Surface Induced Tilt Propagation in Thin Films of Semifluorinated Liquid Crystalline Side Chain Block Copolymers

The supramolecular self-assembly within a liquid crystalline block copolymer thin film with mesogenic semifluorinated alkyl side groups was studied using both grazing incidence X-ray diffraction and small angle scattering. Previous studies employing near-edge X-ray absorption fine structure spectroscopy indicated that mesogens at the film surface are tilted relative to the surface-normal. This tilt may arise from the interplay between the low surface energy of perfluoromethyl groups and the entropic degrees of freedom of each side chain (constrained by neighboring mesogens in the smectic layer and the alkyl linker attachment to the polymer backbone). Here, we show that the mesogen tilt at the surface propagates over several smectic layers into the film interior. We propose that the high grafting density of side chains in combination with a head-to-head arrangement of side chain free ends plays a significant role in transmitting mesogen tilt from the surface to the adjacent smectic layers within the film interior. In addition to scattering from individual mesogens, we observed characteristic scattering patterns in the plane of incidence produced by the horizontal smectic layers. The amorphous polystyrene block formed vertical domains interspersed in the liquid crystalline matrix, resulting in out-of-plane Bragg rods.