FTIR-ATR studies on polymer/polymer and polymer/liquid interfaces.

Abstract

Fourier Transform Infrared Attenuated Total Reflection Spectroscopy (FTIR-ATR) has been employed to study polymer laminates composed of poly (methylmethacrylate) (PMMA) and poly(vinyl alcohol) (PVOH) using different base layer thicknesses and different angles of incidence on a ZnSe substrate. By varying the "barrier" layer of PMMA it has been confirmed that different effective penetration depths into the PVOH are achieved, in very good agreement with the calculated electric field as a function of distance away from the substrate surface. These results show that the two-layered model system can be successfully employed to provide a semi-quantitative depth profile of the laminate. Following the FTIR-ATR study on polymer/polymer interface we have demonstrated how the evanescent field absorption from a IR beam can also be used to monitor in situ the molecular interaction at polymer/liquid interfaces. The chosen system was a series of sulphonated polyethersulphones (SPES) and water. The v(_s)(SO(_3)) and V(_a)(SO(_3)) vibrational modes were monitored as the degree of hydration was varied. It has been found that the v(_3)(SO(_3)) mode increases with hydration suggesting that, with hydration the sulphonic acid groups of SPES are dissociated into SO(_3) ions. It has been found that changes also occur in the benzene ring vibrational band that are dependent on the degree of hydration. Spectral subtraction has been employed to highlight changes in the V(_a)(SO(_3)) mode which were not readily observed in the raw spectra due to overlapping bands. The particular behaviour of V(OH) vibrational mode of water molecules has also been analysed. The main conclusion in this case was that the average hydrogen bond strength of the sorbed water is considerable lower than that in pure water. This would favour an efficient flux of water through the membrane. A method based on monitoring the time dependent change in the v(OH) mode of water has been developed to calculate the diffusion of water on SPES membranes. After reaching a steady state, the normalised absorbance plot versus time has been used for numerical evaluation. Two models have been developed, one for Fickian diffusion and another for the dual sorption mode. The experimental results show a much better agreement for the second model. The diffusion coefficient values have been calculated for different samples and interesting variations of the diffusion coefficient as a function of sulphonation level, solvent and film thickness have been analysed in detail.