification induced by acrylic acid occurred in sample of F¼ 0.25 along with the sol-gel reaction. Another component necessary for esterification is the ethanol derived from sol-gel process, and new esters, such as ethyl acrylate, were hence generated. Since the upfield shift of

C(¼¼O)

was observed in all samples, it was specu-lated that the new anhydrides, such as acrylic anhy-dride, form via condensation of acrylic acid in all sam-ples.

During the sample preparation, we noted that the mixture became muddy when TEOS was added into oligomer. After adding acrylic acid dropwise into the oligomer/TEOS mixture and stirring for a certain pe-riod, the mixture turned into transparent. Because water for sol-gel process was acquired from the ambi-ent, four

Si(OEt) groups in single TEOS molecule would be hydrolyzed by H2O molecules randomly

Figure 5 13_{C NMR spectra of the composite resin samples}

containing 10.0 wt % Si with different F factors.

TABLE II

Variation of 13C NMR Spectra Shown in Figures 3 and 4

Functional groups Chemical shift (ppm) HO

C*

C In free EtOH 57.0 In OA1Si5/10/20 þ0.8/þ0.1/0 In OA0.25/1/2/4Si10 þ0.7/þ0.1/þ0.2/þ0.3

C¼¼O

In free acrylic acid 171.4 In OA1Si5/10/20
2.2/
2.6/
2.4

In OA0.25/1/2/4Si10
2.4/
2.6/
1.7/
1.0

‘‘þ’’ represents downfield shift and ‘‘
’’ represents upfield shift.

adsorbed on the ambient/mixture interface. The exis-tence of Si(OH)x(OEt)4
x transformed from TEOS was

hence expected. Owing to the formation of hydrogen bonds between Si(OH)x(OEt)4
x/acrylic acids/oligomer

during sol-gel process, the degree of phase separation

between organic component and Si(OH)x(OEt)4
x

grad-ually reduced. Subsequently, Si(OH)x(OEt)4
x

mole-cules in the mixture polycondensed each other and formed silica particles cladding in

Si(OEt). The NMR was utilized to analyze the reactions stated above and the

Si

O

Si

structure derived by incomplete hydrolysis process. The 29Si-NMR spectrum shown in Figure 6 evidenced the formation of

Si

O

Si

structure in OA0.25Si10. Referring to the

Si

O

Si

bonding status from 29Si-NMR spectrum reported in previous study,23 there is no free TEOS in OA0.25Si10.

The signal peaks at
83.5 and
85.6 ppm correspond to the

(SiO)Si(OEt)(OH)2 and

(SiO)Si(OEt)2(OH)

structures, respectively; the

(SiO)2Si(OH)2 and

(SiO)2Si(OEt)(OH) were identified by the peaks at

90.1 and
91.2 ppm while the peak at
97.5 ppm was recognized as the formation of

(SiO)2Si(OEt)2. These

results not only evidenced that the

Si

O

Si

structure in OA0.25Si10 was capped by organic

func-tional groups, but also revealed the effects of moisture on the polycondensation of TEOS during sol-gel pro-cess. Although the consumption of

OH groups dur-ing the occurrence of polycondensation/esterification/ Figure 6 29Si-NMR spectrum of OA0.25Si10.

condensation reduced the formation of hydrogen bonds, the degree of phase separation between organic component/silica particles did not occur due to the cap-ping of

Si(OEt). Figure 7 schematically illustrates the microstructure of organic/inorganic phases in the nanocomposite resins.

