Thermodynamic interpretation of domain structure in solvent‐cast films of A–B type block copolymers of styrene and isoprene

Abstract

AbstractElectron microscopic textures of A–B type block copolymers of styrene and isoprene cast from several solvents were investigated by means of the osmium tetroxide fixation technique. The two‐phase structure, i.e., the semimicro heterogeneous structure due to the microphase separation of block segments, was observed to change systematically with the fraction of block segments and the kind of solvent. Three types of fundamental domain structure were found. With the assumption that domain structure originates from micellar structure at a critical concentration in relatively dilute solution during solvent casting, the formation of the three types of domain structure and the sizes of their elements were treated in terms of the equilibria governing the formation of micelles at the critical concentration. This analysis takes into account such thermodynamic and molecular parameters as the incompatibility between the A and B segments, the solvation of the segments, the casting temperature, the total chain length of the block copolymer, and the weight fraction composition of the block copolymer. It was concluded that the block segments are preferentially oriented along the direction perpendicular to the interface between the two phases. This particular orientation‐aggregation of the block segments must make the bulk properties of the block copolymer much different from those of merely mechanical mixtures of the corresponding homopolymers, even if the same semimicro heterogeneous structures are formed in the mechanical mixtures.