Investigation of inorganic network formation in photopatternable hybrid sol-gel films by Si liquid NMR

Liquid ²⁹Si NMR spectroscopy was used to investigate the sol-gel process of methacryloxypropyltrimethoxysilane. This hybrid precursor was involved in the realization of optical elements in laminated crack-free thick films (up to 100 μm), through spatially controlled photopolymerization. Understanding the formation of the inorganic network was of first importance to insure the creation of crack-free photopatterned thick films in a laminated configuration. Material preparation required evaporation of the volatile solvents released during the sol-gel process and limitation of the condensation degree. Both conditions were achieved by a drying process at room temperature. The structure and the composition of the sols were investigated and compared to non-dried sols. Evolution of inorganic species distribution was also studied under increasing aging time or storage temperature. NMR peak fitting of T1 species gave fruitful information on the sol structure evolution during the sol-gel process. It pointed out the presence of a large variety of oligomers in the sol. The study also allowed the identification of more constraint cyclic species in dried sols stored at room temperature. Their amount significantly decreased when increasing the storage temperature and was attributed to ring opening of cyclic species. Consequently, the structure of the dried sol will depend both of the aging time and of the storage temperature. All these results have to be taken into account when the degree of condensation has to be limited to achieved photopatternable hybrid layers for specific optical applications.