Chain-Level Analysis of Reinforced Polyethylene through Stretch-Induced Crystallization

Abstract

Herein, we propose a large-scale simulation approach to perform the stretch-induced crystallization of entangled polyethylene (PE) melts. Sufficiently long (1000 ns) united-atom molecular dynamics (UAMD) simulations for 16000 chains of 1000 consecutive CH₂ united-atom particles under periodic boundary conditions were performed to achieve the crystallinity observed in experiments. Before the isothermal crystallization process, we applied uniaxial stretching as pre-elongation to the embedded strain memory on the entangled PE melts. We confirmed significant differences in the morphologies of crystal domains and scattering patterns for pre-elongation ratios of 400% and 800%. The obtained scattering patterns were consistent with the experimental results. Uniaxial stretching MD simulations revealed that the elastic modulus at 800% pre-elongation was stronger than that at 400% pre-elongation. From this observation, we can derive the structure-property relationship, wherein the magnitude of the pre-elongation governs the crystal domain structures and mechanical properties.

Figure 1

Sliced snapshots and crystallinity of the crystallized PE chains. (a) Crystallinities for preEL = 400% and 800% and t_IC = 1000 and 2000 ns. (b) Snapshot for preEL = 400% and t_IC = 1000 ns. (c) Snapshot for preEL = 800% and t_IC = 1000 ns. (d) Count of square root of the zz-component of the radius of gyration (R_g²) of each PE chain for preEL = 400% and 800% and t_IC = 1000 ns. In (b) and (c), the ith particle is color-coded according to the value of q₆q₆*(i), which is the local order parameter introduced by Zhang and Larson based on the local bond order analysis in hard-sphere colloids by Auer and Frenkel. The thin black lines are a guide to the eye for the boundaries (q₆q₆* = 2.2) generated by image processing. The box dimensions are approximately 600 × 1200 Å. The thickness of the sliced snapshot is 30 Å. The snapshots were created using the OVITO software.

Figure 2

Two-dimensional scattering patterns of WAXS and SAXS for the PE chains with (a) preEL = 400% and (b) 800% at T_IC = 340 K. Pre-elongation is in the z-direction. Results for t_IC = 2000 ns are shown in Figure S4. For WAXS spots, the peak height and fwhm in the q_r- and q_z-direction were (55, 0.074, 0.33) and (87, 0.074, 0.19) for preEL = 400% and 800%, respectively. The contour maps around the peak spots are given in Figures S5 and S6.

Figure 3

Stress–strain curves of the PE crystals under uniaxial stretching for preEL = 400% and 800%. The stretching rate was dε/dt = 0.01/ns.

Figure 4

Average of stress σ_zz in the z-direction, principal stress σ₁, and von Mises stress σ_vM for the PE chains with preEL = 800% at ε = 10%. (a) σ_zz; (b) σ₁; (c) σ_vM. To remove the thermal fluctuations, averaged values were obtained from 100000 snapshots every 50 fs (10 frames). The box dimensions are approximately 600 Å × 1200 Å. The thickness of the sliced snapshot is 30 Å. The snapshots were created using the OVITO software.