Effect of nanostructure on the thermal glass transition and physical aging in polymer materials

Abstract

We review the recent activity aiming to clarify glassy dynamics in nanostructured polymer glasses, in particular thin films, nanocomposites and nanospheres. Special emphasis is devoted to recent results on the out-of-equilibrium dynamics, that is, the way the system leaves equilibrium when cooling, marking the thermal glass transition, or recovers it once in the glassy state – the so-called physical aging. Apart from those systems exhibiting strong interactions at the interface, we show that a huge number of studies probing glassy dynamics in nanostructured glasses finds negative deviations from bulk Tg and accelerated physical aging. Analysis of the dependence of the rate of spontaneous fluctuations – namely the linear dynamics – on nanostructuring indicates that there exists a significant component exhibiting bulk-like dynamics. This is the case even in the most extreme case of nanostructuring, that is, semi-isolated polymer chains and freestanding thin film with thickness ∼10 nm. In the latter case this is found at temperatures around the bulk glass transition temperature (Tg), that is, in the range where deviations of the out-of-equilibrium dynamics are normally observed. All together these results indicate that the linear dynamics alone cannot provide an exhaustive description of the out-of-equilibrium dynamics in nanostructured systems. In this case, purely geometric factors must be included. We discuss recent approaches aiming to capture the phenomenology of glassy dynamics in nanostructured glasses. Special attention is dedicated to the free volume hole diffusion (FVHD) model.