Segmental dynamics of miscible polymer blends: Comparison of the predictions of a concentration fluctuation model to experiment

We recently proposed a concentration fluctuation model to describe the segm ental dynamics of miscible polymer blends [Kumar , J. Chem. Phys. 105, 3777 (1996)]. This model assumes the existence of a cooperative volume, similar to that in the Adam-Gibbs picture of the glass transition, over which segm ents have to reorganize in a concerted fashion to facilitate stress relaxat ion. No molecular theory exists for the cooperative volume. Consequently, h ere we critically compare two alternative functional dependences for this q uantity in the context of the segmental dynamics of the most extensively st udied miscible polymer blend, 1,4-polyisoprene (PI) and polyvinylethylene ( PVE): (a) The Donth model, which assumes the Vogel form for the temperature dependence of relaxation processes, with a relaxation time that diverges a t the Vogel temperature, roughly 50 K below the glass transition, and (b) a more recent dynamic scaling model that predicts the relaxation time diverg es algebraically, only about 10 K below the glass transition. We find that the dynamic scaling model provides a near-quantitative description of the s egmental relaxation in PI/PVE blends. In contrast, the Donth model predicts that the relaxation time spectrum for PI, the faster relaxing component, i s bimodal, in qualitative disagreement with NMR experiments and our dielect ric measurements reported here. Our results therefore emphasize two finding s. First, our model can describe the segmental relaxations of the component s of a polymer blend in a near-quantitative manner. Second, and more fundam entally, it appears that the dynamic scaling model describes segmental dyna mics of polymers near their glass transition. (C) 1999 American Institute o f Physics. [S0021-9606(99)52137-5].