New Photocurable Acrylic/Silsesquioxane
Hybrid Optical Materials: Synthesis,
Properties, and Patterning
Hung-Wen Su, Wen-Chang Chen,* Wen-Chin Lee, Jinn-Shing King
Introduction
Organic/inorganic hybrid materials have been recognized as a new class of advanced materials because of the versatile synthetic approaches and molecular tailing properties.[1–5]Polysilsesquioxanes and their hybrid mate-rials have attracted extensive research interest because of their excellent thermal, mechanical, electronic, and optical
properties.[6] The silsesquioxane functionality employed for preparing the hybrid materials include acrylic,[7] epoxy,[8–13] amine,[14–16] vinyl,[17] hydrido,[17] isocya-nate,[18]halide,[19]and norbonyl.[20]
Extensive studies have been reported on silsesqui-oxane-based polymeric materials for electronic and opto-electronic applications, such as low dielectric constant materials,[21] optical waveguides,[22] or light-emitting diodes.[23]Linear optical planar waveguides fabricated from poly(phenyl silsesquioxane)(PPSSQ),[22a]oligomeric methyl-silsesquioxane-titania,[22b] and phenylsilsesquioxane-titania[22c] materials have shown high thermal stability, surface planarity, and excellent optical transparence. Although such materials show excellent optical and thermal properties, the flexibility is relatively poor due to their high inorganic content and thus limits their applications for flexible electronic and optoelectronic devices. Besides, extra-complicated lithographic processes are required to pattern the above materials into devices
Synthesis, properties, and patterning of new acrylic/silsesquioxane hybrid materials are
reported. PMA-functionalized PHSSQ was synthesized by hydrosilylation and then formulated
with acrylate monomer mixtures to yield the photocurable materials. Experiments suggest
that the thermal/mechanical properties of the
parent acrylic polymers could be significantly
enhanced by incorporating nano-sized
silses-quioxane moieties. The refractive index and
optical loss were reduced by increasing the
silsesquioxane content. The hybrid materials
could be photocured and developed a Y-shape
channel pattern; potential applications
in-clude uses in patterned electronic and
opto-electronic devices.
H.-W. Su, W.-C. Chen
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan
Fax: þ886 2 2362 3040; E-mail: [email protected] H.-W. Su
AGI corporation, Taipei 492, Taiwan W.-C. Chen
Department of Chemical Engineering, National Taiwan Univer-sity, Taipei 106 Taiwan
W.-C. Lee, J.-S. King
since they require additional photoresists for patterning. Therefore, the development of photosensitive silsesquiox-ane hybrid materials is very important for their applications in electronic and optoelectronic devices.[24]
In this study, synthesis and characterization of new photocurable silsesquioxane/acrylic hybrid optical mate-rials are reported. Poly(hydrogen silsesquioxanes) (PHSSQ) precursor solution was first prepared through the hydro-lysis and condensation of triethoxysilane (TES) in the mix solvents of methyl isobutyl ketone (MIBK) and tetrahy-drofuran (THF). Then, propargyl methacrylate (PMA) was incorporated with PHSSQ via Pt-catalyzed hydrosilylation to obtain the methacryloyl-functionalized PHSSQ-PMA, as shown in Scheme 1. Three photocurable materials were prepared by different compositions of the PHSSQ-PMA with methacrylate monomers of benzyl methacrylate
(BzMA), bisphenol A ethoxylate diacrylate (BAEDA), and trimethylolpropane triacrylate(TMPTA)/BzMA (w/w, 80:20), respectively. The above three kinds of hybrid materials depending on their compositions are named as P1-0–P1-75, P2-0–P2-40, and P3-0–P3-40, respectively, as shown in Table 1. Then, photoinitiator of bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (IRGA819) was added to the above precursor solution followed by spin-coating and photolithographic process to form patterned devices. The thermomechanical properties of the prepared hybrid materials were characterized by thermogravimetric analysis (TGA) and thermomechanical analysis (TMA). The surface structure of the prepared films was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The optical teristics of the prepared hybrid optical films were
charac-Si OC2H5 OC2H5 H OC2H5 Si O O Si O Si O Si Si O Si O Si O Si O O O H H H H H OH H O H Si 3.2 O H HO O H Si H O O Si Si O H O O H O H 1 O Si O Si O Si O Si Si O Si O Si O Si O O O H H H OH H O H Si O H HO O Si H O O Si Si O O O H O H O O O O O O O O
Photocurable composition
Patterned devices
Spin coating
PHSSQ
PHSSQ-PMA
(a)
(b)
(c)
3.2 1Photo-patterning
Scheme 1. Reaction scheme for preparing photosensitive PHSSQ-PMA, its photocurable compositions, and patterned devices. (a) Reaction condition: acid catalyzed reaction at 60 8C for 3 h, (b) hydrosilylation reaction of PHSSQ with PMA using Pt(dcp) at 80 8C for 5 h, and (c) formulation with acrylate monomer mixtures and photoinitiator.
