Deformation mechanisms in auxetic calix(4)arene systems

Abstract

Auxetic Materials exhibit the unusual property of becoming fatter when stretched and narrower when compressed (i.e. they have a negative Poisson's Ratio). This unusual behaviour effects beneficially the materials' properties, making auxetic materials superior to conventional ones in a variety of applications. In recent years, various nano-level (molecular) auxetics have been proposed, generally mimicking the behaviour of macrostructures (e.g. the commercially available 'egg-rack' structure investigated by our group). This dissertation is a continuation of a previous study performed by Williams (2003), on networks constructed using calixarene building blocks. Calix( 4)arene networks connected together through rigid or flexible interconnections were modelled using force-field based simulations. Simulations were carried out using the commercially available PC-based systems molecular modelling package, Materials Studio. A validation of the package was performed so as to ensure the package can correctly reproduce existing data. It was shown that the PCFF version within Materials Studio Discover is the most suitable force-field for modelling the calix(4)arene systems being considered here. The main concern for this dissertation was the simulation of the flexible networks of calix( 4 )arene-25 ,26,2 7 ,2 8-tetrol and calix( 4 )arene-25 ,26,27,28-[12]-crown-4 sub units, which were the calix( 4)arene molecules connected together through alkyl chains of different lengths, -(CH2)n- where n = 0,1, ... ,5. These networks were simulated using NPT molecular dynamics. Although this is more computationally intensive than molecular mechanics, it allowed us to study the behaviour of these systems at 300K over a period of time (ps). The dynamics simulations showed that at different loads in the z-direction (0 GPa, 1 GPa and 2 GPa), these flexible networks deformed in a way where the calixarene units rotated with respect to each other. This deformation mechanism mimiced the idealised Rotating Squares Mechanism studied by Grima and Evans (2000), which results in negative Poisson's ratios. Due to the benefits of the negative Poisson' s ratio in these systems it is hoped that they would attract the interest of other scientists and industrialists which may synthesise and in the near future commercialise these chemicals.