Macroscopic phase separation in multicomponent polymer homopolymer blends: general considerations based on studies of AB—crosslinked polymers

Abstract

This paper explores the significance of, and presents additional evidence in support of, our earlier conclusion that molecules of multicomponent polymers (i.e. copolymers consisting of long sequences of different components), such as AB-crosslinked polymers, are normally incompatible with homopolymers of the individual components. The arguments are generalized to include block copolymer systems. After a brief review of relevant previous studies, schematic phase diagrams appropriate to multicomponent polymer/homopolymer blends and common solvent are constructed. The consequences of solvent-casting mixtures of multicomponent species with one homopolymer from homogeneous dilute solution are considered for cases where equilibrium is always achieved, and more practical situations where equilibrium is not attained in bulk polymer. Electron micrographs of ultra-thin sections of solvent-cast blends of AB-crosslinked polymers with homopolymer are presented to substantiate and illustrate points made in the preceding discussion. Conclusions are drawn regarding possible morphologies which can exist in multicomponent polymer/homopolymer blends and it is proposed that unusual morphologies in block copolymer blends reported by various workers are the direct consequences of combinations of macroscopic phase separation and subsequent microphase separation within phases of different composition. We suggest that the incompatibility of chemically identical blocks and homopolymers arises from an unfavourable entropy of mixing as a result of the blocks in the vicinity of microphase interfaces adopting different sets of conformations than randomly-coiled chains in bulk polymer.