Mechanical and thermal properties of SiO2–PMMA monoliths

Organic–inorganic hybrid monoliths of cross-linked polymethylmethacrylate (PMMA) and silica (SiO₂) were prepared by sol–gel, using tetraethoxysilane (TEOS) and methylmethacrylate (MMA) as precursors and 3-(trimethoxysilyl) propylmethacrylate (TMSPM) to make compatible the organic and inorganic components. Two types of monoliths were prepared, H1-type using a molar ratio composition of 16:1:4 for water:TEOS:ethanol and 1:0.1:0.1 for TEOS:TMSPM:MMA and H2-type using 6.3:1:2.52 for water:TEOS:ethanol and 1:0.25:0.25 for TEOS:TMSPM:MMA. Semi-transparent monoliths were obtained in both cases after the gelation–solidification and drying processes, which took about seven days. Thermal properties of the samples were obtained by applying thermal lens spectroscopy and their mechanical properties were measured by depth sensing indentation and Vickers microhardness. FTIR spectroscopy measurements were performed to complement the characterization. We found that the hardness of the hybrid monoliths have values between those of commercial PMMA and a sol–gel SiO₂ monolith. This result is a consequence of the reinforcement produced by the SiO₂ component in the organic–inorganic hybrid matrix. The thermal diffusivity of the hybrid monoliths shows an appreciable increasing in the hybrid system with respect to the pure SiO₂ monolith. Mechanical behavior of monoliths is strong influenced by the SiO₂–PMMA ratio or changing the water:TEOS:ethanol contents.