Plastic Deformation of Semicrystalline Polyethylene by Molecular Simulation

• Published in: Macromolecules 2011-04, Vol.44 (8), p.3096-3108
• Main Authors: Lee, Sanghun, Rutledge, Gregory C
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Mechanical properties; Organic polymers; Physicochemistry of polymers; Properties and characterization
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma1026115

Plastic deformation of semicrystalline polyethylene has been investigated via atomistic Monte Carlo and molecular dynamics simulations. Two deformation modes are considered, both consisting of tensile deformation in the longitudinal direction of the lamellar stack, with either constant lateral dimensions or constant total volume. Stress−strain curves, elastic moduli, yield stresses, and strains are determined at 350 K, with deformation strain rates (in molecular dynamics) ranging from 5 × 106 to 5 × 107 s−1. Fundamentally different yield mechanisms are observed as a function of model constraints, deformation mode, and deformation strain rate. Using Monte Carlo simulations in which the interlamellar noncrystalline material is constrained between two rigid crystalline lamellae, yield stresses in the noncrystalline domain are found to be comparable to those reported previously for amorphous glasses. After yield, in the absence of topological rearrangements of the noncrystalline material, the interphase material hardens postyield with a modulus G R = 3.36 GPa; in the presence of topological rearrangement, the interphase material flows like a liquid. Using molecular dynamics in which the crystalline domains are thermalized and deformable, tensile deformation with constant total volume is accompanied by changes in the crystal stem orientation that can be attributed to crystallographic slip in the (100)[001] slip system. At the slower strain rate, plastic deformation is accompanied by melting and recrystallization. Meanwhile, for the tensile deformation mode with constant lateral dimensions or for the tensile deformation mode at the faster strain rate with constant total volume, yield apparently occurs with cavitation in the interlamellar domain. Calculated stress−strain curves are constructed by balancing the nonbonded and bonded contributions to stress. The density of entanglements is characterized using Kröger’s Z algorithm and found to correlate strongly with the fraction of noncrystalline material within the semicrystalline phase.