Effects of an atomistic modeling approach on predicted mechanical properties of glassy polymers via molecular dynamics

DM Anstine and A Strachan and CM Colina, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, 28, 025006 (2020).

DOI: 10.1088/1361-651X/ab615c

Glassy polymers are utilized in numerous applications ranging from light-weight structural materials to membranes for industrial gas separation. In this study, we quantify the ability of non-equilibrium molecular dynamics (NEMD) simulations to predict mechanical properties of glassy polymers based on different modeling approaches: force field selection, number of polymer chains in the simulation cell, and polymer builder algorithm. The polymers analyzed in this work are poly(methyl methacrylate) (PMMA), poly(propylene) (PP), and a polymer of intrinsic microporosity (PIM-1). PMMA samples were synthesized in silico using three different methods: continuous configurational biased Monte Carlo, pseudo-self-avoiding random walk, and a generalized simulated polymerization approach. Following the application of a consistent equilibration approach for each PMMA sample, stress-strain data from simulated tensile testing revealed that the choice of polymer builder algorithm or number of chains comprising the simulation cell did not have significant impact on the predicted elastic modulus. Force field selection effects have been analyzed by applying generalized force fields (GAFF and DREIDING) to each PMMA and PP sample and were found to be the most influencing factor studied. For these polymers, it was found that the DREIDING force field provides excellent agreement with experimental tensile moduli, similar to 3.25 GPa and similar to 1.53 GPa for PMMA and PP, respectively, while GAFF provides a systematic overestimation of the modulus by approximately 1.0 GPa. However, in the case of the PIM-1 model, with previously validated bonded interactions described by GAFF and non-bonded parameters from the TraPPE force field, the tensile modulus was predicted to be 1.23 GPa, which is well within the range of measured experimental values. Altogether, the simulations performed in this study illustrate the capabilities of atomistic MD simulations to predict the elastic modulus of glassy polymers and highlight energetic potential terms to consider for force field validation.

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