Polypropylene And Polyethylene-Copolymer Blend Miscibility: Slow Chain Dynamics In Individual Blend Components Near The Glass Transition

We systematically evaluate the slow conformational reorientations of polypropylene (PP) and polyethylene-co-1-butene (PEB) chains at temperatures near T-g before and after formation of a miscible blend with chain specific experiments. Solid-state C-13 CODEX and static Xe-129 NMR experiments reveal that aPP and PEB66 (PEB copolymer with 66 wt % 1-butene) are intimately mixed at the chain level. The two pure polymers, differing in T-g by ca. 50 K, exhibit large differences in the central correlation time constant tau(c) for slow chain segmental motion (1-1000 ms) at any temperature but have equal correlation time distributions at/near T-g. In the miscible blend, slow chain dynamics are characterized by essentially equal central correlation time constants tau(c) (ca. 15 ms) at a common temperature corresponding to the maximum exchange intensity for segmental rearrangement in the CODEX experiment, but the widths of the correlation time distributions diverge dramatically at any temperature, including at/near T-g. On the basis of comparisons of quantitative Arrhenius vs WLF models, and using an Adams-Gibbs treatment of the data, we determine that the overall configurational entropy S-c in the aPP/PEB66 blend exceeds that of the unmixed components by 15%, in agreement with previous work (Macromolecules 2007, 40, 5433). On the basis of the experimental data, general conclusions regarding driving forces for polyolefin miscibility and slow chain dynamics in miscible blends are discussed in the context of recent proposals in the literature, recognizing that polyolefins represent a limiting case of macromolecular thermodynamics due to their nonpolar structure. Importantly, all data are measured on individual signals from each polymer component in the solid blend.