Monitoring
of the Degree of Condensation in Alkoxysiloxane
Layers by NIR Reflection Spectroscopy
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Abstract
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 <sup>29</sup>Si NMR spectroscopy.
Results were expressed in terms of different relative ratios of the
T<sup>i</sup> 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<sup>–2</sup>. The root-mean-square error of prediction (RMSEP) for the determination
of the ratio of the T<sup>i</sup> species from the NIR spectra was
found to be less than 3%. The error of the reference data from <sup>29</sup>Si 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<sup>–2</sup> using
gravimetry for calibration (prediction error ∼0.3 g m<sup>–2</sup>)