A SAXS study of microstructure ordering transitions in liquid crystalline side-chain diblock copolymers

For a series of side-chain Liquid crystalline (LC) diblock copolymers we have investigated the overlap of liquid crystalline and morphological phase behaviors. LC thermal transitions were identified using polarized optical microscopy combined with calorimetry. Order-disorder and order-order transitions (ODT's and OOT's) associated with the diblock copolymer morphology were located using elevated temperature SAXS and TEM microscopy. At low LC fractions, samples exhibit pure lamellar morphologies, and the ODT temperature, T-ODT, is correlated to the LC isotropization temperature, T-iso. For samples with higher LC volume fractions, morphologies were found to hare curvature at room temperatures, and the Tom was found to exceed T-iso. In one sample, we observed a thermoreversible OOT transition between a predominately lamellar morphology with cylindrical defects and a completely lamellar morphology. The loss of curvature occurs upon heating, which is unusual for a block copolymer OOT and may be related to the conformational asymmetry of the diblocks. Finally, the sample with the longest LC block has a morphology of hexagonally close-packed PS cylinders which remains ordered well above isotropization. Discussion includes aspects on the coexistence of morphology and LC superstructure. Our analysis indicates that the length of the LC block and the related block copolymer morphology are key parameters to controlling both mesophase order and morphology.