Neutron diffraction on polymer nanocomposites - A tool for structural and orientation studies

A series of Polyvinyl alcohol (Mowiol 5-88) – Bentonite nanocomposite films with predefined clay loading (up to 0-20%), were prepared via solvent casting technique. The developed films, due to the favourable polymer-particle interactions, revealed excellent dispersion of the clay particles in the polymer matrix and improved properties. Furthermore, the properties of PVA/clay nanocomposites as well as their structural changes as a function of the relative humidity were thoroughly investigated using neutron membrane diffraction experiments. The samples prior their measurement were equilibrated at different relative humidity levels (%RH) using either H2O or D2O. The application of contrast variation technique enabled us to investigate the contribution of both the polymer and the clay particles to the diffraction spectra. Thus, the use of H2O enlightened the low Q region, providing information about the structure of the inorganic phase and specifically the stacking of the clay platelets. The diffraction patterns in this region obtained from perpendicular and in-plane sample positions revealed that there is a specific orientation of bentonite plates, parallel to the film surface. This conclusion is in agreement with the results obtained from XRD and gas permeability technique, in which the well organized and dispersed impermeable inorganic layers, increase the resistance in flow through the nanocomposites film, acting as gas barriers. On the other hand, diffraction experiments on pre-equilibrated with D2O samples revealed the structural changes in polymeric matrix, due to hydration. The obtained peak revealed the presence of a new crystalline phase, presumably induced by the presence of the silicates, which is in agreement with DSC data reported in previous studies.