Modeling of diblock copolymers in selective solvents.

Abstract

Phase behavior and micellization kinetics of diblock copolymer surfactants in selective solvents influence many processes. We study the driving forces behind the self-assembly of a diblock copolymer AB, consisting of a solvent-philic block (B) and a solvent-phobic block (A), in selective solvents (S). We investigate this system using self-consistent field theory (SCFT), which is a coarse-grained, approximate theory with a proven track record for polymer mixtures. It discards the effects of fluctuations. Micellar transformations between spherical, cylindrical, and bilayer curvatures are tracked in the dilute regime. We determine thermodynamic and structural properties of these isolated aggregates such as the critical micelle concentration (CMC), the critical micelle temperature (CMT), the solvent penetration of the core, and the core radius of micellar morphologies within the context of SCFT. We also investigate the morphological variation from ordered phases, found in the concentrated regime, to isolated aggregates upon copolymer depletion. Depleting this blend of surfactant causes these stable structures to swell and ultimately unbind. The unbinding transition of the ordered phases is compared with the morphology transformations observed in the dilute regime. We also delineate two phase coexistence regions between ordered phases, and ordered phases and a solvent rich macrophase. Furthermore, we quantify the effective interactions between the aggregates themselves. Intriguingly, for spherical micelles, the free energy of BCC, and FCC phases can be described in terms of a single effective pair potential that depends on micellar aggregation number, however, this aggregation number changes significantly with the concentration and temperature. The kinetic barriers to association and dissociation of diblock copolymers in various selective solvents are calculated. We study the variation of these kinetic barriers for both block copolymers in small molecule solvents and block copolymers in a homopolymer matrix. The kinetic barriers are found to be very sensitive to temperature and surfactant concentration. They also become prohibitive except in a modest range of temperature near the CMT, or in sufficiently highly supersaturated or subsaturated solutions near the equilibrium CMC. The dependence of kinetic barriers upon the chain lengths and solvent quality is also studied.