SELF-CONSISTENT CALCULATIONS OF BLOCK-COPOLYMER SOLUTION-PHASE BEHAVIOR

We present a theoretical study of the influence of solvent on ordered block copolymer solutions. The phase behavior is examined as a functio n of solvent selectivity, temperature, copolymer concentration, compos ition, and molecular weight. Phase maps are constructed using self-con sistent mean-field (SCMF) theory, via the relative stability of the '' classical'' phases, lamellae (L), hexagonally packed cylinders (C), an d a body-centered cubic array of spheres (S). Solvent selectivity and polymer concentration strongly influence phase transitions in copolyme r solutions. When a neutral good solvent is added to a symmetric block copolymer, a direct (lyotropic) transition from L to disordered (D) i s expected, analogous to the (thermotropic) L --> D transition in melt s. Indeed for neutral good solvents the dilution approximation is foll owed: the phase map is equivalent to that in the melt, once the intera ction parameter is multiplied by the copolymer volume fraction. In con trast, for a symmetric block copolymer in the presence of a slightly s elective solvent, the progression L -->C -->S -->micelles -->D is expe cted, although the micellar phase is not treated here. For asymmetric copolymers more elaborate sequences are anticipated, such as the progr ession C-B -->L -->C-A -->S-A --> micelles -->D. The stability limit o f a homogeneous block copolymer solution is also examined via the rand om phase approximation (RPA) method. The effect of polymer concentrati on on the spinodal instability falls into two regimes. When the solven t is not very selective, the stable microphase separation region is re duced as polymer concentration decreases, whereas for very selective s olvents, whereas for very selective solvents decreasing polymer concen tration broadens the region of stable ordered microstructures.