Molecular-weight dependence of the glass transition temperature of freely-standing poly(methyl methacrylate) films

We have used transmission ellipsometry to measure the glass transition temperature, T-g, of freely-standing films of atactic and syndiotactic poly(methyl methacrylate) (PMMA). We have prepared films with different molecular weights, MW, (159x10(3) < M (w) < 1.3x10(6)) and film thicknesses, h, (30nm < h < 200 nm). For the high-MW (M (w) > 509x10(3)) atactic PMMA films, we find that T-g decreases linearly with decreasing h, which is qualitatively similar to previous results obtained for high-MW freely-standing polystyrene (PS) films. However, the overall magnitude of the T-g reduction is much less (by roughly a factor of three) for the high-MW freely-standing PMMA films than for freely-standing PS films of comparable MW and h. The observed differences between the freely-standing PMMA and PS film data suggest that differences in chemical structure determine the magnitude of the T-g reduction and we discuss the possible origins of these differences. Our analysis of the MW-dependence of the T-g reductions suggests that the mechanism responsible for the MW-dependent T-g reductions observed in the high-MW freely-standing films is different than that responsible for the MW-independent T-g reductions observed in the low-MW freely-standing and supported films.