Surface Orientation of Polystyrene Based Polymers:Steric Effects from Pendant Groups on the Phenyl Ring.

Near edge X-ray absorption fine structure (NEXAFS) coupled withmolecular dynamics simulations were utilized to probe the orientationat the exposed surface of the polymer film for polystyrene type polymerswith various pendant functional groups off the phenyl ring. For allthe polymers, the surface was oriented so that the rings are nominallynormal to the film surface and pointing outward from the surface.The magnitude of this orientation was small and dependent on the sizeof the pendant functional group. Bulky functional groups hinderedthe surface orientation, leading to nearly unoriented surfaces. Depthdependent NEXAFS measurements demonstrated that the surface orientationwas localized near the interface. Molecular dynamics simulations showedthat the phenyl rings were not oriented strongly around a particular“average tilt angle”. In contrast, simulations demonstratethat the phenyl rings exhibit a broad distribution of tilt angles,and that changes in the tilt angle distribution with pendant functionalitygive rise to the observed NEXAFS response. The more oriented samplesexhibit a higher probability of phenyl ring orientation at anglesgreater than 60 degrees relative to the plane of the films surface. [ABSTRACT FROM AUTHOR]