Block copolymers in semidilute and concentrated solutions

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

This chapter is concerned with these phases, where a substantial amount of the experimental work has been on poly(oxyethylene)-containing block copolymers in aqueous solution. From another viewpoint, the phase behaviour in concentrated block copolymer solutions has been interpreted using the dilution approximation, which considers concentrated solution phases to be simply uniformly swollen melt phases. Work on styrenic block copolymers in concentrated solution has been interpreted in this framework. There is as yet no unifying theory that treats ordered micellar phases and diluted melt phases coherently. 

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