Chain Ends and the Ultimate Strength of Polyethylene Fibers

TL;DR

This study uses large-scale molecular dynamics simulations to analyze the tensile yield mechanism of polyethylene fibers, revealing that yield occurs via dislocation slip at chain ends rather than chain breakage, and providing a predictive model for ultimate strength.

Contribution

The paper introduces a detailed dislocation-based model for polyethylene fiber strength, linking microscopic dislocation behavior to macroscopic tensile properties.

Findings

Yield stress saturates at 6.3 GPa for long chains

Dislocations nucleate at chain ends and cause slip without chain breakage

Dislocation core size is approximately 25 Å

Abstract

We use large scale molecular dynamics (MD) simulations to determine the tensile yield mechanism of orthorhombic polyethylene (PE) crystals with finite chains spanning $10^2-10^4$ carbons in length. We find the yield stress $\sigma_y$ saturates for long chains at 6.3 GPa, agreeing well with experiments. We show chains do not break but always yield by slip, after nucleation of 1D dislocations at chain ends. Dislocations are accurately described by a Frenkel-Kontorova model parametrized by the mechanical properties of an ideal crystal. We compute a dislocation core size $\xi\approx25$\AA\ and determine the high and low strain rate limits of $\sigma_y$. Our results suggest characterizing the 1D dislocations of polymer crystals as an efficient method for numerically predicting the ultimate tensile strength of aligned fibers.