Molecular Dynamics Simulation on Diffusion of Five Kinds of Chemical Additives in Polypropylene

Molecular dynamics (MD) simulation was used to investigate the diffusion behaviour of five additives [2,6‐di‐tert‐butyl‐4‐methylphenol (BHT), 2‐(2‐Hydroxy‐5‐methylphenyl)benzotriazole (UV‐P), 2,4‐Di‐tert‐butyl‐6‐(5‐chloro‐2H‐benzotriazol‐2‐yl) phenol(UV‐327), 2‐(2H‐benzotriazol‐2‐yl)‐4‐(1,1,3,3‐tetramethylbutyl) phenol (UV‐329) and 2‐hydroxy‐4‐(octyloxy)benzophenone (UV‐531)] in polypropylene (PP) at the temperature of 293, 313 and 343 K. The diffusion coefficients were determined through Einstein relation connecting the data of mean square displacement at different times. The simulated diffusion coefficients were compared with that predicted by Piringer model and by experiments to evaluate the accuracy of MD simulation technique for estimating the diffusion coefficients of chemical additives in PP. Results showed that the simulated values were generally within one order of magnitude of the corresponding experimental values. The activation energies of additives were calculated by plotting the logarithm of diffusion coefficients versus the reciprocal of temperature according to Arrhenius equation. The activation energies calculated from MD simulation were also more closely to experimental values. Subsequently, the diffusion mechanism of additives inside PP was tentatively explored by analysing the interaction energy between diffusion molecules and polymer, free volume, molecular weight, size and shape, and the mobility of polymer chain. The movements of the additives in PP cell models at different simulation time suggested that for a long time, the additive molecules vibrate rather than hopping until they find the equal or larger transport channel to diffuse. It is demonstrated that the MD simulation may be a useful approach for predicting the microstructure and the diffusion coefficient of chemical additive with large molecular size and complex structure in polypropylene. Copyright © 2017 John Wiley & Sons, Ltd. The molecular dynamics simulation technology was used to investigate the diffusion behavior of five additives in polypropylene film. The simulations show that the diffusion of additive is controlled by the interaction energy between diffusion molecules and polymer, free volume, molecular weight/shape and the polymer chain mobility and that the additive molecules vibrate in PP matrix for a long time rather than hopping. All the simulated diffusion coefficients of additives are within one order of magnitude of the corresponding experiment values, and