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Investigating structure property relations of poly (p-phenylene terephthalamide) fibers via reactive molecular dynamics simulations

Summary

Poly(p-phenylene terephthalamide) (PPTA) fibers were built at a realistic scale and subjected to tensile loading using ReaxFF reactive molecular dynamics. The study compares fully crystalline, fully disordered, and core–shell (disordered core with crystalline shell) morphologies, introduces nitrogen vacancies as defects, and extracts tensile moduli, an empirical modulus relation, and a molecular picture of where chains fail under large strain. The work positions reactive MD as a way to connect local bond-breaking chemistry with macroscopic mechanical descriptors for aramid fibers when experimental access to atomic failure sites is limited.

Methods

Force field and morphologies

ReaxFF (parameter lineage discussed in the article) is benchmarked against DFT bond/group energetics for PPTA-relevant fragments (Fig. 2). Fibers are built in three architectures: fully crystalline, fully unordered, and core–shell (disordered core + crystalline shell). Nitrogen vacancies up to 5% probe modulus vs crystallinity and defect density.

MD application (LAMMPS, NVT tensile)

Simulations use LAMMPS as the parallel MD engine (256 CPU cores on a 4×4×16 spatial decomposition) with periodic boundary conditions along transverse directions and the fiber axis along z. Constant-volume NVT integration employs a Nosé–Hoover thermostat (25 fs damping as printed) and a 0.25 fs timestep for all stages. Annealing ramps 300 K → 600 K over 10 ps, holds 600 K for 10 ps, then cools to 300 K over 10 ps to relax artificial construction strain. Quasi-static tension increments the z strain by 1% per macro-step; after each increment the authors run 5 ps NVT to stabilize pressure/energy, then 5 ps NVT to time-average σ\(_{zz}\) for the stress–strain point. Each full 1% strain step spans 10 ps, corresponding to 2×10¹¹ s⁻¹ engineering strain rate (acknowledged as high vs experiment). Loading continues until fiber failure. Representative diameters include 60 Å (153 chains) and 180 Å (1385 chains) in the crystallinity comparison set (§3.1).

  • Barostat / lateral pressure: N/A — NVT at fixed cell volume; lateral stress relaxation is handled via the staged NVT segments described above rather than NPT barostat control.

  • Electric field / replica / metadynamics: N/A — not used.

Findings

  • Modulus vs order: Average tensile modulus increases strongly with crystallinity (abstract: ~192 GPa disordered vs ~289 GPa crystalline).
  • Defect model: An empirical relation predicts modulus from crystallinity and nitrogen vacancy density (abstract).
  • Failure mechanism: Under large tensile load, failure begins with chain scission at boundaries of crystalline domains rather than uniformly through the fiber (HIGHLIGHTS/abstract).
  • Domains: Crystalline domains are observed in the crystalline regions during deformation (abstract).

  • Interpretation: The authors emphasize that interfacial weakness between ordered and disordered regions, rather than uniform bulk scission, dominates the simulated failure picture for the morphologies explored.

  • Comparisons / caveats: Tabulated moduli (~192 GPa vs ~289 GPa for disordered vs crystalline benchmarks in §3.1) should be read next to the authors’ discussion of strain-rate effects—the quoted 2×10¹¹ s⁻¹ loading is far above quasi-static experiment, so versus experiment claims remain contextual rather than pointwise matches.

  • Corpus honesty: Numeric moduli and strain protocol details are authoritative in papers/Yilmaz_Kevlar_Polymer_2018.pdf; abstract-only numbers here should be double-checked against the final Polymer layout.

Limitations

Reactive MD is limited by force-field accuracy for conjugated polyamide chemistry and by accessible time scales; quantitative comparison to macroscopic Kevlar fibers requires caution. Minor differences can exist between proof and version-of-record PDFs—verify critical numbers in the final journal file if needed.

Relevance to group

PPTA (Kevlar-class) reactive MD with ReaxFF from Yilmaz and van Duin—illustrates polymer mechanics and failure at scales inaccessible to QM.

Citations and evidence anchors

  • DOI: 10.1016/j.polymer.2018.09.001