One-dimensional structure of exfoliated polymer-layered silicate nanocomposites: A polyvinylpyrrolidone (PVP) case study

Classical modeling of powdered lamellar structures by X-ray diffraction (XRD) only applies to periodic or quasi-periodic structures and another approach is needed for non-periodic structures. XRD patterns of the non-periodic structures contain a relatively small amount of information and therefore certain initial assumptions are necessary. In case of exfoliated polymer-clay mineral nanocomposites it is possible to assume the chemical composition and the structure of the clay mineral layer while the actual structure of the polymer remains unknown. This paper offers an approach which can be used to provide an approximate solution for the structure of the polymer. This new approach is based on modeling of the LpG(2) factors, recorded from oriented samples in order to obtain the one-dimensional structure of the polymer. Although LpG(2) factors for various smectites are quite different, in the case of polyvinylpyrrolidone (PVP) adsorbed on smectite it was found that the structure of the polymer was insignificantly affected by charge in the tetrahedral or octahedral positions of the smectite. The electron density distribution models of PVP directly adsorbed on the smectite layer suggest that PVP chains directly bound to the surface are more rigid and organized than the molecules occurring farther away. The approximate thickness and distribution of PVP layer adsorbed on the surface was calculated to be equal ca. 5-6 angstrom. (C) 2009 Elsevier B.V. All rights reserved.