terized by their optical transparence, refractive index, and optical loss. It was found that Y-shaped patterned devices with 10 mm line width, 1.5 mm thickness, and a splitting angle of 108 could be achieved from the PHSSQ-PMA/ acrylic hybrid materials.
Experimental Part
Materials
Triethoxysilane (TES; 95%; TCI, Japan), propargyl methacrylate (PMA; 98%; Lancaster, UK), dichloro(dicyclopentadienyl) platinum (II) [Pt(dcp); 99%; Strem, USA] were used without further purification. BzMA (96%; Aldrich, USA), bisphenol A ethoxylate (1.5EO/phenol) diacrylate (BAEDA, Aldrich, USA), TMPTA (techni-cal grade; Aldrich, USA), hydrochloric acid (HCl; 37%; Scharlau Chemie, Spain), IRGA819 (CIBA, USA) were used as received. Poly(methyl silsesquioxane) (PMSSQ; Mw¼ 7 000–8 000, average
OH content ¼ 5%, Gelest) was used as a reference in obtaining the approximate content of Si–OH in the synthesized PHSSQ.
Synthesis of PHSSQ-PMA
The PHSSQ precursor was synthesized using the method reported previously by our laboratory and described briefly as below.[21g]
Briefly, 30 mL of MIBK, 15 mL of THF, and 9.86 g of TES (0.06 mol) were added into a 100 mL three-necked round bottom flask equipped with a condenser and a dean-stark trap. With the reaction conditions of pH ¼ 1 and R(H2O/TES) ¼ 1.5, an aqueous
solution containing 1.62 g (0.09 mol) of deionized water and 0.058 g (5.6 104mol) of HCl was added dropwise over a period
of 10 min into the flask with rigorous stirring. The equipment was then immersed in silicon oil at 60 8C. The hydrolysis and condensation reaction lasted for 3 h under nitrogen atmosphere. The reactive temperature was then increased to 90 8C for another 2 h to remove the THF and the PHSSQ precursor solution was obtained. The synthesis of PHSSQ-PMA used a reaction similar to that of single silsesquioxane cage, reported previously in the literature[7b] although the present study was focused on the polymeric silsesquioxane materials with both cage-like and network-like moieties. In the following, the reaction temperature was decreased to 80 8C, then 5.96 g of PMA (0.048 mol) and 80 mg Pt(dcp) were added into the flask. Upon the completion of addition, the mixture was allowed to react for another 5 h and then cooled down to the room temperature to obtain the homogenous solution of PHSSQ-PMA.
Photocuring and Patterning of PHSSQ-PMA
The photocurable materials were prepared by mixing PHSSQ-PMA with three kinds of acrylic monomers [BzMA, BAEDA, BzMA/ Table 1. Composition and properties of the prepared hybrid materials.
