Polymers and related systems exhibiting anomalous mechanical behaviour

Abstract

Recent research has led to the prediction, discovery and/or synthesis of various materials which exhibit negative Poisson's ratio. Such materials are termed 'auxetic' and exhibit the unusual property of expanding when uniaxially stretched and getting thinner when compressed, thus contrasting with conventional materials which show the opposite behaviour. Considerable advances have been made in research involving auxetic polymers at the molecular level, both from the theoretical and synthetic aspects. The work presented in this thesis reports studies on two classes of polymers, poly(phenylacetylene) networked polymers and potentially auxetic liquid crystalline polymers (LCPs), with the aim of further understanding their behaviour at the molecular level and also to develop novel molecular-level system exhibiting negative Poisson's ratio. Monte-Carlo based sorption simulations have been performed to study the effect of foreign solvent or gas molecules on the mechanical properties of reflexyne (auxetic) and flexyne (non-auxetic) poly(phenylacetylene) networked polymers. Simulations indicate that, while the Poisson's ratio remains more or less constant in the presence of gas of solvent foreign molecules, the stiffness of the networked polymer is significantly affected, particularly by large numbers of solvent molecules which are accommodated within the systems. For example, the Young's modulus of 1,4-reflexyne decreased in the direction perpendicular to the plane of the network, a property which is explained by the fact that the solvent molecules were accommodated in between the layers, thus effectively increasing the inter-layer separation of the material without imparting additional stiffness characteristics. On the other hand, the pores of 2,8-reflexyne and 1,4-flexyne were large enough to accommodate the molecules, whose presence significantly increased the Young's modulus of the networked polymers, something which may be attributed to the densification of the system resulting from increased interaction between the layers. Such results have important implications in the eventuality of the synthesis of such networks in the future and also on possibly tailoring the properties of such polymers through the variation of sorbate concentration. Novel poly(phenylacetylene) systems which are meant to mimic the behaviour of known auxetic macrostructures were also proposed, modelled and discussed. In particular, a novel poly(phenylacetylene) system based on the macromodel designed by Bezazi et al. [Bezazi 2005] was studied. A series of such systems, termed bezaflexynes, have been modelled and studied via static mechanical simulations, which show that these molecularlevel systems behave in a rather versatile manner which is dissimilar to the model on which they are based, particularly due to significant out-of-plane bending of the acetylene Vl chains. This structural behaviour inhibit most of the systems from exhibiting auxetic behaviour, particularly in the case of beza-1,3-flexynes. On the other hand, most beza-1,2- flexynes exhibit a negative Poisson's ratio, a property which is possibly promoted by the 1,2-substitution of the benzene rings. In addition to these systems, the study on poly(phenylacetylene) networked polymers has also been extended to the design of novel systems based on the same chemistry and inspired by Grima's polytriangles-n-yne systems [Grima 2000a]. These novel systems, termed (n1,n2,n3)-polytriangles-Type II systems are also characterised by significant out-of-plane bending, but still exhibit a negative Poisson's ratio in most modelled systems, a property arising through the 'rotating triangles' deformation mechanism, which is not dissimilar to the mechanism shown by Grima' s polytriangles-n-yne systems. A comparative analysis of these novel systems indicates that the polytriangles seem to have the better auxetic characteristics. Also studied were potentially auxetic LCPs based on the concepts proposed by Griffin et al., which are meant to achieve auxetic behaviour through nano-scale rotations of laterally attached nano-rods. The simulations performed in this thesis, which were meant to complement the experimental work by Griffin et al., have elucidated how the system behaves at the molecular level when stress is applied along the direction of the main chain. Static and molecular dynamics simulations have shown that such LCP systems are anisotropic and are indeed likely to exhibit an expansion in at least one direction orthogonal to the stretching main chain direction (negative Poisson's ratio). This direction where the lateral expansion occurs depends on the plane in which the laterally attached rods rotate, which lies at the heart of the deformation mechanism that the polymer undergoes when uniaxially stretched. Simulations have also shown that para-connected phenyl-based laterally attached rods experience significant bending during rotation, something which inhibits the system from showing its full auxetic potential. The anisotropic nature of the Poisson's ratio as well as the bending of the rods had not been looked into before and may have important implications on future testing and use of these highly promising auxetic materials. A solution to the problem of bending rods is proposed through the design of novel quinoid type rods which show a significantly more rigid character and impart enhanced auxetic properties on the LCP system. This resulted in the design of a new generation of potentially auxetic LCPs based on Griffin's concept, which are likely to exhibit better auxetic characteristics. Given the enhanced properties of LCPs incorporating quinoid-based rod types, as well as the interesting results found on the poly(phenylacetylene) systems, it is hoped that future synthesis of molecular-level auxetic polymers will steer towards this direction.