Miscible Polyisoprene/Polystyrene Blends: An Unusual Combination of Heterogeneous Segmental Dynamics and Homogeneous Diffusion

The segmental and terminal dynamics of each component in miscible blends of polyisoprene (PI) and polystyrene (d$_{3}$PS) were characterized over a wide temperature and composition range. Though the system has a large positive interaction parameter \textit{$\chi $} up to 0.15, it is miscible in the temperature range of study due to selected low molecular weight. C-13 and H-2 NMR relaxation measurements were performed to extract the segmental dynamics. Pulse-gradient spin-echo NMR was used to determine the diffusion coefficients. Though the segmental dynamics of PI and PS components differ by more than 2 decades at $T_{g}$+50 K, their terminal dynamics (monomeric friction) are identical. We know of no other system with zero to positive \textit{$\chi $ }showing this feature. The distinct component segmental dynamics can be reasonably interpreted by the Lodge/McLeish model. The unusual homogeneous terminal dynamics are most likely due to a large thermodynamic barrier to diffusion.