Structural control in germania hybrid organic-inorganic materials

The preparation of highly homogeneous hybrid organic-inorganic materials with enhanced functional properties requires a deep understanding of the synthesis process and a high control of the material structure. To achieve this goal it is necessary to apply advanced characterization techniques that can elucidate all the different features of a structure that is complicated by the presence of interconnected organic and inorganic species that cooperate in the building of a complex hybrid network. We have used as a case study a sol-gel synthesized germania-silica hybrid organic-inorganic material with a content of germania up to 30 % in molar concentration with respect to silica. The material has been prepared using 3-glycidoxypropyltrimethoxysilane (GPTMS) as the organically modified alkoxide. Solid-state nuclear magnetic resonance (NMR) spectroscopy, coupled with infrared and Raman spectroscopies, has been used to investigate the structure of the material. In the samples prepared without germania or with a germania content lower than 20 % in molar concentration, the epoxy ring is preserved and some residual mobility is observed in the organic terminal part of GPTMS. When the amount of germania exceeds this limit a significant effect on the epoxy opening was observed, with almost 70 % of the epoxy groups that have reacted during the synthesis. The different characterization techniques have been used to obtain a semiquantitative comparative evaluation of the amount of epoxides. C-13 CP MAS NMR experiments have shown that the epoxy groups give rise to the formation of short polyethylene species. The residual mobility of GPTMS has allowed the use of NMR techniques initially developed for liquids that have brought a better understanding of the hybrid structure. This has been done, in particular, using indirect scalar couplings (1)J(C-H) for H-1 -> C-13 polarization transfers.