Molecular Simulation of Electro-Optical Side-Chain Liquid Crystalline Polymers
Dumitru Pavel, Robert Shanks
RMIT University
Australia
Keywords: Simulation, Liquid Crystalline Polymers, Electro-optical applications
Introduction
Liquid crystallinity occurs when there is order above the melting temperature or in solution. Liquid crystals (LC) are less ordered and more transient than crystals in the solid state. Liquid Crystal Polymers (LPC) represent a particularly important class of materials since they offer a combination of properties unattainable from any other class of materials. In side-chain LCP pendant mesogens are rigid and elongated or planar, but not long such as for a polymer. The mesogens typically contain aromatic groups, but usually only two aromatic rings with a conjugated double bond between them.
Computational details
The side-chain LCP studied are polyacrylates with typical substituted biphenyl and azobenzene mesogens, linked to the main chain by flexible spacers of varying length. An amorphous cell is first constructed with the LCP at a low density to facilitate movement. Dynamics was performed at the low density, then the density was allowed to increase to an equilibrium that was typical of that expected of actual LCP. The LCP in the cell was then subjected to further dynamics. Various temperatures were used above and below expected isotropisation temperatures. Pair correlation analysis was performed to investigate the ordering of mesogens in the cell.
Results & Discussion
Calculation of bulk properties of liquid crystalline polymers requires 3-D representation of the molecular packing. Realistic models of the side chain LCPs would require an unfeasibly large 3D cell to be modelled as well as unrealistic time frame to simulate it's relaxation. Therefore we have used a novel approach to predict side chain LCP crystallisation potential using mesogen assemblies simulated within 3D periodic boundary conditions. The amorphous cell simulation was used to build the low density mesogen assemblies and MD was then performed on the central unit cell using 3D periodic boundary conditions, in an NPT ensemble. MD simulations were performed at atmospheric pressure and a range of temperatures, encompassing the experimental isotropic-nematic and glass transition temperatures of the prototype polymers. The resulting cell structures were used to study intermolecular interactions and packing of the mesogens in the model systems. The pair and orientational correlation functions were used to measure spatial ordering.