PHSSQ-PMA/ acrylate mixture
Ratio Refractive index
at 1 310 nm Film thickness Tda) Residue at (800 -C) CTE Optical loss wt.-% mm -C wt.-% ppm -CS1 dB cmS1 PHSSQ-PMA/BzMA 0:100 (P1-0) –b) –b) 224 1.2 –b) –c) 10:100 (P1-10) –b) 1.03 232 5.1 322 –c) 30:100 (P1-30) –b) 1.54 238 12.5 222 –c) 50:100 (P1-50) –b) 1.62 241 19.1 177 –c) 75:100 (P1-75) –b) 1.75 242 24.3 121 –c) PHSSQ-PMA/BAEDA 0:100 (P2-0) 1.561 11.36 408 6.8 151 0.68 10:100 (P2-10) 1.557 12.74 403 15.1 134 –d) 20:100 (P2-20) 1.552 13.55 411 19.2 121 1.19 30:100 (P2-30) 1.547 13.81 418 23.7 118 0.74 40:100 (P2-40) 1.536 14.31 427 29.8 105 0.43 PHSSQ-PMA/TMPTA-BzMA 0:100 (P3-0) 1.508 7.68 410 3.6 128 0.58 10:100 (P3-10) 1.500 8.34 412 10.4 115 0.51 20:100 (P3-20) 1.498 8.65 416 14.5 103 0.42 30:100 (P3-30) 1.496 10.47 425 17.4 76 0.31 40:100 (P3-40) 1.494 11.35 430 21.1 67 0.29
a)Determined from the intercept of the two tangent lines of the TGA curves;b)The film by spin coating is too thin to measure its refractive
TMPTA (80:20 wt.-%)] with different compositions, as shown in Table 1. IRGACURE819 was used as the photoinitiator due to its outstanding absorption properties and minimum yellowing after the exposure to sufficient amounts of UV radiation. The amount of photoinitiator was 2 wt.-% of the total solid content. By using PHSSQ-PMA/TMPTA-BzMA ¼ 40:100 (P3-40) as an example, 2.5 g of PHSSQ-PMA in MIBK solution (the solid weight of PHSSQ-PMA was about 0.5 g) was mixed with 1 g of TMPTA, 0.25 g of BzMA, and 0.035 g of IRGA819. Then the solution was mixed by rigorously stirring for 1 h, filtered through 0.45 mm polytetrafluoroethylene membrane filter, and then spin-coated onto the split silicon wafer or 10 10 0.7 cm3sheet glass at 750 rpm for 10 s and soft-baked at 80 8C for 1 min, followed by exposure to the UV light of 365 nm (I-line) through a transparent mask or the predefined Cr mask with the dose of 1 000 mJ cm2. Subsequently, the exposed films were
developed by the mixture of MIBK/ethanol in a weight ratio of 60:40. A final thermal treatment was carried out at 150 8C for 10 min and 220 8C for 20 min, respectively, to obtain the patterned optical waveguides.
Characterization
FTIR spectra of the materials on the doubly polished silicon wafers were obtained with a Perkin Elmer PARAGON 1000. Gel perme-ation chromatography (GPC) analysis was performed on Waters GPC system consisting of, Waters 2414 RI detector, Shodex columns (KF-802,803,805), Waters 717 plus auto-sampler, and Waters 515 HPLC pump. The system was calibrated using polystyrene standards. THF was used as the eluent, at a flow rate of 1.0 mL min1.
Thermogravimetric analysis and DSC thermal analyses were conducted on a Perkin-Elmer Pyris 1 TGA and a TA Q100 with a refrigerated cooling system, respectively. Both measurements were performed under continuous flow nitrogen, at a heating rate of 10 8C min1. A Perkin-Elmer Pyris DMA7e was used to characterize the coefficient of thermal expansion (CTE) at a heating rate of 10 8C min1 from room temperature to 200 8C under nitrogen atmosphere. A Model DI 5000 AFM was used to probe the surface morphology of the coated films. The SEM images of the lithographic patterns were obtained by a JEOL JSM-5310 microscope.
A prism coupler (Metricon Model 2100) was used to measure the film thickness and refractive index of the prepared thin films at the wavelength of 1 310 nm. The planar waveguides were fabricated with the structures of negative waveguide resisting thin films on top of a thermal oxide (refractive index ¼ 1.447) using silicon wafers as the substrate. The propagation optical losses of the prepared planar waveguides at the wavelength of 1 310 nm were measured by a cut-back method according to our previous report.[22b,22c]The incident light beam was introduced
into the 50 mm long straight waveguide and the output beam power intensity was measured. The waveguide was cut by 10 mm, while the propagation loss was measured based on the difference between the input and the output light intensities using an optical power meter. For measuring the NIR absorption spectra of the prepared hybrid materials, a sample with a thickness of a few millimeters was obtained by evaporating the precursor solution in a Teflon disk under vacuum. The NIR absorption spectra were
obtained using a UV-vis-NIR spectrophotometer (Jasco, model V-570) in the wavelength range of 1 000–2 000 nm and normalized by dividing with film thickness.
