| dc.description.abstract | Polymers inevitably undergo various types of aging in service, which depend on environmental factors such as UV radiation, heat, and humidity. Previous work on polymer aging has focused on either its chemical or mechanical features; however, little attention has been paid to the chemo-mechanical aspects of oxidative embrittlement. In order to determine to what extent the UV-induced changes at the molecular level are coupled to the mechanical behavior, it is necessary to consider the processes at the mesoscale, which are often overlooked in the literature.
In this work, the effects of UV aging on a semicrystalline polymer are investigated at various scales through a combination of physico-chemical, damage, and mechanical characterization. For this, low-density polyethylene (LDPE) films and plates were submitted to three types of photo-initiated aging processes: UV, hydrothermal-UV (HUV), and natural aging. Various phenomena on the mesoscale were uncovered. Characterization of the chemo-mechanical aspect of aging showed that photo-oxidation leads to microcracking in the absence of mechanical loads. Through damage characterization, we showed that aged LDPE cavitates, in contrast with pristine LDPE that is known not to cavitate. Mechanical experiments evidenced that a UV-induced transient strengthening occurs at intermediate radiation doses. Moreover, this transient strengthening was common to all aging types and specimen geometries (film and bulk). All these phenomena are discussed here in terms of competition between multi-scale processes: chain scission and cross-linking at a fine scale, chemi-crystallization, oxidation-induced cracking, and mechanical damage at the meso and coarse scales. In addition, whether accelerated aging can be correlated to natural aging is not totally settled. A comparison of mechanical results shows a correlation between accelerated aging (HUV) and natural aging. Whether this translates into a correlation in mechanical damage is evaluated through fractography. Lastly, the oxidation of bulk polymer specimens is known to be heterogeneous due to diffusion-limited oxidation. A strategy was implemented to estimate the extent of diffusion-limited oxidation via inverse modeling with a macromolecular model. The results provide insight into how oxidation affects changes in the internal state, which ultimately affects damage and the progression of oxidative embrittlement of semicrystalline polymers. | en |
