Molecular modelling of zeolites with negative Poisson's ratios

Abstract

Materials with a negative Poisson's ratio, i.e. auxetic materials, exhibit the unexpected property of becoming wider when stretched and thinner when compressed. This rare property, which is very much dependent on the geometry of the material, results in the enhancement of various macroscopic properties of the material and makes auxetics superior to their conventional counterparts in many practical applications. It was recently shown through preliminary modelling studies that some zeolite frameworks may exhibit this unusual property. These preliminary studies relied on various assumptions, e.g. no cations or interstitial water molecules were included assuming that they would have no effect on the deformation mechanism. This dissertation re-examines this problem and a methodology for simulating zeolites with cations and water molecules has been developed. This methodology was then applied on a number of zeolites with the scope of assessing the contribution of cations and water molecules on the mechanical properties and on the way the system responds to applied loads (i.e. the deformation mechanism). It was found that cations and water molecules play an important role on the Poisson's ratios although in most cases, the auxetic behaviour is retained. This result is likely to have important commercial repercussions, as it means that the possibility of having molecular sieves where the pore size can be controlled is more likely to become a reality.