Block Copolymers Morphological and Physical Properties

Abstract

It is generally recognized that two dissimilar polymers will be thermodynamically incompatible with one another providing that specific interactions between them do not lead to a negative heat of mixing. This follows from the negligible entropy of mixing of high molecular weight species coupled with a typical positive heat of mixing. It is then not surprising that when such incompatible polymers are joined together to form a block copolymer that this incompatibility should still be manifested — as it is. However, the union of incompatible species in a block copolymer has profound effects on the phase behaviour and morphology of the resulting phase-separated system. There are fundamental differences between a system consisting of a simple mixture of two polymeric species and a system consisting of the same species joined together to form a block copolymer. Whereas the thermodynamically stable state of a simple mixture is the gross separation of the two species to a completely segregated state — with a minimum of interfacial contact between the two separated phases, such gross phase separation cannot occur with block copolymers since the incompatible species are joined together. This factor leads to phase separation that is, restricted to molecular dimensions – “microphase separation” — with the microphases existing in a domain morphology. Such systems are unique in that the dispersed domain structures are thermodynamically stable in the dispersed state.