The effect of architecture on the morphology and crystallization of oxyethylene/oxybutylene block copolymers from micelles in n-hexane

Abstract

The effects of polymer architecture on the morphology and crystallization of oxyethylene/oxybutylene diblock copolymers, E₇₆B₃₈, E₁₁₄B₅₆ and E₁₅₅B₇₆, in a concentration series of n-hexane solution were investigated with simultaneous synchrotron small-angle X-ray scattering and wide-angle X-ray scattering (SAXS/WAXS). It is observed that all the block copolymers form spherical micelles at higher temperature and the core of the micelles is partially ordered in some cases. On cooling to room temperature, the core becomes more anisotropic and there is an increase in crystallinity. After crystallization the three block copolymers behave quite differently. E₇₆B₃₈ precipitates as plates from the solution very easily, E₁₁₄B₅₆ only forms plate-like precipitates at high concentration whereas E₁₅₅B₇₆ remains a micellar solution. Analysis of the SAXS data shows that the B block is highly stretched in the shorter block copolymer. This is interpreted in terms of the aggregation of E₇₆B₃₈ being driven by the need to reduce the total area of the lateral interface because of the unfavorable interaction between the core and the solvent. By contrast, in longer block copolymers the B block is densely packed and an “over-spilling” effect can prevent aggregation by prevention of the interaction between the lateral crystal surfaces and the solvent. The dimensions of precipitated crystals in the presence of solvent are determined and found to be in good agreement with those calculated from self-consistent field theory.