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

QP Yang and WJ Li and ST Stober and AB Burns and M Gopinadhan and A Martini, MACROMOLECULES, 55, 5071-5079 (2022).

DOI: 10.1021/acs.macromol.2c00081

Reactive molecular dynamics simulations were used to model poly(p-phenylene terephthalamide) and related aromatic-aliphatic polyamides derived from p-phenylenediamine and aliphatic diacids with different numbers of carbon atoms in the aliphatic chain (5, 6, 7, or 8). Tensile strain was applied to each polymer crystal in the chain direction, and the mechanical response was characterized. All the polymers with aliphatic segments exhibited strain hardening, transitioning from an initial (low- strain) linear regime to a second (high-strain) linear regime. The modulus at high strain was similar for all polymers, but the modulus calculated at low strain decreased with increasing aliphatic chain length. The decrease in the low-strain modulus with increasing chain length was explained by the observation that polymers with longer aliphatic chains were wavier (i.e., deviated more from the fully extended conformation) in the quiescent state such that they could accommodate low strain without deforming covalent bonds. Extension of wavy chains occurred through an intrachain process for all polymers, quantified by the bond dihedral angles. In addition, for polymers with an even number of non-aromatic carbons, the strain response involved slip between chains within the hydrogen-bonded sheets. The ultimate stress of the polymers exhibited an odd-even effect (even polymers were weaker) which was explained by differences in hydrogen bonding and ring-ring coplanarity prior to failure; polymers with an even number of carbon atoms had less favorable H-bonding and poorer ring alignment. The results revealed direct correlations between aliphatic chain length, intra- and interchain interactions, and the mechanical properties of polyamide crystals.

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