Solid-state nuclear magnetic resonance characterization of PE-PEG/silica hybrid materials prepared by microwave-assisted sol-gel process

Organic-inorganic hybrid materials were prepared by sol-gel processes starting from tetraethoxysilane (TEOS) and a triethoxysilane-terminated polyethylene-b-poly(ethylene glycol) copolymer (PE-PEG-Si). Curing of the initial reactant solution was carried out under different reaction conditions, and the materials so obtained were investigated by solid-state nuclear magnetic resonance (NMR). In particular, the molecular structure resulting from a conventional oven heating was compared with that obtained by unconventional microwave heating. The results highlighted that the extent of condensation reactions occurring over several hours under conventional heating is very similar to that resulting in 1 min under microwave heating. Additionally, Si-29-magic angle spinning (MAS) spectra showed that even though the overall extent of cross-linking in the inorganic network is only slightly affected by the thermal history of the sample, significantly different distributions of silicon sites can be present. C-13-CP/MAS selective spectra revealed the presence of PE "crystalline" domains within the organic phase, not detectable by differential scanning calorimetry (DSC). Finally, H-1-MAS spectra showed that different hydrogen-bond interactions are present in samples obtained under different curing conditions.