Twinning and growth kinetics of lamellar grains in a diblock copolymer solution

The initial stages of growth of the lamellar phase in a block copolymer solution were observed with polarizing optical microscopy (POM). Measurements were made on a poly(styrene-e-isoprene) diblock copolymer with block molecular weights of 15 and 13 kg/mol, respectively, dissolved in dioctyl phthalate with 70% polymer by volume. Upon cooling from above the order-disorder transition temperature, 89.5 °C, to temperatures from 87.5 to 88.5 °C, four distinct types of grain were observed: ellipsoidal single grains, twinned ellipsoidal grains, 2-fold twinned grains, and spherulites. The relative populations were distributed as 50% single ellipsoids, 25% twinned ellipsoids, 10% 2-fold twinned grains, and 15% spherulites. These grain types cover a range of lamellae orientation. For example, the surface of a 2-fold twinned grain is composed of lamellar edges, whereas the spherulite surface is composed of lamellar planes. The specific grain types that arise give insight into the thermodynamic and kinetic forces governing lamellae ordering. Furthermore, growth front velocities of individual grains were measured after rapid quenches from above TODT. These results were compared to the predictions of Goveas and Milner.