The mechanical relaxation behavior of freestanding poly(methyl methacrylate) (PMMA) films and freestanding PMMA supported by a polydimethylsiloxane micrometer film was investigated by using dynamic mechanical analysis (DMA). Results reveal two tiers of enhanced molecular mobility near the surface: a significant enhancement in the nanoscale outer region and a lesser enhancement in a thicker region (thickness, ht ∼ 140 nm) underneath, consistent with observations made in polystyrene (PS) films, where ht ∼ 1000 nm, however. Coarse-grained molecular dynamics simulations of PMMA and PS suggest that fast-moving molecules in the nanoscale surface region activate adjacent molecules, facilitating collective motion in both polymers. For PMMA, this enhancement terminates at a distance consistent with the experimental observation. In contrast, for PS, this dynamic enhancement persists up to the simulated thickness of 250 nm, showing no sign of termination. These findings support the role of near-surface collective motions in driving long-range mobility enhancement, with greater enhancement observed in PS compared to PMMA, accounting for the different ht values.
Nie et al. (Thu,) studied this question.