Crystallization-induced aggregation of block copolymer micelles: influence of crystallization kinetics on morphology

We present a systematic investigation of the crystallization and aggregation behavior of a poly(1,2-butadiene)-block-poly(ethylene oxide) diblock copolymer (PB-b-PEO) in n-heptane. n-Heptane is a poor solvent for PEO and at 70°C the block copolymer self-assembles into spherical micelles composed of a liquid PEO core and a soluble PB corona. Time- and temperature-dependent light scattering experiments revealed that when crystallization of the PEO cores is induced by cooling, the crystal morphology depends on the crystallization temperature (Tc): Below 30°C, the high nucleation rate of the PEO core dictates the growth of the crystals by a fast aggregation of the micelles into meander-like (branched) structures due to a depletion of the micelles at the growth front. Above 30°C the nucleation rate is diminished and a relatively small crystal growth rate leads to the formation of twisted lamellae as imaged by scanning force microscopy. All data demonstrate that the formation mechanism of the crystals through micellar aggregation is dictated by two competitive effects, namely, by the nucleation and growth of the PEO core.