1.11 - Phase Segregation/Polymer Blends/Microphase Separation

Abstract

This is a brief overview on the phase behavior of polymer blends and block copolymers. The self-assembly of flexible diblock copolymers is well understood now, although the recent finding of a new stable phase (Fddd (O70)) suggests that surprises are still possible. The attention has gradually shifted to more complex systems, such as triblock and multiblock copolymers. These systems are characterized by a much larger set of parameters, not only in terms of composition and interaction parameters but also in relation to the molecular architecture, for example, linear versus star. By focusing on specific subsets, considerable progress has been made already. In particular, the development of new numerical procedures to solve the self-consistent field equations has led to a flow of publications predicting mean-field structure formation in complex copolymer systems. This, however, by no means implies that the weak segregation theory (WST) is no longer relevant. Its most remarkable advantage is that it provides an explicit analysis of the architecture dependence of the phase diagrams for rather large families of block copolymers. In particular, even though the G phase is ‘beyond’ the WST framework for diblock copolymers, block copolymer architectures might exist where both the gyroid and some other nonconventional phases could exist ‘within’ the WST framework. And, indeed, it was demonstrated by such a WST analysis based on consideration of the architecture-dependent structure correlators only that some ternary (linear) ABC block copolymers could reveal stable complex morphologies such as gyroid corresponding to Ia3¯d space group, simple cubic (SC), alternating gyroid (GA) corresponding to I4132 space group, and alternating diamond (DA) corresponding to Fd3m space group already in the vicinity of the ordering critical point. Indeed, the latter phase (DA) was first discovered by the WST analysis (even though it was misleadingly called the face-centered cubic (FCC) phase) and its existence was verified by the self-consistent field theory (SCFT) by Morse et al. only very recently.