The Local Structure of Miscible Polymer Blends

1. Introduction The formation of miscible mixtures of two chemically dissimilar polymers is in general unfavourable. For many materials a single phase is never formed, while for others, the miscible state is only obtained at low temperatures. The classic description of the interaction between two polymers is via the Flory-Huggins theory (for example see [l]) in which x, the interaction parameter, together with the molecular weight information defines the phase behaviour. This approach has proved very successful in describing both phase boundaries and the nature of the two phase morphologies. Despite this success there is no firm microscopic model on which to base calculation of the interaction parameter. In part, the lack of a microscopic model arises from the paucity of data describing the local structure of miscible blends. It is clear that a range of interactions may exist. For example atactic poly(methylmethacrylate) (PMMA) and atactic poly(styrene) (PS) do not form a single phase unless oligomeric systems are considered. However, copolymerisation of styrene with-10 mol% acrylic acid leads to a miscible blend with PMMA. The natural conclusion is that the miscible state is stabilised through a dilute set of strong interactions. In contrast, the blend system of PS and poly(phenyleneoxide) exhibits a strongly negative volume of mixing and some evidence for changes to the local structure on mixing [a]. In other words, the system is stabilised by a concentrated set of relatively weak local interactions. If such variations in local structure exist, then clearly, the interaction parameter must embrace many factors.