Complex layered phases in asymmetric diblock copolymers

The phase behavior of diblock copolymers near the order-disorder transition is studied using transmission electron microscopy, small-angle neutron scattering (SANS) and dynamical mechanical spectroscopy. For a series of polyolefin diblocks with volume fraction, $f_1$, of the block with the larger segment length in the range $f_1 = 0.63 - 0.75$, we find that the transition from lamellae to hexagonally packed cylinder phases occurs via intermediate layered phases that are always characterized by in-plane hexagonal order. One class of diblock forms a hexagonally modulated lamellar (HML) phase, then a hexagonally perforated layer (HPL) phase upon heating from lamellae, while another class forms a modulated hexagonally packed cylinder phase from the HML phase. We attempt to rationalize this based on the differences in inter-layer ordering in the HML phase evidenced by SANS patterns. Detailed phase diagrams for the diblocks depend on molecular weight, and a conformational asymmetry parameter, which is discussed. These results are similar to those found in certain smectic liquid crystal polymorphs, suggesting an analogy between thermotropic liquid crystalline behavior and the intermediate layered phases of diblock copolymer melts