Thermal degradation of transparent poly(methyl methacrylate)/silica nanoparticle hybrid films

Using thermogravimetric (TG/DTG) analysis, we evaluated the thermal degradation characteristics of transparent poly(methyl methacrylate) (PMMA)/silica nanoparticle hybrid films obtained by well dispersing surface-unmodified silica (SiO2) nanoparticles, of average particle size approximately 15 nm, in poly(methyl methacrylate), of weight-average molecular weight (Mw) 9.6 × 104 as synthesized by radical solution polymerization of methyl methacrylate (MMA). The TG/DTG curve of a secondary aggregated opaque hybrid film, comprising SiO2 nanoparticles of size greater than several hundreds of nanometers, was in substantial agreement with that of pure PMMA. What was surprising, however, was that in the transparent hybrid, weight reduction due to cleavage of head-to-head bonds in PMMA occurring around 200 °C was nearly completely absent, and DTG peaks originating from cleavage of head-to-tail bonds and allylic-position cleavage of terminal vinylidene double bonds increased. These thermal degradation behaviors can be explained by the hypothesis of a specific chemical reaction between tertiary terminal macroradicals produced by cleavage of head-to-head bonds and silanol (SiOH) on the SiO2 nanoparticle surface.