The relationship between dynamic and spatial heterogeneity in polymer blends is still not fully understood, yet it plays a crucial role in our understanding of their viscoelastic properties. Here, we report the results of an investigation of the viscoelastic response of a set of poly(ethylene oxide) (PEO)-poly(methyl methacrylate) (PMMA)-dimethylacetamide (DMAc) ternary blends, with emphasis on the nominally miscible regime. The work reveals interesting deviations from time-temperature superposition (TTS) as evidenced through van Gurp-Palmen (vGP) analysis. While previous rheological studies on such systems have focused on the terminal regime, our work extends through the Rouse- and towards the glassy-regime to broaden the range of dynamic behavior probed. We find that PEO:PMMA(DMAc) blends show thermal and rheological compatibility at PEO mass fractions below 30 % in the rubbery and terminal regimes. However, as the system dynamics go through the Rouse-regime and approach the glassy or α -relaxation (glass transition regime) the vGP plots no longer form a single curve, thus revealing a breakdown in TTS. These results both suggest that small-scale heterogeneities with distinct temperature dependencies exist, i.e., the blends are immiscible, and that the vGP plots are much more sensitive to breakdown of TTS than is the creation of the master curves themselves. Thus, it appears that the vGP analysis provides a sensitive measure of dynamic heterogeneity, highlighting subtle deviations that are not seen in standard TTS analysis. Additionally, in the system studied here, the ternary nature of the blends (PMMA-DMAc-PEO) includes a solvent and this alters the T g and relaxation dynamics, and we describe how this impacts the reported outcomes. Our findings suggest that, as the polymer blends approach the α -relaxation, small-scale heterogeneities influence the dynamics such that they show different temperature dependencies, leading to dynamic heterogeneity. We also show results from wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and ultra-small-angle X-ray scattering (USAXS) investigations that support the observations from the viscoelastic measurements.
Tantorno et al. (Fri,) studied this question.