Pa. Drake et Pw. Bohn, LOCALIZED RAMAN-SCATTERING PROBES OF MOLECULAR-SCALE MOTIONS IN CASE-II SWELLING OF POLYSTYRENE IN N-HEXANE, Analytical chemistry, 66(1), 1994, pp. 79-84
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.