Computer Simulation of Side-Chain Liquid Crystal Polymer Melts and Elastomers

Abstract

Liquid crystalline polymers combine the optical activity and mesophase formation of liquid crystals with the visco-elastic properties of polymers. The synergy and interplay between both give rise to a number of phenomena, namely: memory effects, photo-induced deformations, formation of surface relief gratings, etc. Modelling of these effects should retain relevant features of the inter-particle interaction potentials and, simultaneously, be able to address the relatively large length scale on which these phenomena take place. A reasonable compromise between the two can be achieved by employing the so-called semi-atomistic force-field modelling, in which relevant groups of atoms are united into single sites with appropriately parameterised interaction potentials. Such simulations, performed by means of the molecular dynamics method, are considered in this chapter for the particular case of side-chain liquid crystal polymers (SCLCPs). The first part of the chapter is focused on the phase behavior of SCLCPs, covering existing experimental data and computer simulation studies. We consider in detail two particular architectures, the weakly- and strongly-coupled SCLCPs, respectively. The former category has a flexible backbone and long side chain (spacer) of ten monomers, and exhibits isotropic as well as poly- and monodomain smectic phases. The latter exhibits no ordered liquid crystalline phase, but a disordered glass-like low temperature phase. The structure of SCLCPs in these phases is characterised via the orientational order of the mesogens, as well as metric properties of backbones and side-chains, their respective orientational order, relaxation times and diffusion anisotropy. The second part of the chapter extends the analysis to the case of the reaction of SCLCPs to external perturbation. In particular, we assume the mesogens to represent the azobenzene chromophores, and the photo-isomerization of the latter under suitable illumination is reduced to the effect of reorientation of the trans-isomers, followed by the deformation of the polymer subsystem. The coupling between the chromophores and polymer backbone plays the vital role here, yielding contraction (extension) of the SCLCP volume element along the polarization vector for the case of weakly (strongly) coupled SCLCPs. This result, obtained by means of computer simulations, explains the experimentally observed differences in opto-mechanical behavior of azobenzene polymers with different molecular architectures. Finally, in the third part of the chapter, we consider the effect of crosslinking on the coupling between the liquid crystallinity and the mechanical state of the SCLCP. Crosslinking is performed in the smectic-A phase of a weakly coupled SCLCP to form a liquid crystal elastomer. We reproduce the effect of stabilisation of the smectic phase, as well as the memory effects in liquid crystalline order and in sample shape, when the elastomer is driven through the smectic–isotropic transition. Above this transition, in the isotropic phase, the polydomain smectic phase is induced by a uniaxial load. Below the transition, in a monodomain smectic-A phase, both experimentally observed effects of homogeneous director reorientation and stripe formation are reproduced when the sample is stretched along the director. When the load is applied perpendicularly to the director, the sample demonstrates reversible deformation with no change of liquid crystalline order, indicating elasticity of the two-dimensional network of layers.