Hydrogen-bond interactions in organically-modified polysiloxane networks studied by 1D and 2D CRAMPS and double-quantum 1H MAS NMR

Hydrogen-bonding interactions, distribution of various hydroxy groups, and surface morphology in organically modified polysiloxane networks were studied by solid-state NMR techniques based on H-1 spin-exchange, double-quantum, and H-1-Si-29 heteronuclear MAS NMR spectroscopy. H-1 CRAMPS experiments revealed four main types of OH groups differing in hydrogen-bond strength, order, and dynamics, which are mutually dipolar-coupled (their interatomic distances are not larger than 0.5 nm), however, not involved in fast chemical exchange even in the fully hydrated state at room temperature. The resulting hydrogen-bonding network is inhomogeneous in the entire set of hydroxyl groups. These findings were correlated with the quantum chemical geometry optimization of hydrogen-bonded local structures and subsequent calculations of H-1 NMR chemical shifts. 2D H-1 spin-diffusion experiments were used to determine the H-1-H-1 interatomic distances and to probe the average size of OH clusters, which is ca. 1-2 nm. Intimate mixing of strongly hydrogen-bonded OH and methyl groups was confirmed by 2D double-quantum H-1 MAS NMR spectra. Hydrogen-bonding strength of various hydroxyl clusters was evaluated with respect to the type of siloxane structure units and geometry of the siloxane matrix by 2D H-1-Si-29 heteronuclear experiments.