Dynamic studies of Ormosil membranes

Abstract

With the help of nuclear magnetic resonance (NMR) and quasi-elastic neutron scattering (QENS) we have carried out dynamic studies of Ormosil membranes presenting protonic conductivity properties. These nanocomposite hybrid materials prepared by sol–gel process are based on an organic–inorganic network [polymer chains (poly(propylene glycol), PPG) and (poly(dimethylsiloxane), PDMS) crosslinked by silica clusters] in which polyoxometalates (POM, also named heteropolyacid) are dissolved. Selective deuterium exchange of mobile protons was performed, allowing to probe selectively the dynamics of the protons and the polymer, respectively by 1H and 2H NMR. It confirmed that chemical structure of the polymer matrix plays a crucial role in the properties of water incorporated in the membrane, which agrees well with QENS results. Hydrophobic matrix composed of PDMS chains allows only dispersion of POM clusters inside the matrix, resulting in water mobility behavior similar to thus of pure POM. On the contrary, PPG-silica host dissolves well the POM units and consequently alters water mobility when they are incorporated in the hybrid material. For polymeric materials with numerous hydrogen content, quasi-elastic neutron scattering is dominated by the large incoherent cross section of the hydrogen atoms. QENS studies can provide both temporal and spatial information on the diffusive processes taking place in these membranes. Diffusion coefficients of 2.0×10−6cm2/s and 1.8×10−6cm2/s at room temperature were obtained for pure PDMS and PPG polymer-silica hosts, respectively.