Low-shrinkage, high-hardness, and transparent hybrid coatings: Poly(methyl methacrylate) cross-linked with silsesquioxane

Copolymers of trimethoxysilylpropyl methacrylate (TMSPM) and methyl methacrylate (MMA) with varied composition and molecular weight were prepared as building blocks for transparent organic-inorganic hybrid materials. Thermally and chemically induced condensation of these network precursors after solvent evaporation results in optically clear coatings as thick as 1.0 mm with extremely low lateral shrinkage. Many interacting variables such as acid catalyst concentration, polymer molecular weight, polymer composition, and thermal treatment combine to produce materials having controllable shrinkage, hardness, transparency, and thermal stability. By selectively defining these variables, the syntheses of these materials are optimized such that silsesquioxane-cross-linked acrylic polymers are produced that exhibit desirable properties such as low optical loss in the visible spectrum, low lateral shrinkage, high scratch hardness, and no glass transition temperature. The available combinations, interrelationships, and limitations of these properties are discussed.