Results and Discussion
Polymer Structure Characterization of PHSSQ-PMA and its Hybrids
Figure 1 shows the FTIR absorption spectra of the PHSSQ, PHSSQ-PMA, P1-30, P2-30, and P3-30, respectively. In the PHSSQ spectra, the absorption peaks at 1 128 and 1 072 cm1are attributed to the Si–O–Si stretching band of cage-like and network-like structure, respective-ly.[21g,22b]The ratio of cage-like/network-like is about 3.2 by curve fitting of the IR spectra in the range of 1 250–950 cm1 under the assumption of two Gaussian peaks at 1 128 and 1 072 cm1. The Si–OH content is determined by integrating the peak areas at 1 300–1 000 and 930–900 cm1, which are assigned to the Si–O–Si and Si–OH groups, respectively. By a comparison with a refe-rence PMSSQ sample from Gelest with a reported 5% OH content, the approximate content of Si–OH is about 2.3%. The peak around 2 255 and 829–864 cm1are assigned to the Si–H stretching and H–Si–O hybrid vibration, respec-tively. After hydrosilylation, the peak intensity of the Si–H bands at 830 and 2 255 cm1decreases significantly, which suggests that the Si–H have reacted with the propargyl group of PMA. Moreover, the methacrylate group peaks: around 2 900 (–CH3), 1 720 (C––O), and 1 640 cm1(C––C) are
4000 3500 3000 2500 2000 1500 1000 Wavelength (cm-1) P3-30 P2-30 1640cm-1 1720cm-1 P1-30 PHSSQ-PMA PHSSQ T ran sm issio n (a.u .)
Figure 1. FTIR transmittance spectra of PHSSQ, PHSSQ-PMA, P1-30, P2-30, and P3-30 on doubly polished silicon wafers.
observed in Figure 1, indicating the bonding of PMA monomers with PHSSQ. The FTIR result suggests that the methacrylate group is successfully incorporated into the PHSSQ.
The number-average molecular weight (Mn) of the PHSSQ precursor was 5 030 with the polydispersity index of 1.82 and the ratio of cage-like/network-like structures was 3.2. If the size of cage-like and network-like structures is assumed to be similar, the diameter of PHSSQ could be estimated to be 11 nm based on the ratio of Mnof PHSSQ and (SiO1.5)8. After hydrosilylation, the Mn of the PHSSQ-PMA precursor was 12 280 with the polydispersity index of 2.69. The hydrosilylation ratio estimated by TGA was about 48% by the 1:0.8 loading mole ratio of SiH:PMA in the present study. That means the side group of the PHSSQ-PMA contained 48% of PMA and 52% of Si–H moieties. We also attempted other loading ratio at the low loading ratio of 1:0.3 for Si–H/PMA but the gelation would have occurred due to the intramolecular hydrosilylation reaction.[7b]However, if the high mole ratio of PMA/Si–H (e.g., 1:1) was used, the hydrosilation ratio was not increased but was with a large PMA residue. Hence, the PHSSQ-PMA precursor prepared from the hydrosilation reaction with the SiH:PMA mole ratio of 1:0.8 was used for preparing hybrid materials.
Three kinds of acrylate functional monomers with different weight ratios are used to prepare hybrid materials with PHSSQ-PMA, PHSSQ-PMA/BzMA (P1), PHSSQ-PMA/BAEDA(P2), and PHSSQ-PMA/TMPTA/BzMA (P3). As shown in Figure 1, the intensity of 1 640 cm1(C––C of methacrylate group) of the hybrid materials, P1-30, P2-30, and P3-30 is almost disappeared after UV irradiation and thermal curing, implying the nearly complete poly-merization of the hybrid materials. Hence, the prepared hybrid materials could be used for photopatterning device applications.
