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 1 H spin-exchange, double-quantum, and 1 H− 29 Si heteronuclear MAS NMR spectroscopy. 1 H 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 1 H NMR chemical shifts. 2D 1 H spin-diffusion experiments were used to determine the 1 H− 1 H 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 1 H 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 1 H− 29 Si heteronuclear experiments.