Monitoring of the Degree of Condensation in Alkoxysiloxane Layers by NIR Reflection Spectroscopy

This paper introduces a novel analytical approach for monitoring the degree of condensation of thin siloxane films, which is potentially suitable for in-line process control during the deposition of such layers, e.g., to polymer films. Near-infrared (NIR) reflection spectroscopy in combination with chemometric methods was used as a process monitoring tool. The state of the formation of the inorganic Si–O–Si network in partially condensed 3-methacryl­oxypropyl­trimeth­oxysilane batches was analyzed by inverse gated ²⁹Si NMR spectroscopy. Results were expressed in terms of different relative ratios of the Tⁱ species (i.e., structures with different numbers of Si–O–Si units per Si atom). These data were used for calibration of the NIR method, which was applied to thin layers printed on a polymer foil with a thickness of ∼2.2 g m^–2. The root-mean-square error of prediction (RMSEP) for the determination of the ratio of the Tⁱ species from the NIR spectra was found to be less than 3%. The error of the reference data from ²⁹Si NMR spectroscopy is 4%, which results in an overall error of 5%. Moreover, the thickness of siloxane layers was determined by this method in a range from 2.5 to 5.5 g m^–2 using gravimetry for calibration (prediction error ∼0.3 g m^–2).