Resolving the Puzzle of Two Glass Transitions in Miscible Polymer Blends

The existence of two glass transitions has widely been observed in experiments for miscible polymer blends. Qualitative explanations postulate models of local concentration enhancements (depletions) of component 1 (2) in the neighborhood of a chain of species 1 (2). The occurrence of two glass transition temperatures is analyzed and explained by using a merger of three statistical mechanical theories: the generalized entropy theory for glass-formation in binary homopolymer blends, the lattice cluster theory for the thermodynamics of binary polymer blends, and Kirkwood-Buff theory for concentration fluctuations in binary mixtures. Specific computations of glass transition temperatures are provided for blends of semi-flexible linear chains with varying stiffness.