Control of Solution Phase Behavior through Block-Random Copolymer Sequence

The phase behavior of polymers in solution is of both fundamental and practical interest. Previous work using coarse-grained molecular simulations suggests that the critical temperature (T_c) of macromolecules in solution can be controlled by the monomer sequence. Here, we experimentally investigated the solution phase behavior of a series of styrene-isoprene copolymers in both styrene- and isoprene-selective solvents. Across the series, the copolymers had a similar overall composition and molecular weight but subtle changes in the monomer sequence obtained by systematically placing a short homopolymer block of either polystyrene or polyisoprene at the end or the center of an otherwise random styrene-isoprene copolymer chain. Compared with a fully random copolymer, sequences that microphase-separate to form starlike or crew-cut micelles produce a lower T_c. Conversely, sequences that do not form micelles exhibit a higher T_c. Through a delicate balance of solvent/polymer compatibility, we demonstrate the spontaneous and thermoreversible formation of unusually large (aggregation number, ∼1000), stable crew-cut micelles. Despite the unusual structure, the thermodynamics of formation of these crew-cut micelles is similar to that of starlike block copolymer micelles.