Neutron scattering experiments were performed on three molecular weight pairs of symmetric, isotopic blends of poly(dimethylsiloxane) (PDMS) of near-critical composition. Scattering data covering close to 3 decades in size were globally fit using the random phase approximation (RPA) and the Debye function for Gaussian polymer coils using the interaction parameter, χ, and statistical segment length, b, as free parameters. These wide q range fits differ from the standard, narrow q range RPA fits in that the power-law scaling regime and exponential decay regimes, related to b, are accounted for. Values for χ showed a well-behaved linear dependence on inverse temperature. Critical temperatures were estimated from these data. Direct observations of the miscibility limit, through neutron cloud points, were made in several cases which agree to some extent with the extrapolated critical points. Monotonic dependencies in temperature of the coil expansion factor, α, as calculated from the statistical segment length, were observed. Under the assumption that the thermal dependence of α can be described in a Flory−Krigbaum form, this offers a second measure of the critical point in these blends. If coil expansion is accounted for in this way, the noncombinatorial entropic component of χ is observed to vanish in the high-molecular-weight limit in keeping with a Flory−Huggins/Hildebrand description of χ as B/T. The molecular weight dependence of χ supports the view that, after accounting for coil expansion, end-group effects are the sole source of noncombinatorial entropy in this model system.

No abstract

No abstract

Surface-enhanced Raman scattering (SERS) was observed from thin films formed by 1,4 and 1,3dinitrobenzene (DNB) spin-coated onto silver island films from dilute solutions. During laser irradiation of 1,QDNB films on silver substrates at relatively high laser powers, new bands appeared near 1152, 1350, 1412 and 1460 cm-' and bands near 1118, 1358 and 1372 cm-' decreased in inensity. A band near 1598 cm-' shifted to near 1605 cm-'. The bands near 1152 and 1412 cm-' were assigned to stretching modes of +N and -N=N-bonds of azo compounds, respectively, indicating that polymerization of 1,QDNB had occurred. The extent of polymerization was greatest when the surface coverage of adsorbed 1,dDNB was greatest awl when the SERS experiments were carried out in a nitrogen atmosphere, indicating that reducing conditions were required for the polymerization. No polymerization was observed for 1,3-DNB under similar circumstances. The difference was attributed to orientation of the adsorbed species. It was concluded that 1,4-DNB was adsorbed with a horizontal coofiguration in which the aromatic rings were parallel to the substrate surface and both nitro groups were in contact with the surface while 1,3-DNB was adsorbed with a vertical configuration in which the rings were perpendicular to the surface and only one nitro group was in contact with the surface.

No abstract