The silica-polymer hybrid materials have been extensively studied for application of chemical and mechanical protection coatings on glasses, metals and plastics. Because they show outstanding abrasion resistance as well as chemical stability and are endowed with various functionality by tailoring functional compounds. The most representative silica-polymer hybrid material is silica-poly(ethylene oxide) hybrid material prepared by using γ-glycidoxypropyltrimethoxysilane (GPTS) as a precursor.
The silica polymeric network is formed by hydrolysis and condensation of methoxyl radical in the presence of water and PEO polymeric network is made by polymerization of epoxy ring with addition of polymerization initiator. However, in the presence of water and acid or base, the epoxy group could react with water to form glycol by opening epoxy ring, which terminated the polymerization. Thus, in both thermal and UV curing systems, the two step process to hydrolyze and condense was used to polymerize epoxide without hydrolytic ring opening in GPTS.
In thermal curing system, 1-methylimidazol(MI) was used as hydrolysis and condensation catalyst in sol state and thermal polymerization initiator in gel one. It was found that the silica network was formed without epoxy ring opening in sol state by $^1H$- and $^{13}C$- nuclear magnetic resonance (NMR) spectroscopy.
The epoxide polymerization was confirmed by using differential scanning calorimetry(DSC). The polymerization was stated at 80℃ and finished at 237℃. The maximum polymerization occurred at 180℃ and its enthalpy change was 82.2 J/g. In order to the structural change with thermal curing of the GPTS/MI gel, the solid state cross polarization-magic angle spinning (CP-MAS) $^{13}C$-NMR was used. The epoxy peak decreased and the new peak appeared on heat treating at higher temperature and longer time. The new peak was due to the initiation of polymerization by MI. The molecular structures were investigated by Fourier transformed-infrared (FT-IR) spectroscopy. As the curing time increased, the epoxy groups were polymerized to poly(ethylene oxide) and the silica condensation is accomplished with increasing curing time.
The optical transmittance of the silica-PEO hybrid films was examined by using UV/VIS/NIR spectrophotometer. Its optical absorption edge shifted to longer wavelength as heat-treated at higher temperature and longer time. The refractive index and the extinction coefficient were investigated by spectroscopic ellipsometer. The refractive index and extinction coefficient increased in the range of 400-800nm through thermal curing due to epoxide polymerization and densification.
In UV curing system, mixed triarylsulfonium hexafluorophosphate salt (UVI-6990) was used as a cationic polymerization initiator. The prehydrolzed GPTS was refluxed at 80℃ for 12 hours to form the silica network because the cationic initiators couldn't work in base condition. It was found that the silica network was formed without epoxy ring opening in sol state by $^1$H- and $^{13}$C- NMR spectroscopy.
The molecular structures were investigated by FT-IR spectroscopy. The refluxed GPTS/UVI coating film was irradiated by 254nm light and then heat-treated at 180℃ for 30 minutes. As the UV dose increased, the epoxy groups were polymerized to poly(ethylene oxide).
The refractive index was measured by using prism coupler. The refluxed GPTS/UVI solution was coated on fused silica glass. The refractive index increased from 1.501 to 1.507 as the UV dose increased from 0mJ/㎠ to 8100mJ/㎠, respectively.
