LOCALIZED RAMAN-SCATTERING PROBES OF MOLECULAR-SCALE MOTIONS IN CASE-II SWELLING OF POLYSTYRENE IN N-HEXANE

Surface-enhanced Raman scattering (SERS) active metal island films pla ced within micrometer-sized polymer sandwich thin film structures were explored as molecular-level probes of case II diffusion events for n- hexane swelling of polystyrene. The fabrication and use of these struc tures in an internal reflection geometry is detailed. A comparison of Ag and Au films indicates that strong SERS signals are obtained from A u island structures interspersed to various depths within polystyrene, while simultaneously avoiding photooxidation problems typically encou ntered with Ag-enhancing structures. The physical spread of the Au isl and structure in the depth direction was determined to be less-than-or -equal-to 200 angstrom by Auger depth profiling and physical profilome try. Combining this physical spread and the SERS-distance dependence d (1/e) almost-equal-to 50 angstrom yields a spatial resolution for this technique of less-than-or-equal-to 300 angstrom. Although the SERS fe atures of the solvent within the swollen polymer film were generally s ubstantially lower than those of the polystyrene, in situ observation of permeant progression was possible for unannealed samples. Large cha nges in the vibrational spectrum with solvent influx were Doted for th e polystyrene features at 1070 and 1371 cm-1. In particular, changes i n the latter band are interpreted in terms of an increase in chain mob ility, with solvent passage, which allows the pendant phenyl moieties to align parallel to the semimajor axes of the oblate spheroidal parti cles, thus increasing their effective scattering cross section. Interp retation of the time course of solvent front passage is hampered by cr aze formation, but it is clear that polymer relaxation continues long after the solvent signal has saturated.