Grain shapes and growth kinetics during self-assembly of block copolymers

With block copolymer equilibrium phase behavior becoming increasingly better understood, a more detailed description of ordering kinetics is feasible. This article highlights recent experimental observations that provide new insight into ordering kinetics and grain structures when a block copolymer liquid is first cooled below the order-disorder transition. Measurements have been made on several concentrated poly(styrene-/b-isoprene) solutions, with concentrations and temperatures adjusted to access the cylinder, lamellar, and gyroid phases. A diverse set of grain types were observed using polarized optical microscopy (POM). Cylinder grains were either oblate ellipsoids or spherulites, whereas the lamellar phase formed prolate ellipsoidal grains, twinned ellipsoidal grains, twofold twinned grains, and spherulites. The orientation of the cylinders and lamellae within the grains were unambiguously identified. The shapes of the ellipsoidal and twinned grains were dictated by the relative interfacial tensions of the available growth surfaces with respect to the disordered state. Grain growth front velocities were also quantified before grain impingement, and consistent values were obtained among different grains for a given sample and quench temperature. The experimental growth velocities were compared to an expression derived by Goveas and Milner. There was good agreement in terms of the temperature dependence, and the absolute magnitude was predicted with some success. The rates of nucleation and overall conversion to the ordered phase were also determined.