Simulation of Azobenzene Copolymers
Dumitru Pavel
RMIT University
Australia
Keywords: Simulation, Azobenzene polymers, Optical Divices
Introduction
Recently, electro-optical polymers have gained a relevant interest for potential applications in various fields ranging from photonics to integrated optics and from electro-optical devices to telecommunication systems.
Molecular simulation methods are useful in solving research problems in chemical and pharmaceutical industries. Techniques like energy minimization, Monte Carlo and molecular dynamics have become valuable tools for materials and drug design. Molecular simulation techniques have been applied to investigate conjugated liquid crystalline polymers containing azobenzene and diphenyl mesogenic groups within the main chain. Computations and conformational analyses were carried out using molecular simulation software for material science, Cerius2 version 4.0, designed by Accelrys, Inc. Single chains and amorphous unit cells of aromatic polymers with a degree of polymerisation (DP) ranging from 100 to 300 were used containing propylene and diethyletheric (oxydiethylene) spacers. The energy was minimised and then molecular dynamics were performed for 30 ns at 7 temperatures between 10 and 600 K.
Results & Discussion
The axial ratio or coefficient of asymmetry was calculated from the computer-generated structures in order to determine the stiffness of the simulated polymer chains. The orientational order parameter was used to estimate the degree of orientation and the liquid crystalline-isotropic transition temperature of the polymers. According to Maier-Saupe mean field theory for a polymer, it is possible to generate a stable mesophase if the order parameter is greater than 0.6. The simulated results for the monotropic polymers agreed very well with Maier-Saupe mean field theory and experimental data. In general, the simulations of the studied polymers with DP 100, 200 and 300 have shown that the order parameter slightly decreases with an increase of DP.
The predicted glass transition and decomposition temperatures of the simulated polymers are also reported. Series of NPT molecular dynamics (MD) simulations have been applied in order to calculate glass transition temperatures of the amorphous structures of the simulated polymers. The glass transition temperature was determined from specific volume versus temperature diagrams by analysing the trajectory file data generated by MD simulation. In order to predict the decomposition temperature the empirical correlation based on QSPR technique has been employed. The simulations of the studied polymers predict a very good thermostability despite the fact that the azobenzene groups act to reduce this.