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Monitoring of the Degree of Condensation in Thin Alkoxysiloaxane Layers by NIR Reflection Spectroscopy

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Introduction
Inorganic-organic hybrid polymers are widely applied for high grade coatings, adhesives and planarization layers in various areas of applications. Common precursors are functionalized alkoxysilanes. Generally, the preparation of such networks proceeds via a combined hydrolysis / condensation process of alkoxysilane-based materials in a sol-gel process and consecutive curing, which adjusts the inorganic Si-O-Si network for optimal material properties.
We intended to develop a novel analytical method for monitoring the state of the formation of the inorganic network, which has the potential for in-line monitoring of technical deposition processes. The statistical distribution of the number of Si−O−Si bonds per Si atom is a mea-sure for the current state of the condensation process. 29Si NMR spectroscopy is the only technique that is able to directly determine the ratio of these different Ti structures. However, it is obvious that this method is inapplicable for direct process control. Rather, there is a need for a cost-efficient and fast alternative. In this study, it was tested if NIR spectroscopy was suitable to predict the ratio of Ti species in thin layers.

Experimental
Siloxane batches with different degrees of condensation were prepared from 3-methacryloxypropyltrimethoxysilane. Analytical data of the percentages of Ti species in these precursors were obtained by 29Si NMR spectroscopy, which served as reference values for the calibration of the NIR spectra. Calibration samples were made by printing the precursors on polymer film with a thickness of 2.20±0.05 g/m². The methacrylate functionalities were crosslinked by UV irradiation.
NIR spectra were recorded with a process spectrometer equipped with a separate probe head and an InGaAs photodiode array detector covering a spectral range from 1100 to 2200 nm. NIR spectra were taken in transflection mode using a ceramic reflector and a diffuser plate in front of the probe head to suppress interferences. Quantitative analysis of the spectral data with respect to concentrations of various siloxane structures was carried out with chemometric approaches based on the PLS2 algorithm.

Results and Discussion
The state of the formation of the inorganic Si−O−Si network in UV-cured alkoxysilane-based layers was determined by NIR reflection spectroscopy. In particular, the relative concentrations of the species T1 to T3 were predicted simultaneously from each recorded spectrum. The root-mean-square error of prediction (RMSEP) for the determination of the ratios of the Ti species was found to be less than 3 %. The error of the reference data from 29Si NMR spectroscopy was 4 %, which resulted in an overall error of 5 %.
In a further study, the thickness of the siloxane layers on the polymer film was determined by a similar procedure using the PLS1 algorithm and gravimetry as reference method. Coating weights in the range from 2.5 to 5.5 g/m² were predicted with a RMSEP of ∼ 0.3 g/m².