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 an d within the bulk of thin polystyrene films were studied by ultra soft X-ray absorption spectroscopy above the carbon K edge. The polystyren e films, which were produced by spin casting from solvents, were of va rying thicknesses (1.5 to 100 nm) and molecular weights (3.8 kDa to 10 00 kDa). Electron-yield and fluorescence-yield ultra soft X-ray absorp tion 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 polys tyrene films. For the films studied, the pendant phenyl groups were fo und to be preferentially oriented toward the surface normal, independe nt of casting solvent, molecular weight, and film thickness. Additiona lly, the preferential orientation of the phenyl rings was slightly enh anced 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 indic ate that the intramolecular forces may be more important than the inte rmolecular forces in determining polymer conformations at the surface and in bulk polystyrene. (C) 1998 Elsevier Science B.V. All rights res erved.