Molecular Dynamics Simulation of the Stress–Strain Behavior of Polyamide Crystals

Abstract

Molecular dynamics simulations modeled the aramid poly(p-phenylene terephthalamide) (PPTA) and a related aromatic-aliphatic polyamide derived from a five-carbon aliphatic diacid (PAP5) with nine different reactive and non-reactive force fields. The force fields were evaluated based on crystal structures as well as intermolecular Hbonding and π-molecular interactions. The optimum force field was then used to simulate stress-strain behavior in the chain and transverse-to-chain directions. In the chain direction, PAP5 had higher ultimate stress and failure strain than PPTA; however, the stiffness of PAP5 was lower than PPTA at low strain (0-2%) while the reverse was observed at high strain (last 5% before failure). This contrast, and differences in the transverse direction properties, were explained by the methylene segments of PAP5 that confer conformational freedom, enabling accommodation of low strain without stretching covalent bonds. The simulation approach demonstrated here for two poly-1 Macromolecules 2021 10.1021/acs.macromol.1c00974 mers with distinct chemistry but similar atomic interactions may be extended to other polyamides.

Effect of Aliphatic Chain Length on the Stress–Strain Response of Semiaromatic Polyamide Crystals

“…The unit cell was then replicated in the x-, y-, and z-directions to create larger simulation boxes (4 × 4 × 4) to maintain MD simulation fidelity while maximizing the computational efficiency, as demonstrated in our previous work. 49 Periodic boundary conditions were applied in all three directions to mimic ideal crystalline polymers with infinite chain length and without defects or chain ends. Although the models in this study are approximations of realistic crystalline polymers that have finite length chains with defects and chain ends, the results here represent upper bounds on the mechanical properties and the simulation methods developed form the basis for more realistic models.…”

“…In our previous study, 49 two nonreactive force fields (Optimized Potentials for Liquid Simulations, OPLS, 51 and Consistent Valence Force Field, CVFF 52 ) and seven different ReaxFF parametrizations were tested for PPTA and PAP5. The results indicated that the ReaxFF force field developed by Liu et al 53 was best for studying structure− property relationships of PPTA and PAP5.…”

“…The details of the method were reported in our previous paper. 49 The low-strain elastic modulus was calculated by applying a linear fit to the stress−strain data from 0 to 2% strain, and the highstrain modulus was calculated from the last 5% strain before failure. The ultimate stress was the stress at the failure strain.…”

“…The chain direction was aligned with the x -axis (corresponding to crystallographic axis c ), and the H-bonding and π-stacking directions were aligned with the y - and z -axes, respectively. The unit cell was then replicated in the x -, y -, and z -directions to create larger simulation boxes (4 × 4 × 4) to maintain MD simulation fidelity while maximizing the computational efficiency, as demonstrated in our previous work . Periodic boundary conditions were applied in all three directions to mimic ideal crystalline polymers with infinite chain length and without defects or chain ends.…”

“…However, MD simulations have not been used to understand the effect of the length of flexible aliphatic moieties on the mechanical properties of semi-aromatic polyamide crystals. In our previous study, 49 we compared PPTA with one PPTA-related aromatic-aliphatic polyamide, PAP5, where 5 refers to the number of carbon atoms in the diacid monomer used in the reaction with p-phenylene diamine. The results showed that some mechanical properties (tensile strength and failure strain) of PAP5 were superior to those of PPTA.…”