Cosolvent-free synthesis and characterisation of poly(phenyl-co-n-alkylsilsesquioxane) and poly(phenyl-co-vinylsilsesquioxane) glasses with low melting temperatures

摘要

Thermoplastic poly(phenylsilsesquioxane) [poly(Ph-SQ)] and its copolymers with R-SQ units of linear aliphatic R groups [poly(Ph-co-R-SQ) (R = Me, Et, Pr, and Vi)] were synthesised by cosolvent-free hydrolytic polycondensation from acid-catalysed water-organotrimethoxysilane binary systems. These compounds became transparent glasses with coefficents of linear thermal expansion of similar to 1 x 10(-4) K-1 and Vickers hardnesses of 50-110 MPa when thermally treated at or above 100 degrees C. Poly(Ph-SQ) formed a fragile melt with kinetic fragility (F-1/2 greater than or similar to 0.8) as high as that of molecular glasses. The low melting temperature and high fragility probably arise from weak attractive pi interactions between phenyl groups that readily dissociate on heating, disordered Si-O frameworks, and low average molar mass (similar to 10(3) g mol(-1)). Proton NMR measurements confirmed other weak attractive CH/pi interactions between phenyl and R groups in poly(Ph-co-R-SQ). Poly(Ph-SQ) and poly(Ph-co-R-SQ) glasses were hydrophilic and strongly bonded to a glass plate because of the presence of residual SiOH groups, while thermosetting by polycondensation was insignificant at or below 140 degrees C. The poly(Ph-SQ) glass was brittle, and macroscopic cracks were formed when melted on a glass plate as a result of a thermal expansion coefficient mismatch. Such crack formation was suppressed by incorporating Et-SQ, Pr-SQ, or Vi-SQ units and enhancing structural relaxation.