Segmental dynamics and the correlation length in nanoconfined PMMA

We studied the segmental dynamics of poly(methyl methacrylate) (PMMA) in multi-layered films with polycarbonate (PC) having layer thicknesses as small as 4 nm. Such a design provides macroscopic dimensions enabling macroscopic measurements, while maintaining nm-scale confinement. Of particular significance, we were able to determine correlation length scales for the polymer under nanoconfinement. Increases of the local segmental relaxation time, ta, and glass transition temperature, Tg, were observed with decreasing layer thickness. However, neither the fragility (Tg-normalized temperature dependence of ta) nor the breadth of the relaxation dispersion were affected by the geometric confinement. More significantly, the dynamic correlation volume, a measure of the degree of cooperativity of the dynamics, was also unaffected; that is, values of the correlation volume for confined PMMA were equal to those of the bulk polymer when compared at the same ta. This absence of an effect of geometric confinement on dynamic correlation, even when the confinement length scale approaches the correlation length scale, suggests a nonspherical correlation volume. The slowing of the segmental dynamics of PMMA confined to thin layers is due to admixing of the high Tg polycarbonate. A negligible mixing enthalpy gives rise to an extended interfacial region, which comprises a significant fraction of each layer. Published by Elsevier Ltd.