Lamellar Orientation Analysis and Mechanical Properties of Polyethylene in Stretch-Induced Crystallization

Polyethylene films prepared from orientation-dependent methods are strong and resilient, have reduced permeability, and possess higher tensile strength. A molecular dynamics investigation is performed to reveal the emergence of chain folding and lamellar crystal axis alignment along the stretching axis (tilt angle) in the stretch-induced crystallization (SIC) of high-density polyethylene (HDPE), which mimics the internal structure of the fiber. The morphology in phase transition is assessed by the total density (<i>ρ</i>), degree of crystallinity (%χc), average number of entanglements per chain (<<i>Z</i>>), elastic modulus of the mechanical property, and lamellar chain tilt angle (θ) from the stretch-axis. The simulation emphasizes crystal formation by changing the total <i>ρ</i> from 0.85 g·cm^-3 to 0.90 g·cm^-3 and by tracking the gradual increase in % χc during stretching (~40%) and relaxation processes (~50%). Moreover, the primitive path analysis-based <<i>Z</i>> decreased during stretching and further in the subsequent relaxation process, supporting the alignment and thickening of the lamellar chain structure and chain folding from the random coil structure. The elastic modulus of ~350-400 MPa evidences the high alignment of the lamellar chains along the stretching axis. Consistent with the chain tilt angle of the HDPE in SAXS/WAXS experiments, the model estimated the lamellar chain title angle (<i>θ</i>) relative to the stretching axis to be ~20-35°. In conclusion, SIC is a convenient approach for simulating high stiffness, tensile strength, reduced permeability, and chain alignment in fiber film models, which can help design new fiber morphology-based polymers or composites.