Photopatternable hybrid sol–gel films: A liquid 29Si NMR investigation of the inorganic network formation

Methacryloxypropyltrimethoxysilane precursor has been involved in the realization of optical elements in crack free thick films (ranging from 15 to 100 μm), through spatially controlled photopolymerization. First, alkoxysilane functions were partially hydrolyzed and condensed. Then, using a photoinitiator, free radical photopolymerization was proceeded by irradiating the sample under UV or visible light. Since an organic network is formed in the matrix of the primarily formed inorganic network, understanding the formation of the silicate backbone was of first importance to ensure the creation of crack free thick films through efficient polymerization.Liquid 29Si NMR spectroscopy was used to investigate the inorganic network formation. Material preparation required evaporation of the volatile solvents released by 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 dried sols were investigated and compared to non-dried sols. NMR peak fitting pointed out to the presence of a large variety of cyclic and linear oligomers in the sol. The structure of the dried sol appeared to depend both on the aging time and on the storage temperature. All these results have to be taken into account when the condensation degree has to be limited for specific optical applications.