During the sol-gel reaction with relatively small amount of water molecules, the nanocomposite resin comprising of acrylate functional groups was subse-quently photo-polymerized when exposed to UV irra-diation. It is known that the C¼¼C double bonds among acrylate groups of oligomer/monomer will be broken by the radicals derived from the breakage of photo-initiators during UV exposure24 and subse-quently oligomer and monomer crosslink each other to complete the polymerization. Figure 8 shows the FTIR spectra of OA0.25Si10before and after

photo-poly-merization. The2HC¼¼C

stretching band of acrylate

groups were identified around 1630 cm
1. The reduc-tion of 2HC¼¼C

peak in Figure 8 was clearly

observed and the conversion ratio of 2HC¼¼C

in

OA0.25Si10 calculated from the following formula was

approximately equal to 41% while that of oligomer was about 78%. These results evidenced the occur-rence of photo-polymerization during UV irradiation; however, the relatively low ratio was attributed to the intercalation of silica transformed from TEOS.25

Conversion ratioð%Þ

¼ 1
FTIR absorbance of2HC¼¼Cafter UVcuring FTIR absorbance of2HC¼¼C before UV curing

 

100: Microstructure of nanocomposite thin films

The silica particle size distribution of OA0.25Si10 and

TEM micrographs of OA0.25Si10 and OA0.25Si20

nano-composite thin films are shown in Figure 9(a–c). The diameters of silica in OA0.25Si10are in the range from

9.9 to 12.8 nm (99%) with average value equal to 11.5 nm, as indicated by the result of particle size

Figure 8 FTIR spectra of OA0.25Si10: (a) before and (b) after

photo-polymerization.

Figure 9 (a) Silica particle size distribution of OA0.25Si10

and (b) corresponding TEM micrograph. (c) TEM micro-graph of OA0.25Si20 with selected area electron diffraction

analysis shown in Figure 9(a). The TEM observation for OA0.25Si10not only confirmed the result of particle

size analysis, it also revealed a uniform dispersion of silica particles in polymer matrix. In contrast to OA0.25Si10, the size of silica particles in OA0.25Si20

exhibits a wider distribution ranging from 20 to 100 nm although the distribution of silica particles remains uniform. Previous studies pointed out that the diameter of silica domain catalyzed by acid is much smaller than that catalyzed by base in sol-gel process,16 and the intermolecular hydrogen bonds might enhance the compatibility between organic– inorganic phases.26–29 With the NMR/FTIR analyses mentioned above, the clad

Si(OEt) resulted from partial hydrolysis of TEOS apparently benefited the size reduction and enhance the dispersion of silica particles in polymer matrix although hydroxyl groups might cap part of the silica particles. The conforma-tional hindrance from molecular chains also restricted the growth of silica. Furthermore, the excess Si(OH)x(OEt)4
x in OA0.25Si20 not only increased the

number of nuclei for particle formation but also pro-moted the growth of silica particles. A wider distribu-tion of nano-sized silica in OA0.25Si20 consequently

obtained.

Thermal properties

Experimental results of TGA, DTG, TMA, and DMA analyses of oligomer, OA0.25Si10 and OA0.25Si20 are

shown in Table III and Figure 10, respectively. The 5.0% weight loss decomposition temperature (Td
5%)

of oligomer is about 367 8C in N2and 342 8C in air

am-bient while the 50% weight loss decomposition tem-perature (Td
50%) of oligomer reaches 4688C in N2and

4588C in air ambient. After the formation of nanocom-posite resins, Td
5% of samples decreases both in N2

and air ambient with the increase of Si content and vice versa for Td
50%. As to the residual weight

analy-sis, good consistence was observed in OA0.25Si10.

However, in OA0.25Si20, the residual weight was only

15.72 wt %, which is lower than the theoretical value. As revealed by above analysis, this is resulted from the partial hydrolysis of TEOS by moisture in the am-bient so that the

Si(OEt) did not completely trans-form into

Si(OH) during sol-gel process to form the

silica particles. Incomplete hydrolysis of TEOS might affect the thermal stability of OA0.25Si20. As to the

sam-ples subjected to heat during TGA test, thermal prop-TABLE III

Thermal Properties of Oligomer and Nanocomposite Resins Td
5%(8C) Td
50%(8C) _Residual weight (wt %) In N2 In air In N2 In air Oligomer 367 342 468 459 – OA0.25Si10 180 187 507 574 11.12 OA0.25Si20 154 158 497 589 15.72

