Relaxation in nanometre-size polymers and glass formers: application of the coupling model to some current problems

Two recent experiments have shown dramatic changes in molecular dynamics when the size of the glass formers is reduced to the nanometre scale but in opposite directions. The first is a dielectric relaxation study by Anastasiadis et al. of the local segmental alpha relaxation in 1.5-2.0 nm poly(methyl phenyl siloxane) (PMPS) films confined between parallel solid surfaces of silicate nanocomposites. The alpha relaxation was observed to be much faster and had a significantly weaker temperature dependence compared to the bulk polymer. The other experiment is a molecular dynamics computer simulation by Scheidler et al. of a binary Lennard-Jones liquid confined either in a cylindrical pore or as a thin film confined between two walls. In the simulation, the walls were made of an amorphous structure similar to that of the confined liquid but the particles in the walls were fixed in position, The mobile particles inside the pore or in the thin film interact with each other and with the wall particles by the same potential. Scheidler et al. found the relaxation rates of particles in the pore or the film are slowed down and the degree of slowing down increases rapidly with decreasing distance between the particle and the wall. In this work, the coupling model is used to explain quantitatively the large changes in relaxation rates in the PMPS films and in the confined binary Lennard-Jones liquid, and why they go in opposite directions. Further, the observed effects in these two cases are shown to have analogues in the dynamics of probe molecules introduced in dilute quantity to glass-forming hosts and the component dynamics of miscible blends of two glass formers.