Interplay between Smectic Ordering and Microphase Separation in a Series of Side-Group Liquid-Crystal Block Copolymers

Abstract

Hierarchical ordering in a series of side-group liquid-crystal block copolymers was investigated in the bulk via differential scanning calorimetry (DSC), polarized light microscopy, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The diblock copolymers comprise a polystyrene block and a block of poly(methyl methacrylate) bearing a chiral biphenyl ester mesogenic unit linked to the backbone by a dodecyloxy spacer. A series of copolymers with different volume fractions of mesogenic block were prepared by atom transfer radical polymerization. Ordering of mesogens into a smectic phase is characterized by a period 3.5 nm. Glass transition temperatures and the clearing temperature for each sample were determined by DSC. Additional ordering occurs due to microphase separation of the block copolymer at a length scale of 22−27 nm, as confirmed by SAXS and SANS. The order−disorder transition was found to be coincident with the smectic−isotropic transition for a sample comprising PS cylinders. A hexagonal morphology was determined for samples with both a minority and a majority liquid-crystal block. Remarkably, the morphology comprising liquid-crystal (LC) cylinders in a polystyrene matrix could be oriented by slow cooling through the clearing temperature, in the presence of a strong magnetic field. The inverse morphology of cylinders formed by the PS block in an LC matrix was not oriented in this way. This is ascribed to the nucleation of defects around the nanorods in the LC matrix. The thin film nanostructure was investigated by atomic force microscopy (AFM) and X-ray reflectivity for a sample comprising PS cylinders. AFM confirmed a hexagonal-packed cylinder morphology in thin films with coexisting parallel and perpendicular orientations of rods with respect to the substrate. The presence of Bragg peaks in specular X-ray reflectivity intensity profiles indicates a proportion of smectic layers lying parallel to the substrate, with a spacing similar to that in bulk. Our results provide a comprehensive picture of hierarchical ordering in the bulk and in thin films.