Dynamics of poly(ethylene oxide) in a blend with poly(methyl methacrylate): A quasielastic neutron scattering and molecular dynamics simulations study

In this paper, we have addressed the question of the dynamic miscibility in a blend characterized by very different glass-transition temperatures, $T_g$, for the components: poly(ethylene oxide) and poly(methyl methacrylate) (PEO/PMMA). The combination of quasielastic neutron scattering with isotopic labeling and fully atomistic molecular dynamics simulations has allowed us to selectively investigate the dynamics of the two components in the picosecond—10 nanoseconds scale at temperatures close and above the $T_g$ of the blend. The main focus was on the PEO component, i.e., that of the lowest $T_g$, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference. In the region investigated, the dynamics of PMMA in the blend is strongly affected by the $\alpha$-methyl rotation; an additional process detected in the experimental window 65 K above the blend-$T_g$ can be identified as the merged $\alpha \beta$ process of this component that shows strong deviations from Gaussian behavior. On the other hand, pure PEO displays entropy driven dynamics up to very large momentum transfers. Such kind of motion seems to freeze when the PEO chains are in the blend. There, we have directly observed a very heterogeneous and moreover confined dynamics for the PEO component. The presence of the hardly moving PMMA matrix leads to the creation of little pockets of mobility where PEO can move. The characteristic size of such confined islands of mobility might be estimated to be of $\approx 1\mathrm{nm}$. These findings are corroborated by the simulation study, which has been an essential support and guide in our data analysis procedure.