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−(CH 2 ) x - 1 −(CF 2 ) 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 −CF 2 − tilt angle with the surface normal lies in the range 29−46°. 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 −(CH 2 ) x - 1 − group ( x increases) but decreases with increasing the length of the −(CF 2 ) y − part of the molecule ( y increases) at constant x .