A direct comparison of surface and bulk chain-relaxation in polystyrene

Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy was used to measure simultaneously the relaxation rates of polystyrene (PS) molecules at the free surface and in the bulk. The samples were uniaxially stretched and annealed at temperatures below the bulk glass transition temperature of PS. The surface and bulk chain relaxation was monitored by measuring the partial-electron and the fluorescence NEXAFS yields, respectively, both parallel and perpendicular to the stretching direction. The decay of the optical birefringence was also measured to provide an independent measure of the bulk relaxation. Relaxation of PS chains was found to occur faster on the surface relative to the bulk. The magnitude of the surface glass transition temperature suppression over the bulk was estimated based on the information on the temperature dependence of the rates

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-A to the isotropic phase.This research was supported by the Office of Naval Research, Grant No. N00014-92-J-1246 and the Division of Materials Research, NSF Polymers Program, Grant DMR92-23099. Partial support from Division of Materials Research, NSF Polymer Program, Grant DMR93-214573, is also appreciated

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