Thermal degradation of poly(methyl methacrylate) on SiO2 nanoparticles as a function of SiO2 size and silanol density

The degradation behavior of amorphous polymethyl methacrylate (PMMA) on silica (SiO2) nanoparticles (PMMA–SiO2) as a function of silanol density and nanoparticle size was investigated by derivative thermogravimetric analysis (dTGA). The thermal degradation temperature, Td, was measured for PMMA adsorption amounts below plateau adsorption, which enhanced the contribution of chains in direct contact with the silica. Td increased as the number of hydrogen bonds between the silanol (SiOH) groups of the SiO2 and the carbonyl (Cdouble bond; length as m-dashO) of the PMMA increased, either from varying the heat treatment of the SiO2, which changed the silanol density, or increasing the nanoparticle size, thus increasing the number of sites where the PMMA chain contacted the nanoparticles. The increases in Td correlated with increases in Tg for the PMMA–SiO2, suggesting that the enhanced thermal stability was attributable to decreased segmental mobility of PMMA chain segments, which in turn decreased thermally induced degradation. Td and Tg did not change for polar PMMA on hydrophobically modified SiO2, (CH3)–SiO2, as expected based on this mechanism, since there were no specific hydrogen-bond interactions between the Cdouble bond; length as m-dashO of PMMA and the Si–O–(CH3)3 of SiO2.