Organic-inorganic hybrid materials: relations of thermal and mechanical properties with structures

Two series of polymethacrylate-silica covalent hybrid materials, i.e., poly[methyl methacrylate-co-3-(trimethoxysilyl)propyl methacrylate]-silica (PMCM-SiO2) and poly(2-hydroxyethyl methacrylate)-silica (PHEMA-SiO2), have been synthesized via two different sol-gel routes. The materials are highly transparent to visible light. Rapid formation of covalent bonds between the polymer and silica components during the sol-gel reactions prevents thermodynamically induced organic-inorganic phase separation. The presence of organic components does not alter the condensations of silanol groups. When extracted with an organic solvent, compositions of the hybrid materials remained largely unchanged. Bulk density and hardness of the materials can be controlled between those of the pure silica and polymers by varying the compositions. The hardness is not significantly affected by the molecular weight of PMCM polymers. The glass transition of the polymer components in all the hybrid materials is much less pronounced than that of pure polymer, as indicated by lower Tan δ peak values and smaller changes in storage modulus. As the silica content is increased, the Tg increases for the PHEMA-SiO2 hybrids while it remains essentially the same for the PMCM-SiO2 materials. As the silica content is increased, both the storage modulus and thermal stability of the hybrids are improved.