Crystallization-induced switching of the morphology of poly(ethylene oxide)-block-polybutadiene micelles

Abstract

We studied the morphology of micelles formed by a well-defined poly(1,2-butadiene)-block-poly(ethylene oxide) diblock copolymer (PB-b-PEO). Dissolved in n-heptane at 70 °C, that is, above the melting point of PEO, spherical micelles are formed due to the selectivity of the solvent for the PB-block. If the solutions are cooled down to low temperatures, the liquid PEO-block crystallizes within the cores of the spherical micelles that remain stable. If, however, the solutions are quenched to 30 °C, the spherical micelles aggregate to a novel meander-like structure within several minutes. In its final state, the meander-like super-structure is crystalline, as revealed by time-resolved wide-angle X-ray scattering. The super-structure is shown to result from crystallization-induced aggregation of spherical micelles. Moreover, crystallization leads to well-defined angles between subsequent aggregating units. A quantitative Avrami-type analysis of the crystallization kinetics demonstrates that the formation of the meander-type structure resembles a 2D growth process combined with a breakout crystallization, showing an Avrami-exponent of 2.5. In contrast to this, crystallization at low temperatures resembles a confined crystallization with a low Avrami-exponent of 0.7. All data demonstrate that the morphology of block copolymers having a crystallizable block can be switched by the competition of aggregation and crystallization.