Figure 10 (a) DTG curves, (b) coefficient of thermal expansion (CTE) and (c) dissipation factors (tand) of (———) oligomer, (

) OA0.25Si10, and (- - -) OA0.25Si20deduced from DMA

analy-sis. The dot lines in (a) represents the baseline in each curve and the numerals in (c) represent the glass transition temperature (Tg).

erty change of nanocomposite films in comparison with that of oligomer could be more clearly seen when TGA data were transformed into the DTG curves shown in Figure 10(a). The information about intermo-lecular interactions and bonding formation could also be deduced from the plots of DTG curves. In Fig-ure 10(a), there is a broad peak around 1908C for both nanocomposite resins. In addition to the major peak about 4608C, the peak ‘‘shoulder’’ also appears around 5708C for OA0.25Si10and OA0.25Si20. Since we

adopted sol-gel process for sample preparation, short-chain segments in co-polyacrylate backbone derived from photo-polymerization between oligomer and small molecules might decompose at the early stage of heating.29,30 Another explanation for this phenom-enon proposed by Huang and Qui28and Ma and co-workers31,32is that the intermolecular hydrogen bonds might form via the

OH groups of (SiO2)xand

car-bonyl groups of PMMA in the case of PMMA/SiO2

hybrids prepared by sol-gel process. In this study, although previous FTIR analysis indicated that a few free

OH exists in nanocomposite resins, the peak around 1908C should still attribute to the possible breakage of interchains hydrogen bonds. The peak around 4608C corresponds to the decomposition of photo-polymerized oligomer. Owing to two 28

OH groups within oligomer, acrylic acid might also react with them to esterify the oligomer even if it is gener-ally understood that 28

OH is less reactive than 18

OH.22The ‘‘shoulder’’ of DTG peak hence corre-lated to the decomposition of esterified oligomer. TMA analysis shown in Figure 10(b) revealed that the CTE of the samples decreases from 99.2 ppm/8C for pure oligomer, to 30.6 ppm/8C for OA0.25Si10 and to

22.7 ppm/8C for OA0.25Si20. Organic materials usually

possess higher CTE values than those of the inorganic. This implies that the formation of silica in polymer is

able to reduce of the CTE of organic–inorganic compo-sites. Interlaced nano-sized silica particles hence reduce the volume expansion of polymer network in polymer/silica composites. As shown in Figure 10(c), the dissipation factor (tand) deduced from DMA anal-ysis indicates that OA0.25Si10possesses less dissipation

in comparison with oligomer and the glass transition temperature (Tg) of OA0.25Si10( 1078C) is higher than

that of oligomer ( 868C). Hu et al. reported that clad-ding of organic groups on nano-sized particles restricts the mobility of polymer chains because poly-mer chains were tethered to the organic portion.33 Owing to the existence of uniformly dispersed nano-silica particles capped with

O

CH2CH3and

OH

groups in polymeric matrix, it is apparent that the fric-tions between polymer chains and

O

CH2CH3/

OH groups capped on silica particles effectively restrict the mobility of co-polyacrylate chains so as to form a more rigid structure. The nanocomposite sam-ples hence possessed smaller dissipation factor and higher Tg.

Other properties

After photo-curing and post baking, all samples retained good transparency. The transmittance in the wavelength range of visible light is shown in Fig-ure 11. Our calculation showed that the average trans-mittances between 400 to 800 nm are 82.33%, 81.92%, and 89.61% for oligomer, OA0.25Si10, and OA0.25Si20,

respectively. Since the sizes of silica are in nanometer scale, the degree of light scattering is reduced so that there is no severe deterioration of transmittance of the nanocomposite samples.