Properties of the Hybrid Materials
Figure 2 shows the TGA curves of P1-0, P1-30, P2-0, P2-30, P3-0, and P3-30 under nitrogen atmosphere at the heating rate of 10 8C min1, respectively. Note that the P1-0, P2-0, and P3-0 are the parent polymers of poly(benzyl meth-acrylate), poly(bisphenol A ethoxylate dimeth-acrylate), and poly[(trimethylolpropane triacrylate)-co-(benzyl meth-acrylate)], respectively. As shown in the figure, the hybrid materials have a slightly higher thermal decomposition temperature but a much higher char yield than their parent polymers. The thermal decomposition tempera-tures as well as other physical properties of the studied materials are summarized in Table 1. Figure 3 shows the TMA curves of P2-0, P2-30, P3-0, and P3-30, respectively, in which the CTE are 151, 118, 128, 76 ppm 8C1, respec-tively. It suggests that the CTE could be significantly reduced by incorporating the PHSSQ-PMA. The CTE of the other hybrid materials shown in Table 1 also suggests a similar trend. It suggests that the extensive crosslinking and incorporating nano-size silsesquioxane structures are the major factors contributing to the dimensional stability of the hybrid films.
The film thickness measured by prism coupler are in the range of 1.03–1.75, 11.36–14.41, and 7.68–11.35 mm, for the P1, P2, and P3 hybrid materials, respectively. The molecular weight and viscosity of the BAEDA moiety are higher than the other two acrylate monomers and thus the film thickness of P3 is higher than those of P1 and P2. Besides, the film thickness of P1 could not be over 2 mm due to the low molecular weight and viscosity of BzMA. Figure 4 shows the AFM images for the waveguide films P2-30 and P3-30, respectively. The average roughness (Ra) for P2-30 and P3-30 is 1.33 and 0.53 nm, respectively, while the mean square roughness (Rq) is 1.68 and 0.68 nm. The relatively low surface roughness in comparison with the
200 400 600 800 0 20 40 60 80 100 P1-30 P1-0 P3-0 P3-30 P2-0 Weig ht (%) Temperature (oC) P2-30
Figure 2. TGA curves of P1-0, P1-30, P2-0, P2-30, P3-0, and P3-30 at a heating rate of 10 8C min1under nitrogen atmosphere.
40 60 80 100 120 140 160 180 200 99.5 100.0 100.5 101.0 101.5 102.0 102.5 103.0 P3-30 P2-30 P3-0 P2-0 Dimensio nal c h ange (%) Temperature (o C)
corresponding film thickness suggests the uniform hybrid films. The SEM images of the hybrid materials also do not show any significant domain size, indicating uniform film quality of the hybrid materials.
Figure 5 shows the UV-vis absorption spectra of PHSSQ-PMA, P1-30, P2-30, and P3-30, which exhibit high transparency in the wavelength range of 400–1 000 nm. The relatively small silsesquioxane domain and chemical bonding between the organic and inorganic moiety prevent phase separation and thus explain the excellent optical quality of the prepared hybrid films. The refractive index at 1 310 nm is also similar, 1.561–1.536 and 1.508–1.494, for the P2-0–P2-40 and P3-0–P3-40, as shown in Table 1. The refractive index of the hybrid film is decreased as the content of the PHSSQ (or) silsesquioxane is enhanced, which is due to the low refractive index of the PHSSQ (n ¼ 1.402) measured by the prism coupler.[21g]The
refractive index would be decreased with an increase in the amount of the PHSSQ since it created the free volume in the hybrid materials due to its cage-like structure. It explains the low refractive index of the silsesquioxane-based hybrid materials.
The film thickness of the P1-0 P1-75 hybrid materials is too thin for preparing optical waveguide and thus the waveguide characteristics of the other two kinds of hybrid materials are reported. The multimode planar waveguides were fabricated with the structures of the hybrid films on top of the thermal oxide (refractive index ¼ 1.447) using silicon wafers as the substrate. The cut-back method was used previously by our laboratories to determine the propagation optical losses at 1 310 nm.[22b,22c] Figure 6 shows the variations of transmitted optical power (10 log P) with the lengths for the hybrid materials of P2-0, P2-40, P3-0, and P3-40, respectively. The estimated optical losses of the above four materials are 0.68, 0.43, 0.58, and 0.29 dB cm1, respectively, while those of the other hybrid materials are listed in Table 1. The comparison on optical losses of the P3 hybrid materials suggests that the incorporation of the PHSSQ-PMA could reduce the optical loss. For pure organic polymer-based optical waveguides reported in the literature, polyguides (acrylic-based poly-mer) could achieve optical losses of 0.2 dB cm1 at 1 310 nm. Other kinds of polymers could even achieve much lower values.[25,26] As discussed previously, the hybrid materials have the nano-size silsesquioxane domain and uniform film quality and thus scattering loss is insignificant in the film. Besides, the incorporation of the PHSSQ could reduce the C–H number density and thus the optical loss is decreased. Note that the optical loss of pure organic polymers is mostly originated from the C–H number density.[21,22b,22c,25] Figure 7 shows the near infrared absorption spectra of P3-0, P3-20, and P3-40. The absorption bands in the range of 1 120–1 260 and
200 400 600 800 1000 10 20 30 40 50 60 70 80 90 100 110 T ( % ) Wavelength (nm) PHSSQ-PMA P2-30 P1-30 P3-30
Figure 5. UV-visible transmission spectra of the prepared hybrid films on quartz substrates: PHSSQ-PMA, P1-30, P2-30, and P3-30.
