Modeling the visco-hyperelastic–viscoplastic behavior of photodegraded semi-crystalline low-density polyethylene films

Abstract

In this work we propose to model the mechanical and fracture response of semi-crystalline low-density polyethylene (LDPE) films exposed to accelerated ultraviolet (UV) ageing using a physically based viscohyperelastic-viscoplastic approach. UV irradiation induces an alteration of the chemical and structural properties of the semi-crystalline polymer, which affects significantly its mechanical behavior. In this work, pristine and oxidized low-density polyethylene films are characterized by conventional physicochemical and mechanical techniques (FTIR spectroscopy, DSC, HT-GPC, and uniaxial tensile testing). Polyethylene exhibits an oxidation-induced strengthening for a low range of UV radiation doses and a cavitation-induced softening for higher UV radiation doses. A competing multi-scale phenomena induced by UV radiation are incorporated into the constitutive model to capture the macroscopically observed mechanical and fracture behavior. Namely, the model will incorporate at the nano scale, chain scissions and cross-linking and at the meso and macro scales, oxidation-induced cracking, chemi-crystallization, and mechanical damage. The model used the high-temperature gel permeation chromatographymeasured molecular weight as degradation indicator. The model was able to capture accurately the evolution of the macroscopically observed mechanical and fracture behavior over a wide range of UV irradiation doses.

Physics-based Constitutive Modeling of Photo-oxidative Aging in Semi-Crystalline Polymers based on Chemical Characterization Techniques

“…The ability of the developed constitutive framework to accurately predict the mechanical test results of aged LDPE independently of any mechanical tests constitutes the important contribution of the proposed framework. Indeed, many, if not all of the existing works use several mechanical tests to fit and obtain fitting parameters that carry no physical meaning within the overall material behavior (Ayoub et al, 2020;Lamnii et al, 2021;Belbachir et al, 2010). Doing so renders the constitutive approach essentially a fitting algorithm with numerous fitting parameters that applies only to the specific problem for which calibration was performed.…”

“…The resulting polymer films had thicknesses ranging between 30 and 80 𝜇𝑚. This range of thickness was intentionally selected to allow for homogeneous oxidation and prevent diffusion-limited-oxidation (DLO) conditions (Ayoub et al, 2020;Tavares et al, 2003;Tireau et al, 2009;Hsueh et al, 2020). with thicknesses ranging between 30 and 80 𝜇𝑚).…”

“…Due to this embrittlement, the polymers harden and visible cracks can potentially occur on their surface (Rodriguez et al, 2020). A common consensus in the literature is that in semi-crystalline polymers, photo-oxidation reactions occur in the amorphous region that is favorable to oxygen diffusion (Ayoub et al, 2020;Rodriguez et al, 2020). The random coil structure of the amorphous region favors chain linking/unlinking.…”

“…However, to the best knowledge of the authors, only a few studies tried to develop constitutive equations to study the behavior of semi-crystalline polymers in response to photo-oxidation. Belbachir et al (2010) and Ayoub et al (2020) used physics-based elasto-viscoplastic constitutive relationships to incorporate the effects of UV radiation on the mechanical properties of polylactic acid (PLA) and low-density-polyethylene (LDPE), respectively. More recently, Lamnii et al (2021) captured the effect of UV radiation on the fatigue life of a bulk semi-crystalline polymer based on two fatigue indicators: the maximum true stress and the dissipated energy.…”

A rate-dependent constitutive model predicting the double yield phenomenon, self-heating and thermal softening in semi-crystalline polymers

“…The model was validated with isotactic polypropylene (iPP) under quasi-static and dynamic loads. Recently, Ayoub et al [63] developed a physically-based approach to model the mechanical and fracture response of LDPE with ageing effect through the acceleration of A semi-crystalline polymer constitutive model ultraviolet radiation (UV). The UV ageing effect was incorporated by the alteration of the molecular weight, which updates the evolution of the crystal content, network and intermolecular resistances.…”