Crystallization and Heterogeneous Local Stress Distribution in Hydrogen-Bonded Polymers: Molecular Dynamics Simulations of Polyamide 6 (PA6)

We present a coarse-grained (CG) model of polyamide 6 (PA6) that captures the hydrogen bonding interactions between amide groups by embedding small charged beads within larger CG beads. This CG model balances the necessary atomic details and efficient coarse-graining, enabling crystallization simulations of PA6 on larger length and time scales. The results reveal a two-step structural adjustment during crystallization: hydrogen bonding layers form rapidly, followed by significant ordering and elongation of the stem length. Moreover, we explore the heterogeneous distribution of local stress across the semicrystalline structure for various polymer systems. A strong correlation between the local stress and order parameters is observed in systems without hydrogen bonding interactions, while the one in the PA6 system is notably weaker. By decomposing the local stress contributions from different bead types, we attribute this weak correlation to the superposition of varying correlations from the backbone and amide groups, which highlights the influence of hydrogen bonds on the local stress distributions. Our analysis of local stress tensors at the atomic level in semicrystalline polymers represents a critical step toward bridging the gap between microscopic structural properties and macroscopic mechanical behavior.