Dielectric Relaxations in Ultra-Thin Films of PMMA: Assessing the Length Scale of Cooperativity in the Dynamic Glass Transition

The local and cooperative dynamics of supported ultra-thin films $(L=4-170\ {\rm nm})$ of poly(methyl methacrylate) was investigated by dielectric spectroscopy at frequencies from 0.1 Hz to 106 Hz and temperatures from 250–423 K. To assess the effect of stereoregularity, three tacticities ($i, a$ and $s$-PMMA) of similar molecular weight $M_{n}=120-140\times 10^{3}\ {\rm g}\ {\rm mol}^{ -1}$ were studied. A second $M_{n}$ value for $s$-PMMA was …