Microscopic temperature-dependent structural dynamics in polymer nanocomposites: role of the graft-matrix chain...

We present a systematic study of the structural dynamics in bulk entropic polymer nanocomposites (PNCs) with deuterated-polymer-grafted nanoparticles (DPGNPs) using quasi-elastic neutron scattering (QENS). We observe that the wave-vector-dependent relaxation dynamics depend on the entropic parameter f as well as the length scale being probed. The entropic parameter can be defined in terms of the grafted-to-matrix polymer molecular weight ratio and controls the extent of matrix chain penetration into the graft. Dynamical cross-over from Gaussian to non-Gaussian behavior at the wave vector Q_c, which depends on temperature and f, was observed. Further insight into the underlying microscopic mechanism responsible for the observed behavior revealed that when interpreted using a jump-diffusion model, in addition to the speeding-up in local chain dynamics, the elementary distance over which sections of the chain hop is strongly dependent on f. Interestingly, we also observe dynamic heterogeneity (DH) in the studied systems, characterized by the non-Gaussian parameter α₂, which reduces for a high-f (f = 0.225) sample compared with the pristine host polymer, indicating reduced dynamical heterogeneity, while it is mostly unchanged for the low-f sample. The results highlight that, unlike enthalpic PNCs, entropic PNCs with DPGNPs can modify the host polymer dynamics due to the subtle balance of interactions that occur at different length scales in the matrix.