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, (159 x 10(3) < M (w) < 1.3 x 10(6)) and film thicknesses, h, ( 30 nm < h < 200 nm). For the high-MW ( M (w) > 509 x 10(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.