Computational study of mechanical properties of poly-phenylene terephthalamide

We explore the mechanical properties of poly-phenylene terephthalamide (PPTA) by density functional theory calculations. We compare the results from LDA, PBE, PBE+rVV10, SCAN, and SCAN+rVV10, with the experiment. Among them SCAN shows the best performance in predicting the lattice constants of PPTA along the two crystal directions involving van der Waals (vdW) interaction and hydrogen-bond interaction. We study the mechanical response of PPTA as well by applying strain along three lattice directions. A clear observation is that the PBE functional does not capture any vdW interaction and exhibits a non-bonding energy curve. Due to the inclusion of vdW interaction, SCAN, PBE+rVV10, and SCAN+rVV10 all exhibit bonding energy curves. The equilibrium lattice constants obtained by SCAN and PBE+rVV10 are close to experimental data while SCAN+rVV10 slightly over-binds the system. We compute the Young’s modulus and yield strength of PPTA, and find that the experimental data are much smaller than computationally predicted values, partly due to the fact that these samples consisting of fibers are mechanically weaker than perfect molecular crystals.