Understanding the Microstructure Connectivity in Photopolymerizable Aluminum-Phosphate-Silicate Sol-Gel Hybrid Materials for Additive Manufacturing

In this paper, we report the synthesis and structural characterization of transparent and photopolymerizable aluminum-phosphate-silicate hybrid materials obtained via the sol-gel route, with different aluminum/phosphate (Al/P) ratios. We explored the system Si(1-x)-(Al/P) (x) with x varying from 0.3 to 1, and atomic ratios of Al/P are 1:3, 1:1, and 3:1. All compositions contain high inorganic mass content (up to 40 wt %). Furthermore, they are compatible with vat-photopolymerization platforms. The structural evolution of the hybrid materials with the silicon concentration was investigated by SEM, phase-contrast AFM, and solid-state NMR techniques, using single-and double-resonance experiments. The structure follows the build-up principle using aluminum phosphate species and alkoxysilane chains as fundamental building blocks. These aluminum-phosphate species were identified as monomeric and dimeric chain structures by comparing different parameters obtained from NMR data to compound models. Monomeric and dimeric aluminum-phosphate chain structures were predominant in 3:1 and 1:3 Al/P ratio samples, respectively, promoting and hindering the heterocondensation with the alkoxysilane precursor, respectively. The photopolymerization mechanism leads to the percolation of the inorganic networks through a parallel polymethylmethacrylate network, resulting in a material with structural heterogeneities in the range of 5 nm, evidenced by phase-contrast AFM.