Functional group orientation in surface and bulk polystyrene studied by ultra soft X-ray absorption spectroscopy

The composition and orientation of functional groups at the surface and within the bulk of thin polystyrene films were studied by ultra soft X-ray absorption spectroscopy above the carbon K edge. The polystyrene films, which were produced by spin casting from solvents, were of varying thicknesses (1.5 to 100 nm) and molecular weights (3.8 kDa to 1000 kDa). Electron-yield and fluorescence-yield ultra soft X-ray absorption spectroscopy were used to probe simultaneously the surface (less than or equal to 3.6 nm) and the bulk (similar to 300 nm) of the polystyrene films. For the films studied, the pendant phenyl groups were found to be preferentially oriented toward the surface normal, independent of casting solvent, molecular weight, and film thickness. Additionally, the preferential orientation of the phenyl rings was slightly enhanced in the bulk versus the surface. The preferred orientation of the phenyl groups implies an orientation of the polymer backbone that is preferentially parallel to the surface of the films. The results indicate that the intramolecular forces may be more important than the intermolecular forces in determining polymer conformations at the surface and in bulk polystyrene. (C) 1998 Elsevier Science B.V. All rights reserved.