The orientation of semifluorinated alkanes attached to polymers at the surface of polymer films

The surface molecular orientation of a liquid crystalline (LC) layer made up of semifluorinated (SF) single side groups [-CO-(CH2)(x-1)-(CF2)(y)F] (single SF groups) attached to polyisoprene homopolymer or the isoprene block of a styrene-isoprene diblock copolymer was determined by analyzing the partial electron yield C-edge NEXAFS signal. The results show that the surfaces of thin SF polymer films are covered with a uniform layer, consisting of the SF-LC groups whose average -CF2- tilt angle with the surface normal lies in the range 29-46 degrees. This is in direct contrast to the bulk, where the directors of the SF-LC mesogens are aligned parallel to the polystyrene/SF-polyisoprene interface of the block copolymers. This average tilt angle increases with increasing the length of the -(CH2)(x-1)- group (x increases) but decreases with increasing the length of the -(CF2)(y)- part of the molecule (y increases) at constant x.