Molecular simulation of structure, thermodynamic and transport properties of polyacrylonitrile, polystyrene and their alternating copolymers in high temperatures

This manuscript focuses on the molecular simulation of an industrially important copolymer, namely poly(styrene-co-acrylonitrile) (usually known with the acronym SAN). This polymer is glassy at ambient temperatures with glass transition temperature around 100–110 °C. In our case, the copolymer is studied at rather high temperature, in the range 150–300 °C, corresponding to industrial processing conditions. In the initial stage of the work, emphasis was put on the molecular dynamics (MD) code development for the simulation of SAN and on the development of an optimum atomistic force field. Polar interactions were accounted implicitly through appropriate adjustment of the Lennard–Jones parameters. All simulations and optimizations were performed using the GROMACS simulation package that is used widely by the polymer and biopolymer research community. Both homo-polymers (polyacrylonitrile and polystyrene) and the copolymer at different compositions were simulated. The force field was tuned to high temperature experimental data at 1 bar and used subsequently for simulations at higher pressures. Volumetric and structural properties of the homo-polymers and copolymers were calculated. Simulation results are in very good agreement with limited experimental data available. In the second stage of the work, the solubility of five gases (namely He, Ne, Ar, Kr and Xe), four n-alkanes (namely CH4, C3H8, n-C4H10 and n-C5H12) and toluene in the homo-polymers and copolymers was calculated using the Widom particle insertion methodology in the temperature range 450–550 K. Finally, the diffusion coefficient of acrylonitrile and styrene in the same polymers and temperatures was calculated. Due to lack of available experimental data for the mixtures, the accuracy of the reported calculations cannot be quantitatively assessed. Nevertheless, the trends observed with respect to the effect of temperature and polymer chemical structure on the physical properties evaluated should be useful for future polymer selection for specific applications. Clearly, additional experimental data are needed in order to refine the models developed here and validate the predictions reported.