Sphere-to-Rod Transition of Micelles formed by the Semicrystalline Polybutadiene-block-Poly(ethylene oxide) Block Copolymer in a Selective Solvent.

We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)-block-poly(ethylene oxide), in the selective solvent n-heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5 kg · mol(-1) , respectively. In this solvent, micellization into a liquid PEO-core and a corona of PB-chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at -30 °C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (-196 °C; liquid nitrogen) leads to the transition from spherical to rod-like micelles. With time these rod-like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO-cores of the rod-like micelles increases. The transition from a spherical to a rod-like morphology can be explained by a decrease of solvent power of the solvent n-heptane for the PB-corona chains: n-Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology.