Two-dimensional solid state NMR characterization of physisorbed siloxane polymer (OV-225) on silica

Physisorbed cyanopropyl-methyl-phenyl-methyl-siloxane polymer on a silica surface was characterized by one- and two-dimensional solid state NMR techniques including heteronuclear proton-silicon correlation spectroscopy. Spin-lattice relaxations of protons of the siloxane polymer exhibited only small changes upon anchoring to the silica surface indicating somewhat altered molecular dynamics of proton moieties that contribute to the relaxation process. However, the same relaxation rates of the siloxane polymer's silica atoms were reduced due to restricted mobility of the polymer. Proton-silicon heteronuclear correlation spectroscopy (HETCOR) revealed strong correlations of silanol protons with both Q3 and Q4 sites of the silica surface. In addition, a correlation between methyl protons and the Q3 site of the silica surface was observed when HETCOR experiments with very small mixing time (5 ms) were performed. The presence of these correlations is indicative of the coherent magnetization transfer mainly through dipolar mechanisms. Since magnetization transfer through the dipolar mechanism is 1/r3 dependent, methyl protons must lie in close proximity to the silica surface. Hydrogen bonding of the silica surface's hydroxyl protons with the bridging oxygen of the siloxane polymer is most likely responsible for positioning the methyl protons closer to the surface. Additional correlations between 29Si nuclei and methylene protons next to cyano group was also observed with mixing time indicating the closer proximity of these protons to the silica surface as well. This juxtaposition of methylene protons is most likely due to hydrogen bonding of the siloxane polymer through the cyano moiety. Furthermore, the hydrogen bonding through the cyano group is most likely to be in parallel orientation to the surface. Finally, the aromatic protons exhibited weak correlations only with Q3 sites, indicating that these protons must also lie in close proximity of the silica surface.