Sum frequency (SF) vibrational spectra in the C-H stretching region of polydimethyl siloxane (PDMS) and of the comb copolymer cetyl dimethicone copolyol (CDC), consisting of a PDMS backbone with grafted poly(ethylene oxide) and cetyl side chains, have been recorded in air after deposition onto a gold-coated substrate. The polymers were deposited over a range of thicknesses (up to 70 nm) by spin coating from chloroform solutions of different polymer concentrations. Film thicknesses were determined by ellipsometry. The methyl symmetric (r + ) stretching modes appeared as peaks in the SF spectra of both polymers at all film thicknesses investigated, indicating that the constituent methyl groups have a net orientation toward the air. However, the phase of the methyl anti-symmetric (r -) stretching mode displayed a dependence upon film thickness, changing from a peak (positive phase) to a dip (negative phase) as film thickness was increased. The phase behavior of the rmethyl resonance has been successfully modeled by the extension of a previously developed interference theory to include multiple reflections and a resonant contribution from the polymer/ gold interface.

The sum frequency (SF) vibrational spectrum in the C−H stretching region of a comb copolymer, cetyl dimethicone copolyol (CDC), consisting of a poly(dimethylsiloxane) backbone and grafted poly(ethylene oxide) (PEO) and cetyl side chains has been recorded after deposition on a gold-coated substrate. Resonant assignment of the SF spectrum of CDC has been achieved by examining the spectra of several structural analogues of the polymer. Since sum frequency generation is dependent upon both infrared and Raman activity, assignment of the SF spectrum of CDC was aided by recording complementary infrared and Raman spectra. Sum frequency resonances attributable to the methyl groups of the polymer backbone and the cetyl side chains of the polymer were observed, implying conformational order of these moieties. Further, it was concluded from the phase of the cetyl resonances that the grafted alkyl chains were oriented away from the polymer/air interface. Conversely, the PEO side chains of CDC were found to be SF inactive.

A novel biologically relevant composite substrate has been prepared consisting of a calcium phosphate (CaP) layer formed by magnetron sputter-coating from a hydroxyapatite (HA) target onto a gold-coated silicon substrate. The CaP layer is intended to mimic tooth and bone surfaces and allows polymers used in oral care to be deposited in a procedure analogous to that used for dental surfaces. The polymer cetyl dimethicone copolyol (CDC) was deposited onto the CaP surface of the substrate by Langmuir Blodgett deposition, and the structure of the adsorbed layer was investigated by the surface specific technique of sum frequency generation (SFG) vibrational spectroscopy. The gold sublayer provides enhancement of the SFG signal arising from the polymer but plays no part in the adsorption of the polymer. The surface morphology of the substrate was investigated using SEM and AFM. The surface roughness was commensurate with that of the thermally evaporated gold sublayer and uniform over areas of at least 36 mum(2). The chemical composition of the CaP-coated surface was determined by FTIR and TOF-SIMS. It was concluded that the surface is primarily calcium phosphate present as a mixture of amorphous, non-hydroxylated phases rather than solely stoichiometric hydroxyapatite. The SFG spectra from CDC on CaP were closely similar, both in resonance wavenumbers and in their relative intensities, with spectra of thin films of CDC recorded directly on gold. Application of previous analysis of the spectra of CDC on gold therefore enabled interpretation of the polymer orientation and conformation on the CaP substrate.