Materials properties of clay nanocomposites: onset of negative Poisson ratio in large-scale molecular dynamics simulation

Clay-polymer nanocomposites are a new range of particle-filled composites possessing enhanced mechanical properties. Transmission electron microscopy studies reveal that many single and double clay sheets are bent when immersed in the polymer matrix. To understand the elastic properties of the sheets we investigate their behaviour under compression through molecular dynamics simulations in both aqueous and poly(ethylene oxide) environments. We examine the sheets before and after buckling and find that the elastic properties of the clay sheets are much reduced when buckled in each case. Buckling occurs with only small out-of-plane displacement and we ascertain that any observably bent sheet will have reduced elastic properties. The sheets possess a negative Poisson ratio during buckling, which allows the clay sheet to regain its uncompressed area. We present a simple analytical model which explains the buckling through the competition between in-plane strain energy and out-of-plane displacements. Knowledge of how the elastic properties of the clay sheets change with shape is important for the design of new nanocomposites.