Figure 4. AFM images of the prepared hybrid films on the silicon wafers: (a) P2-30 and (b) P3-30.
1 600–1 850 nm are assigned to be the third (n3) and second (n2) harmonic stretching vibration absorption bands of the C–H bond. The absorption band in the range of 1 320– 1 530 nm results from a combination of the second harmonic stretching vibration (n2), the bending vibration (d) of the C–H bond and the second harmonic stretching vibration of OH bond (n2). Hence, the intrinsic absorption loss at 1 310 nm is mostly from C–H vibration absorption and the C–H bonding density could be decreased with an increase in PHSSQ content. The smaller intensities of these absorption bands in P3-40 than those of P3-0 are probably due to the reduction of C–H number density as explained above. In the case of the P2 hybrid materials, the correlation between the optical loss at 1 310 nm with the PHSSQ content is not as clear as that of the P3 although the optical loss of P2-40 is significantly lower than that of P2-0. The interface between the hybrid materials and the
thermal oxide could result in such an unexpected trend. Figure 8 shows the SEM diagrams of the developed patterns of P2-40 and P3-40 with 10 mm width, 1.5 mm thickness, and 108 splitting angle. Both P2-40 and P3-40 show clear developed patterns. Based on the optical properties and patterning resolution, the prepared hybrid materials could have potential applications for patterned optical devices. The present study demonstrates that the incorporation of silsesquioxane domain could not only significantly enhance the thermomechanical properties but also enhance the optical characteristics.
Conclusion
We have successfully synthesized three series of new acrylic/silsesquioxane hybrid materials through the precursor of methacrylate-functionalized PHSSQ. By incorpo-rating the nano-size silsesquioxane moiety, the thermo-mechanical properties of the parent acrylic polymers could
1000 1200 1400 1600 1800 2000 P3-40 P3-20 P3-0 No rma li z ed Ab so rbanc e (a.u .) Wavelength (nm)
Figure 7. Near infrared absorption spectra of P3-0, P3-20, and P3-40 bulk sample in the wavelength range of 1 000–2 000 nm.
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -10 *lo gP Length (cm) P3-40 P3-0 P2-0 P2-40
Figure 6. Variation on transmitted optical power with the lengths of the prepared waveguide films, P2-0, P2-40, P3-0, and P3-40, respectively.
Figure 8. SEM images of the developed patterns with 10 mm line width and a 108 splitting angle, left: P2-40; right: P3-40.
be significantly enhanced. Besides, the refractive index and optical loss were reduced by such moiety. Relatively low optical loss of 0.29 dB cm1at 1 310 nm was obtained with the PHSSQ-PMA/TMPTA/BzMA hybrid material-based opti-cal waveguide. The hybrid materials could be photocured to develop Y-shaped pattern with 10 mm line width and a 108 splitting angle. The newly prepared photocurable hybrid materials could have the potential applications for pat-terned electronic and optoelectronic devices.
Acknowledgements: We thank the financial supports from National Science Council of Taiwan, Ministry of Economics Affairs of Taiwan, and Industrial Technology Research Institute. Received: November 16, 2006; Revised: February 1, 2007; Accepted: February 21, 2007; DOI: 10.1002/mame.200600442 Keywords: composites; hybrid; optical films; photopolymeriza-tion; silsesquioxane
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