P. Shajesh

Sol-gel synthesis of biocompatible silica-chitosan hybrids and hydrophobic coatings

A new organic–inorganic hybrid synthesized through a sol-gel process starting from alkoxysilane and chitosan is reported. Functionalization of the hybrid was effected through in situ hydrolysis–condensation reaction of methyltrimethoxysilane (MTMS) and vinyltrimethoxysilane (VTMS) in the reaction medium. The process yields highly transparent and hydrophobic silica–chitosan hybrids. The hybrid gel was investigated with respect to chemical modification, thermal degradation, hydrophobicity, and transparency under the ultraviolet-visible region. The extent of hydrophobicity had been tailored by varying the precursor ratio. SiO2–chitosan–MTMS hybrids showed a higher thermal stability than SiO2–chitosan–VTMS (SCV) hybrids with respect to hydrophobicity. Condensation of silsesquioxanes generated from the hydrolysis of MTMS and VTMS over the silica-chitosan particles impart hydrophobicity to the hybrid. The coatings of functionalized SiO2–chitosan precursor sol on glass substrates showed nearly 100% optical transmittance in the visible region. The present hybrid material may find application in optics and other industries.

UV curable hydrophobic inorganic–organic hybrid coating on solar cell covers for photocatalytic self cleaning application

A process for obtaining a low temperature, UV curable inorganic–organic hybrid coating having excellent photoactive properties which is suitable for glass surfaces such as solar cell covers, automobile rear view mirrors and several electronic devices is presented. Nano titania sol having an average particle size of 19 nm has been synthesized using the aqueous sol–gel route starting from titanyl sulphate and was made highly photoactive by crystallizing through a simple microwave treatment which is of considerable importance in the synthesis of hybrid coatings where conventional heat treatment is not possible. The photoactive titania particles were then partially covered by silica particles by optimizing the amount of the silane moiety. A multifunctional monomer composition was further prepared and coated on a solar cell cover glass followed by a second layer coating using photoactive titania particles by dip coating and was cured under UV for six hours. The coated substrate shows photocatalytic efficiency as well as transparency >90% and further, a water contact angle >90°. Titania particles were uniformly distributed on the surface of the top coating as revealed by AFM and the coating had a total thickness of approximately 2.5 μm. The appropriate ratio of organic silanes/acrylates imparts strong adhesion to the substrate, while the inorganic components composed of a hybrid mixture of silica/titania enhanced the self cleaning efficiency. Such multifunctional coatings on solar cell covers have considerable advantage in preventing surface contamination from the exposed atmosphere which decreases the efficiency of such panels in the 15–25% range.

Synthesis, structure and properties of cross-linked R(SiO1.5)/SiO2 (R = 3-glycidoxypropyl) porous organic inorganic hybrid networks dried at ambient pressure.

Highly porous organic inorganic hybrid networks have been prepared from tetraethoxysilane (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) precursors by drying at ambient pressure. Hydrolysis-condensation reactions of the alkoxy groups on the precursors provide the inorganic network and polymerization of the epoxy group on GPTMS, the organic network. The effects of varying the molar ratios, water/OR, and TEOS/GPTMS (precursor ratio) on the gel structure were investigated. Infrared spectroscopy, thermal analysis, nitrogen adsorption measurements, and tunneling electron microscopy were used to characterize the structure and porosity features of the hybrids. At low precursor ratios, the organic networking is characterized by highly cross-linked polymer chains of low molecular weight and at high precursor ratios it consists of mainly linear chains of high molecular weight. Organic networking reinforces the inorganic network at high precursor ratios and the monoliths effectively withstand the stress produced during drying at ambient pressure. But as the proportion of the organic precursor increases, there is a higher shrinkage of the gel network, increasing the density of the dried gel. Variation in precursor ratio affects the volume of larger pores, while varying water content influences the volume of smaller pores. Monolithic gels with low shrinkage and density could be prepared through ambient pressure drying by this method.