Glass transition temperature of freely-standing films of atactic poly(methyl methacrylate)

We have used ellipsometry to measure the glass transition temperature T-g of high molecular weight (M-w=790 x 10(3)), freely-standing films of atactic poly(methyl methacrylate) (a-PMMA), as well as films of the same polymer supported on two different substrates: the native oxide layer of silicon (Si) and gold-covered Si. We observe linear reductions in T-g with decreasing film thickness h for the freely-standing PMMA films with 30 nm <h<100 nm, which is qualitatively similar to previous results obtained for freely-standing polystyrene (PS) films. However the magnitude of the T-g reductions for PMMA is much less than for freely-standing films of PS of comparable molecular weight and thickness. We also find that for films supported on either substrate, with thicknesses as small as 30 nm, the T-g values do not deviate substantially from the value measured for thick films.