Glass transition of polymers: Memory effects in structural relaxation of polystyrene

The dynamics of the $\alpha$ relaxation in polystyrene is investigated by applying specific multidimensional solid-state NMR echo techniques to evaluate multitime correlation functions. A broad distribution of correlation times characteristic for amorphous polymers above the glass transition temperature is analyzed with respect to temporal fluctuations within the relaxation time distribution. These fluctuations are quantified by a rate memory parameter $Q$. The experimental correlation functions reveal $Q$ to be almost at its theoretical minimum, proving a strong coupling among different relaxation modes. Similar results obtained for polyvinylacetate and ortho-terphenyl indicate that this behavior is a characteristic feature of the dynamics in amorphous systems above the glass transition as seen by NMR experiments. Furthermore, the multitime correlation functions allow one to quantify the heterogeneous and homogeneous contributions to the relaxation process and to determine to what extent the relaxation can be regarded as a heterogeneous superposition of different relaxation processes as opposed to a homogeneous scenario, in which the nonexponentiality is intrinsic. The analysis reveals that the relaxation is close to the heterogeneous limit and that remaining deviations indicate the presence of a small fraction of correlated back-and-forth jumps.