Experimental results of moisture absorption listed in Table IV show that the OA0.25Si10/OA0.25Si20

absorbs less moisture in comparison with oligomer. The results of moisture permeability measurement further confirmed that the nanocomposite sample pos-sesses higher resistance to moisture penetration. It is believed that the finely dispersive, nano-scale silica in polymer may effectively serve as a barrier to moisture diffusion so as to suppress the moisture permeability of nanocomposite materials.

Figure 11 UV-Vis spectrum of (———) oligomer, (

) OA0.25Si10 and (- - -) OA0.25Si20. The thickness of samples

 100 mm.

TABLE IV

Moisture Absorption, Moisture Permeability, and Adhesion Strengths of Oligomer and

Nanocomposite Resins Moisture absorption (%) Moisture permeabilitya (g/m224 h) Adhesion strength (kgf/cm2) Oligomer 1.15 13.59 20.23 OA0.25Si10 1.01 10.41 42.76 OA0.25Si20 1.07 – 102.27 a_{The amount of moisture penetrating through resin film}

Adhesion test results shown in Table IV indicate that the adhesion strengths of OA0.25Si10 and

OA0.25Si20are higher than that of oligomer. For

exam-ples, the adhesion strengths of OA0.25Si10 and

OA0.25Si20 are two- and five-folds higher than that of

oligomer, respectively. It is known that the hydrogen bonds formed at the nanocomposite-glass interface might enhance the adhesion strength.32 In nanocom-posite resin samples, both free

OH groups and car-bonyl groups in polyacrylate backbone form hydrogen bonds with

OH groups on glass substrates to improve adhesion strength; moreover, nano-sized silica particles clad in ethoxy did not suppress the ad-hesion strength. Hence the enhancement of adad-hesion property was observed.

CONCLUSIONS

Photo-curable co-polyacrylate/silica nanocompo-sites were prepared via sol-gel process. The FTIR and 13C NMR analyses indicated that segmental esters and anhydride were formed in OA0.25Si10/

OA0.25Si20 nanocomposite resin samples along with

the sol-gel process, and the inorganic silica cladded in ethoxy groups. During UV exposure, the C¼¼C double bonds among acrylate groups were broken by radicals derived from photoinitiators to complete the formation of nanocomposite films via photo-po-lymerization. TEM observation revealed that the silica particles with average sizes 11.5 nm were uni-formly distributed in the polymer matrix for OA0.25Si10 sample. In OA0.25Si20, which contained

higher Si wt %, nano-scale silica particles with wider size distribution were observed. Such fine particle dispersions were attributed to the formation of inter-chain hydrogen bonds and/or the compatible ethoxy groups in the surrounding of silica resulted from the partial hydrolysis of TEOS. The TGA/DTG analysis showed that the thermal stability of OA0.25Si10 and

OA0.25Si20 decreases due to the breakage of

hydro-gen bonds and newly formed short segments. The thermal analysis also implied the formation and crosslinking of esterified oligomers via photo-poly-merization. Depends on the Si content, addition of silica effectively reduced the CTE’s of the composites up to one-third of the value of oligomer. Other prop-erty characterizations indicated that the co-polyacry-late/silica composites possessed satisfactory trans-mittance above 80% in visible light region and lower moisture absorption/permeability. This evidenced that, in addition to serve as an effective moisture permeation barrier, the nano-scale silica particles did not cause severe light scattering and hence could be applied to direct encapsulation of optoelectronic devices such as OLEDs. Adhesion tests indicated

that

OH and carbonyl groups in polyacrylate

back-bone might form hydrogen bonds with the

OH

groups on the surface of glass substrate so a sub-stantial improvement of adhesion strength was achieved.

