| dc.description.abstract | The vast production and disposal of synthetic polymers have led to widespread microplastic (MP) pollution. MPs are nominally defined as polymer-based particles with at least one size dimension in the 1 μm to 1 mm range. MPs pose risks to ecosystems, human health, and they may impact climate processes. Understanding MP formation is important to mitigating these impacts. A primary source of MP generation is the fragmentation of larger plastics through environmental weathering, with UV radiation likely playing a significant role due to its effects on polymers. The overarching goal of this research is to bridge the gap between UV-mediated molecular-scale changes in polymers and surface cracking by applying principles from fracture mechanics. MPs extracted from the environment show signs of UV-induced changes, and observational studies in the lab confirm that UV exposure accelerates fragmentation, producing smaller, more abundant particles. Studies of polymer degradation in the lab and in the field also show that UV, combined with factors such as temperature and humidity, lead to property gradients and sometimes surface cracking. However, there is disagreement about the depth and severity of these gradients, and it is difficult to isolate the effect of UV on surface cracking. Residual stress arising from gradient formation may lead to cracking, but residual stress generated by UV exposure has not been studied. Thus, while UV-induced molecular changes are well-known, the mechanisms by which UV exposure causes surface cracking and fragmentation remains unclear, and predictive models are lacking. We address this gap by: 1) quantifying property gradients, residual stress, and surface cracking in a model polymer, additive-free low-density polyethylene (LDPE), exposed to UV radiation, and 2) applying a film-on-substrate fracture mechanics model to relate surface cracking to mechanical property gradients, residual stress (if present), geometry, and external loading in UV-exposed LDPE.
In the first part of the thesis, we quantify UV-mediated property gradients and residual stresses in LDPE subjected to varying durations of UV exposure in a custom-built benchtop chamber. A depth-profiling method, based on a glancing angle cut, enabling high-resolution property analysis as a function of depth, is developed. Chemical, morphological, and mechanical property gradients are measured using x-ray photoelectron spectroscopy (XPS), Raman microscopy, and nanoindentation. Our results show that although a gradient in mechanical properties emerges, it is not sufficient to induce residual stress or surface cracking in the absence of external loading, under the examined conditions. Crystallinity increases with UV exposure, but no depth-varying trend emerges. At the chemical level, O2 content varies in a way that suggests UV in combination with other factors contributes to oxidation, since similar changes in oxidation emerge for irradiated samples and for control samples placed in the chamber that are shielded from UV.
In the second part of the thesis, we evaluate the relevance of a film-on-substrate model to predict surface crack density in UV-irradiated LDPE. Film-on-substrate models offer valuable insights into how material heterogeneities, such as residual stress and elastic property gradients, influence fracture behavior. We select a finite fracture mechanics model that predicts surface crack density as a function of mechanical property gradients, geometry, and applied tensile or bending strain. UV-irradiated LDPE samples with known property gradients and geometry are subjected to varying levels of tensile strain using a universal tensile tester and then observed using scanning electron microscopy (SEM). Surface crack density is determined by visual inspection and compared to model predictions. A pattern of parallel surface cracks emerges in specimens exposed to UV, but not in controls. Crack density increases with increasing strain, with the direction of crack propagation orthogonal to the direction of tensile loading. Crack density and orientation are consistent with model predictions.
To our knowledge, this is the first experimental study to explicitly connect UV-mediated molecular-scale changes to cracking behavior in a predictive manner by applying fracture mechanics principles. The mechanical property gradient formation observed is consistent with literature, showing surface softening followed by stiffening as a function of depth. We found that the formation of mechanical and chemical property gradients did not coincide with residual stress or surface cracking under the conditions of this study, suggesting that additional factors, such as external bending and tensile loads, in combination with UV exposure, are required for cracking to occur. Surface cracking patterns are consistent with patterns observed for LDPE exposed to UV and other factors. This study demonstrates that film-on-substrate fracture models are promising to improve our understanding of cracking behavior in UV-irradiated LDPE. In the future, this framework could be expanded to investigate cracking under a broader range of weathering conditions (e.g., temperature, humidity, or cyclic exposure), as well as in different polymers exhibiting similar responses to UV, such as high-density polyethylene (HDPE) or polypropylene (PP). Additionally, the influence of polymer additives on gradient formation and subsequent cracking behavior could be explored. This study used an additive-free polymer, and many commercial polymers, which end up as MP pollution, use various additives and stabilizers. This thesis serves as a stepping stone for clarifying the relationship between UV exposure and the generation of MP pollution via UV-mediated fragmentation. | |
