EFFECT OF HYDROGEN-BONDING ON THE ENTHALPY OF MIXING AND THE COMPOSITION DEPENDENCE OF THE GLASS-TRANSITION TEMPERATURE IN POLYMER BLENDS

A classical thermodynamic theory is presented for the composition dependence of the glass transition temperature for miscible polymer blends that involve strong specific interactions, such as hydrogen bonds. The derived T(g) equation is separated into three terms, a nonspecific interaction component, a heat of mixing in the liquid-state term, and a term that accounts for that part of the temperature dependence of the specific heat that is due to self-association. This third term is assumed to dominate the overall temperature dependence of the specific heat, so that the nonspecific interaction component is assumed independent of temperature. We also argue that the heat of mixing in systems with strong specific interactions can be positive, depending upon the balance between self-association and association between segments of the different components of the blend (interassociation). Using parameters reported in previous studies, we find the predicted mixtures' T(g)'s are in good agreement with experiment, allowing an understanding of the unusual variations of T(g) with composition that are observed for some blends that have strong specific interactions.