Chain Ends and the Ultimate Strength of Polyethylene Fibers

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

Molecular Dynamics Modeling of the Effect of Axial and Transverse Compression on the Residual Tensile Properties of Ballistic Fiber

“…This could be attributed to the low strain rate used in the experiments as well as other factors associated with the experimental fibers including the random orientation of crystals and different microstructural and topological defects. For PE, the axial modulus of 257 GPa in the Zdirection obtained from the present simulation is comparable with the experimental and simulation results, which are in the range of 235-300 GPa, reported in the literature [25,[43][44][45]. The axial modulus of PE reported by O'Connor et al [25] from MD simulation is 258 GPa, which is in excellent agreement with the present simulation value.…”

“…For PE, the axial modulus of 257 GPa in the Zdirection obtained from the present simulation is comparable with the experimental and simulation results, which are in the range of 235-300 GPa, reported in the literature [25,[43][44][45]. The axial modulus of PE reported by O'Connor et al [25] from MD simulation is 258 GPa, which is in excellent agreement with the present simulation value. Figures 11-14 show the snapshots of damage for Kevlar ® and PE under transverse and axial tension.…”

“…The effect of AC and TC deformation modes on the fiber tensile failure strength has not been studied extensively. There are few atomistic simulations studies [20][21][22][23][24][25] that focused on the elastic and strength properties of Kevlar ® and PE crystal. Rutledge et al [20] have determined the elastic properties of Kevlar ® crystal through atomistic simulations.…”

“…Recently, Yilmaz et al [23] and Mercer et al [24] have studied the anisotropic elastic and strength properties of Kevlar ® crystal using reactive force field ReaxFF [27]. O'Connor et al [25] have studied the effects of chain ends on the elastic and strength properties of PE crystal using a modified version of reactive force field AIREBO (Adaptive Intermolecular Reactive Empirical Bond Order) [28].…”

Nanoscale interfibrillar adhesion in UHMWPE fibers

“…From high resolution AFM analysis of the microfibril morphology, we know that physical connections take three main forms: domains of tie molecules, actual branching points that form the 3 D network reported by McDaniel et al and epitaxial crystallization that spans multiple fibrils as seen in Figure and reported by Strawhecker et al Based on this, it is reasonable to assume that the additional work of adhesion required to split the fibrils, after partitioning energies through the double indent methodology, is due to yielding and deformation of these morphological features. Ultimately failure of these material bridges can occur in two possible ways: Breaking of molecules, and/or pull out of chain ends from the crystal lattice . While further partitioning of the excess work of adhesion to account for both mechanisms is beyond the scope of this work, we can bound the problem by estimating the number chains involved for either extreme‐bond breaking or chain pull out.…”