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

Abstract

Reactive molecular dynamics simulations were used to model poly(p-phenylene terephthalamide) and related aromatic-aliphatic polyamides derived from p-phenylene diamine 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 1 state such that they could accommodate low strain without deforming covalent bonds.Extension of wavy chains occurred through an intra-chain 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 inter-chain interactions, and the mechanical properties of polyamide crystals.

Probabilistic Approach to Low Strain Rate Atomistic Simulations of Ultimate Tensile Strength of Polymer Crystals

“…The BDE results are given in Table 1. The bond between the aliphatic C and the N, C a −N, is the weakest, in agreement with previous results for PPTA from ReaxFF MD simulations 13,14 and BDE calculations. 8 The BDE of the other three bonds increases as C c −C a < C a −O < N−C c .…”

“…This ultimate stress is much higher than experimental values, which are in the range of 2.7−3.4 GPa, 1,33 but it is consistent with previous ReaxFF MD simulations run at high strain rates. 13,14 The failure is analyzed in terms of which bonds broke at each strain, as shown in Figure 2b. The first bond type that breaks is C a − N, which is in agreement with our BDE results.…”

“…The difference observed between the experimental data and the low-strain simulation results is likely attributable to the nature of the present model PPTA. As mentioned in previous work, 13 this model comprises a perfect, infinite crystal of infinitely long chains (due the presence of periodic boundary conditions). It does not include morphological defects, imperfect chain alignment, or chain ends and thus yields an upper bound on the predicted mechanical properties of the polymer.…”

“…9 MD simulations of PPTA have been conducted to study stress−strain behavior 10−12 and to model bond scission. 10,13,14 MD simulations that involve bond scission are of particular interest 15,16 because they can be used to understand and possibly suggest ways to improve polymer strength. 17,18 This type of simulation necessarily has a reactive force field that captures the breaking and formation of chemical bonds.…”

Methanol-Assisted ADMET Polymerization of Semiaromatic Amides

“…Assuming this peak corresponds to the preferred interchain spacing orthogonal to the chain axis, this result is consistent with either chain tilt with respect to the lamellar interfaces or helical chain conformations. The chain conformations in the crystal were further probed by molecular dynamics (MD) simulations (see the Supporting Information for details). , The crystal structure was naïvely initialized by fully extending the chains and aligning the aryl amide groups to facilitate H-bonding and π–π interactions. Relaxing the simulations in the NPT ensemble yielded a chevron-like packing motif with fully extended aliphatic segments, which may explain the splitting of the principle high- q Bragg peak in the 2D fiber pattern.…”