Reflection-absorption Fourier transform infrared spectroscopic study of transferred films of poly(dimethylsiloxane) using the Langmuir-Blodgett technique

Reflection-absorption Fourier transform infrared spectroscopy (RA-FTIR) has been utilized to investigate the tertiary structures of a series of poly(dimethylsiloxane) (PDMS) films transferred to silver substrates using the Langmuir-Blodgett technique. A surface pressure-area isotherm of PDMS of molecular weight of 2400 Da with low polydispersity (PDI of 1.09) was measured on a water subphase at 20 degrees C. Films were transferred onto silver substrates at various surface pressures associated with regions on the isotherm where structural changes are indicated. The infrared data from the films indicated the structural changes identified by analysis of the isotherm could be related to polymeric backbone orientational adjustments. This was evidenced by relative intensity changes of the asymmetric and symmetric Si-O-Si stretches located at 1050 and 1110 cm(-1), respectively Infrared data from the films also showed increases in absorption consistent with varying film thickness. Ratios of the peak intensity of 1265 cm(-1) for the various films were compared with theoretical and experimentally observed film thicknesses reported by Fox, Taylor, and Zisman.(1) There was a positive correlation between the peak intensity ratios and structural thickness ratios reported by these authors which supports the proposed structures of caterpillar, zigzag, and helical coil. Furthermore, we report new infrared data supporting the model of structural change in the isotherm proposed by Lenk and Koberstein(2) based on the formation of helical coils with varying number of monomeric units per turn which form based on the surface pressure.