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)yF] (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°. 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.This research was supported by the Office of Naval Research, Grant No. N00014-92-J-1246. Partial support from Division of Materials Research, NSF Polymer Program, Grants No. DMR92-23099 and DMR93-214573, is also appreciated. The work at NC State University was supported by the NCSU COE start-up funds and the NSF CAREER award, Grant No. DMR98-75256. The authors thank Dr. B. Glu¨ sen and Ms. S. Yang (Cornell University) for their assistance during the course of the NEXAFS experiments. K.C. greatly acknowledges the financial support from the LG-Yonam Foundation for a sabbatical visit to Cornell University (1997-1998). NEXAFS experiments were carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. This work made use of MRL Central Facilities at UCSB supported by the National Science Foundation under Grant No. DMR96-32716