Concentration fluctuation induced dynamic heterogeneities in polymer blends

Abstract

The presence of two distinctly different local segmental mobilities found in the case of several phase mixed polymer blends by two-dimensional 2H-NMR, dielectric spectroscopy and depolarized dynamic light scattering is rationalized through a simple concentration fluctuation model. Our primary hypothesis is that, although the probability of the occurrence of concentration fluctuations is symmetric about the mean value in a given volume, the ‘‘cooperative volume’’ over which a fluctuation must occur for it to be detected by a dynamic probe is not a constant, but rather depends on the composition of the cooperative volume. Consequently, we suggest that the cooperative volume associated with a concentration fluctuation be determined by the local composition in a self-consistent manner. In the case of systems with weak interactions and large Tg contrast, these ideas are shown to create a bimodal probability density function for dynamic concentration fluctuations, which has a local maximum corresponding to small cooperative volumes rich in the more mobile component, in addition to the bulk composition itself. We also predict the broadening of both the segmental relaxation time spectrum and the single calorimetric glass transition of the blends with increasing concentration of the slow component. Comparisons of the predictions are made for two experimental systems with different thermodynamic and kinetic effects to illustrate the present approach. Further, we predict the effects of concentration fluctuations in the case of three other blends whose segmental dynamics have not been examined experimentally to date. We predict that the dynamics of the two components approximately follow time–temperature superposition either in the case of strongly interacting blends or for systems that have closely matched Tg values and Williams–Landel–Ferry (WLF) coefficients.