Micromechanical modeling of the visco-hyperelastic–viscoplastic​ behavior and fracture of aged semicrystalline polymers

This work presents a micromechanical-based visco-hyperelastic–viscoplastic constitutive model for low-density polyethylene (LDPE) films subjected to accelerated ultraviolet (UV) aging. The model captures the evolution of the mechanical and fracture properties of the material. Our proposed framework assumes that the semicrystalline polymer’s macromechanical behavior is the sum of the intermolecular and macromolecular network contributions. The intermolecular part of the model follows a nonlinear composite mixture rule, combining the contribution of the crystalline and amorphous phases, each represented rheologically by a linear elastic spring in series with a nonlinear viscous dashpot. The rheological representation of the macromolecular network consists of a series of two Langevin springs capturing deformation-induced conformation change and interatomic bond deformation between Kuhn segments, as well as a viscous dashpot for chain relaxation. We have developed a novel framework that reformulates the eight chain model to include the energy densities associated with both conformation change and interatomic bond deformation, providing a more accurate description of macromolecular network mechanical and fracture behavior. The aging of semicrystalline polymers results from two competing mechanisms: oxidation-induced strengthening resulting from chemicrystallization and crosslinking, and oxidation-induced softening resulting from chain scissions and chemicavitation. Our proposed model accurately captures the effect of UV aging on the mechanical behavior of the semicrystalline material. Additionally, we introduce the novel concept of a ”healthy network” to predict the evolution of the strain to fracture. The model accurately predicts the evolution of the mechanical behavior and elongation at break over a wide range of UV irradiation doses.