Effects of Polymer Tacticity and Molecular Weight on the Glass Transition Temperature of Poly(methyl methacrylate) Films on Silica

The glass transition temperature ( T g ) of poly(methyl methacrylate) (PMMA) films supported by silica was studied as a function of film thickness at two molecular weights (MW = 2.5 and ∼50 kg/mol) and two syndiotacticities (s% = 56 and 69 or 79%). We found that the result depends on both the MW and s% in unconventional ways. For the 2.5 kg/mol films, the T g was depressed for the lower s% films but enlarged for the higher s% films. For the ∼50 kg/mol films, however, the signs of the T g change were reversed. To understand these observations, we constructed bilayer films from pairs of the studied PMMAs with roughly the same MW but different s%. Our result suggests that for the higher MW films, both the effects of the free surface and the substrate interface on the T g are more significant on the higher s% PMMAs than on the lower s% PMMAs. For the lower MW films, however, the significance of the surface and substrate effects are modified, probably due to the effect of chain stiffness on the T g and different degrees of substrate segregation of the higher s% chains at different MW’s, respectively.