INFRARED DICHROISM AND SURFACE CONFORMATIONAL DYNAMICS OF ADSORBED POLY(DIMETHYLSILOXANE)

We examined polymer conformations at a strongly adsorbing solid surfac e and their evolution with elapsed time during the adsorption process. Poly(dimethylsiloxane) (PDMS: M-w = 118 000; M-w/M-n = 1.16) was allo wed to adsorb onto oxidized germanium from dilute carbon tetrachloride solutions. The mean backbone orientation was found to change even aft er the mass adsorbed had saturated; its conformation was determined us ing infrared dichroism spectroscopy in attenuated total reflection (FT IR-ATR). The dichroic ratio (D) of the Si-O-Si and Si-CH3 stretch vibr ations showed the PDMS backbone to be preferentially horizontal to the surface (D congruent to 0.9 at early adsorption times) and to grow in creasingly flattened with increasing adsorption time. However, as the adsorbed amount increased with higher solution concentrations, the ave rage orientation was less flattened. Chain flattening during the adsor ption process was also slower, the higher the adsorbed amount, reflect ing molecular crowding within more densely occupied layers. During des orption into the pure solvent, the evolution of the dichroic ratio cou ld be described by the hypothesis of a bimodal configurational distrib ution.