References

1. Popovic, Z. D.; Xie, S.; Hu, N.; Hor, A.; Fork, D.; Anderson, G.; Tripp, C. Thin Solid Film 2000, 363, 6.

2. Tak, Y. H.; Kim, K. B.; Park, H. G.; Lee, K. H.; Lee, J. R. Thin Solid Film 2002, 411, 12.

3. Jeong, Y. S.; Ratier, B.; Moliton, A.; Guyard, L. Synth Met 2002, 127, 189.

4. Kim, G. H.; Oh, J.; Yang, Y. S.; Do, L. M.; Suh, K. S. Polymer 2004, 45, 1879.

5. Liu, T.; Lim, K. P.; Tjin, W. C.; Pramoda, K. P.; Chen, Z. K. Poly-mer 2003, 44, 3529.

6. Huang, J. C.; He, C. B.; Xiao, Y.; Mya, K. Y.; Dai, J; Siow, Y. P. Polymer 2003, 44, 4491.

7. Zimmerman, A. F.; Palumbo, G.; Aust, K. T.; Erb, U. Mater Sci Eng A 2002, 328, 137.

8. Isik, I.; Yilmazer, U.; Bayram, G. Polymer 2003, 44, 6371. 9. Messersmith, P. B.; Giannelis, E. P. Chem Mater 1994, 6, 1719. 10. Pereira, A. P. V.; Vasconcelos, W. L.; Ore´fice, R. L. J Non-Cryst

Solid 2000, 273, 180.

11. Motomatsu, M.; Takahashi, T.; Nie, H. Y.; Mizutani, W.; Toku-moto, H. Polymer 1997, 38, 177.

12. Ghosh, N. N.; Pramanik, P. NanoStruc Mater 1997, 8, 1041. 13. Brinker, C. J.; Scherer, W. G. Sol-Gel Science/the Physics and

Chemistry of Sol-Gel Processing, Chapter 3; 1990.

14. Oh, E. O.; Chakrabarti, K.; Jung, H. Y.; Whang, C. M. Mater Sci Eng B 2002, 90, 60.

15. Landry, C. J. T.; Coltrain, B. K.; Wesson, J. A.; Zumbulyadis, N.; Lippert, J. L. Polymer 1992, 33, 1496.

16. Landry, C. J. T.; Coltrain, B. K.; Brady, B. K. Polymer 1992, 33, 1486.

17. Nagata, H.; Shiroshi, M.; Miyama, Y.; Mitsugi, N.; Miyamoyo, N. Opt Fiber Technol 1995, 1, 283.

18. Baikerikar, K. K.; Scranton, A. B. Polymer 2001, 42, 431.

19. Wang, Y. Y.; Hsieh, T.-E.; Chen, I. C.; Chen, C. H. IEEE Trans Adv Packaging, to appear.

20. Innocenzi, P. J Non-Cryst Solids 2003, 316, 309.

21. Me´ndez-Vivar, J.; Mendoza-Bandala, A. J Non-Cryst Solids 2000, 261, 127.

22. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991.

23. Hook, R. J. J Non-Cryst Solids 1996, 195, 1.

24. Decker, C.; Nguyen Thi Viet, T.; Decker, E.; Weber-Koehl, E. Polymer 2001, 42, 5531.

25. Salmi, A.; Benfarhi, S.; Donnet, J. B.; Decker, C. Eur Polym J 2006, 42, 1966.

26. Stevens, N. S. M.; Rezac, M. E. Polymer 1999, 40, 4289. 27. Ha, C. S.; Park, H. D.; Frank, C. W. Chem Mater 2000, 12, 839. 28. Huang, Z. H.; Qiu, K. Y. Polymer 1997, 38, 521.

29. Salahuddin, N.; Shehata, M. Polymer 2001, 42, 8379.

30. Xu, M.; Choi, Y. S.; Kim, Y. K.; Wang, K. H.; Chung, I. J. Polymer 2003, 44, 6397.

31. Chiang, C. L.; Ma, C. C. Eur Polym J 2002, 38, 2219.

32. Chiang, C. L.; Yih, F. Y.; Ma, C. C.; Chang, H. R. Polym Degrad Stab 2002, 77, 273.

33. Hu, Y. H.; Chen, C. Y.; Wang, C. C. Polym Degrad Stab 2004, 84, 545.