Physical properties and structure of organic-inorganic hybrid materials produced by sol-gel process

Two approaches of the sol-gel process to prepare organic-inorganic hybrids are reviewed. One method is simple and involves mixing an organic polymer with a metal alkoxide such as tetraethoxysilane (TEOS). During the sol-gel process the inorganic mineral is deposited in the organic polymer matrix forming hydrogen bonding between organic phase and inorganic phase. In the small angle X-ray scattering (SAXS) profiles of such hybrids, the scattering intensity increases with increasing amount of inorganic mineral incorporated into the organic matrix, but no peak occurs. The dynamic modulus of such hybrids increases with increasing TEOS content and is accompanied by a decrease in tan δ peak intensity. No significant change in the position of the tan δ peak occurs, because the molecular motions of the organic polymer are not restricted by the deposited silica component. Hydrogen bonded hybrid systems of hydroxypropyl cellulose (HPC), poly(vinyl alcohol) (PVA), and poly(vinylidene fluoride) (PVDF) are described. Another method is to introduce triethoxy silyl groups into the organic polymer prior to the sol-gel reactions with TEOS. In this review two methods are described. One method involves terminating the ethoxy silanes on both ends of the organic polymer with urethane or urea linkages using isocyanatopropyltriethoxysilane. Another method is to copolymerize the organic monomer with vinyl triethoxysilane. The SAXS profiles of covalently bonded hybrids are much different than those for hydrogen bonded hybrids. A large sharp peak occurs in the SAXS curves and the peak height increases with increasing TEOS content. This is perhaps due to micro phase separation between silica-rich domains and the organic polymer matrix. The dynamic modulus increases with increasing TEOS content and the tan δ peak height decreases. The tan δ peak position shifts towards higher temperatures due to the molecular motions of the organic polymer being restricted by the silica rich domains. Highly functional hybrids derived from the sol-gel process are being developed for practical applications such as coatings, contact lenses and non-linear optical, electrochromic, and electroconductive materials.