Different bonding type along each crystallographic axis: Computational study of poly(p-phenylene terephthalamide)

Poly(p-phenylene terephthalamide) (PPTA) exhibits van der Waals (vdW) bonding along its a axis, hydrogen bonding along its b axis, and covalent bonding along its c axis. We explore the structural and mechanical properties of PPTA using density functional theory with various functionals including LDA, PBE, PBE-rVV10L, SCAN, and SCAN+rVV10, compared with available experiments. The hierarchy of nonempirical semilocal functionals LDA, PBE, and SCAN (not fitted to any multicenter bonded system) includes differing amounts of intermediate-range vdW interaction. rVV10 is the long-range vdW correction. (rVV10L differs from rVV10 only in the value of a range parameter.) Among the tested functionals, SCAN shows the best performance for the lattice parameters of PPTA along the two crystal directions involving vdW or hydrogen-bond interaction. The equilibrium lattice constants obtained by SCAN and PBE-rVV10L are closest to experimental data, while SCAN-rVV10 slightly overbinds the system. We study the mechanical response of PPTA by applying strain along three lattice directions. Due to the inclusion of vdW interaction, SCAN, PBE-rVV10L, and SCAN-rVV10 all exhibit correct bonding strain-energy curves. On the contrary, PBE strongly underestimates the vdW interaction needed to resist uniaxial stretching along the a axis. The Young's modulus and yield strength of PPTA are computed and compared with previous results. The experimental values are much smaller than the computed ones, mainly due to the fact that the PPTA fiber samples used for measurements are mechanically weaker than the perfect molecular crystals considered in the simulations. Interestingly, when a compressive uniaxial stress of 25 Kbar is applied along the b axis, a structural phase transition, in which the hydrogen bonds reform along one diagonal of the ab rectangle, is predicted by SCAN.