Temperature dependence of molecular orientation on the surfaces of semifluorinated polymer thin films

Near-edge X-ray absorption fine structure is used to investigate the temperature dependence of molecular orientation of semifluorinated liquid crystalline (SF-LC) mesogens, which are attached to the modified isoprene backbone of (i) a poly(1,2-isoprene) homopolymer and (ii) a diblock copolymer consisting of polystyrene and poly( 1,2-isoprene) blocks. Our experiments reveal the existence of two temperature regions in which the surface orientation of the SF-LC mesogens changes abruptly, but even 30 K above the highest such temperature region the surface orientation does not become isotropic. The lower temperature surface transition for both homopolymer and block copolymer occurs close to the temperature of the bulk homopolymer smectic-B to smectic-a transition and well above the bulk smectic-B to smectic-a transition in the block copolymer. It seems to be controlled exclusively by the ordering phenomena originating from the surface. In contrast, the change in the surface organization of the SF-LC mesogens at higher temperatures can be associated with the bulk LC transition from the smectic-ii to the isotropic phase.