Infrared dichroism and surface conformational dynamics of adsorbed poly(dimethylsiloxane)

We examined polymer conformations at a strongly adsorbing solid surface and their evolution with elapsed time during the adsorption process. Poly(dimethylsiloxane) (PDMS: M_w = 118 000; M_W/M_n = 1.16) was allowed to adsorb onto oxidized germanium from dilute carbon tetrachloride solutions. The mean backbone orientation was found to change even after the mass adsorbed had saturated; its conformation was determined using infrared dichroism spectroscopy in attenuated total reflection (FTIR-ATR). The dichroic ratio (D) of the Si-O-Si and Si-CH₃ stretch vibrations showed the PDMS backbone to be preferentially horizontal to the surface (D ≅ 0.9 at early adsorption times) and to grow increasingly flattened with increasing adsorption time. However, as the adsorbed amount increased with higher solution concentrations, the average orientation was less flattened. Chain flattening during the adsorption process was also slower, the higher the adsorbed amount, reflecting molecular crowding within more densely occupied layers. During desorption into the pure solvent, the evolution of the dichroic ratio could be described by the hypothesis of a bimodal configurational distribution.