Measurement of Trace Silanol in Siloxanes by IR Spectroscopy

Abstract

Two novel approaches for the measurement of trace SiOH moieties in siloxane polymers by IR spectroscopy are described. Both methods are based on the idea of selective spectral modification of SiOH absorbance bands so that the silanol absorbance bands can be isolated in the presence of much larger, overlapping background absorbances. Modification of the SiOH groups is made to occur either through H-bonding using mixed solvent systems or through deuteration resulting from isotope exchange in the presence of excess D₂O. Both techniques have high specificity for SiOH groups over other portions of the molecules and produce only minimal changes in the IR absorbances due to non-silanol moieties. Subtraction of a modified sample spectrum from an unmodified spectrum then allows the SiOH absorbances to be isolated and quantitatively measured to low concentrations. These techniques allow one to circumvent a major limitation of IR spectroscopy for trace analysis, namely, the need to rely on a separate reference material for the removal of interfering background absorbances. Of the two methods described in this paper, the deuteration approach is superior in terms of ease and sensitivity. Measurements can be made down to 10 ppm (wt OH/wt sample), which is close to the noise level of the spectral measurement.