We recently proposed a concentration fluctuation model to describe the segmental dynamics of miscible polymer blends [Kumar et al., 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 segments have to reorganize in a concerted fashion to facilitate stress relaxation. No molecular theory exists for the cooperative volume. Consequently, here we critically compare two alternative functional dependences for this quantity in the context of the segmental dynamics of the most extensively studied 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 at the Vogel temperature, roughly 50 K below the glass transition, and (b) a more recent dynamic scaling model that predicts the relaxation time diverges algebraically, only about 10 K below the glass transition. We find that the dynamic scaling model provides a near-quantitative description of the segmental relaxation in PI/PVE blends. In contrast, the Donth model predicts that the relaxation time spectrum for PI, the faster relaxing component, is bimodal, in qualitative disagreement with NMR experiments and our dielectric measurements reported here. Our results therefore emphasize two findings. First, our model can describe the segmental relaxations of the components of a polymer blend in a near-quantitative manner. Second, and more fundamentally, it appears that the dynamic scaling model describes segmental dynamics of polymers near their glass transition.

The linear viscoelasticity of miscible blends of a random copolymer of 80% styrene and 20% acrylonitrile and poly(methyl methacrylate) has been investigated using oscillatory shear. The Flory−Huggins interaction parameter of this blend is weakly negative. The glass transitions of the pure components are very close (ΔT g = 20 K). The blends are thermorheologically simple, in that the oscillatory shear response at different temperatures can be superimposed with the empirical time−temperature superposition principle with a precision similar to that for the pure component polymers. These results are anticipated by a theory of concentration-fluctuation-induced dynamic heterogeneities in miscible polymer blends. While sizable concentration fluctuations are present in this blend system, they do not complicate the dynamics, because all compositions have similar local dynamics. We suggest a simple phase diagram based on this model, that should be useful for deciding whether time−temperature superposition will be valid for a given blend with weak energetic interactions. Regions of thermorheological complexity are separated from regions of thermorheological simplicity on a plot of the range of blend free volume studied against the glass transition contrast of the components (ΔT g).

We present neutron spin echo and structural measurements on a perdeuterated miscible polymer blend: poly(ethylene oxide)[PEO]/poly(methyl methacrylate)[PMMA], characterized by a large difference in component glass transition temperatures and minimal interactions. The measurements cover the q range 0.35 to 1.66 A(-1) and the temperature range Tg -75 to Tg +89 K, where Tg is the blend glass transition. The spectra, obtained directly in the time domain, are very broad with stretching parameters beta approximately 0.30. The relaxation times vary considerably over the spatial range considered however at none of the q values do we see two distinct relaxation times. At small spatial scales relaxations are still detectable at temperatures far below Tg. The temperature dependence of these relaxation times strongly resembles the beta-relaxation process observed in pure PMMA.

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