Evaluation of opto-mechanical properties of UV-cured and thermally-cured sol-gel hybrids monoliths as a function of organic content and curing process

We fabricated sol-gel monoliths using a fast sol-gel (FSG) technique with either UV-curing or thermal-curing, producing highly dense, stable monoliths in a short time. This paper focuses on the preparation and the characterization of different FSG compositions as a function of the inorganic-organic molar-ratio and curing-mechanism. The fabrication of the monoliths was based on three precursors, tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMS), and methacryloxypropyl-trimethoxysilane (MAPTMS). Three series of FSG monoliths with different material compositions were synthesized. The organic content in the final monoliths varied from 17 to 52 weight percent (wt%). We established the range of precursor's molar-ratios that permit formation of stable low-organic content optical monoliths by UV-curing. The physical properties of the monoliths formed under different curing conditions were examined. The refractive-index increased with increased thermal-curing temperature and for combined thermal and UV-curing. On the other hand, the coefficient of thermal expansion (CTE) increases with the organic content for thermally-cured samples and decreases for UV-cured samples, due to full crosslinking. Additional important physical properties for sol-gel applications such as weight-loss, porosity and adhesive strength are also presented. The results obtained give guidelines for fabricating useful optical monoliths with lower CTE using a UV-curing process for applications such as optical bonding and encapsulation.