Segmental relaxation in miscible polymer blends

Abstract

Usually the shape of the glass transition dispersion in the mechanical or dielectric spectra of pure polymers is skewed toward higher frequencies. In miscible polymer blends not only is this peak broader than in pure polymers, the broadening is often asymmetric towards lower frequencies. Concentration fluctuations are the obvious source of the broadening; however, a simple distribution of relaxation times, corresponding to a distribution in local compositions, would not account for the reversal in the asymmetry of the dispersion. Miscible blends are also thermorheological complex, with the temperature dependence of segmental relaxation exhibiting idiosyncratic composition dependencies. A model to describe the composition dependence and shape of the relaxation spectra of miscible polymer blends in the glass transition zone is described. The fluctuations in local composition inherent to a miscible blend give rise to a distribution in both the relaxation time and degree of cooperativity of segmental relaxation. Generally, the intermolecular cooperativity will be amplified by a high relative abundance of the component of the blend with the higher glass transition temperature; at least for the blends studied herein, higher TG is associated with a stronger capacity for intermolecular coupling of the segments. At fixed blend composition this effect governs the shape of the dispersion, as well as being manifested in the composition dependence of the segmental relaxation time. Since the response at lower frequency reflects the contribution of segments residing in regions richer in the high TG component, it is anticipated that the glass transition dispersion in miscible blends will be asymmetrically broadened towards lower frequencies. Application of the model to some miscible blends, including poly(vinylethylene)/polyisoprene, polyvinylmethylether/polystyrene, and tetramethyl polycarbonate/polystyrene), is demonstrated to successfully describe their most prominent features.