The research objectives of this work aim to contribute to the understanding and assessment of ageing in semi-crystalline polymers through destructive and non-destructive tests, and to study the feasibility of acoustic means to be utilized for future age-monitoring systems. This called for a thorough evaluation of the governing material properties that can be practically used to infer material alteration, i.e., ageing, through systematic ageing-induced experiments that exposed polyethylene-based specimens to gaseous and supercritical CO₂ at elevated temperatures (up to 90 ℃) and pressures (up to 400 barg).

Polymer ageing is an umbrella term that summarizes all irreversible chemical or physical alterations that lead to a change in measurable material properties. The superstructure of polyethylene is presented with regards to polymer crystallinity and its measuring systems, thermal expansion, elasticity, relaxation processes and acoustic attenuation. This is along with literature reported effects of supercritical CO₂ on polymer plasticization.

The first type of induced-ageing experiments comprised of long-term (up to 60 days) continuous flow permeation tests, conducted on polyethylene of raised temperature and polyvinylidene fluoride specimens exposed to CO₂. The tests included a differential pressure setup of a polymer membrane exposed to a high-pressure gas inlet from one side, and a gas chromatograph that measures the permeant content at the other side. The fluid transport coefficient analysis of permeability, diffusivity and solubility aimed to study the flux behaviour and infer from it signs for membrane ageing, at various pressure steps. It was shown that the diffusion and permeation coefficients of supercritical CO₂ to polyethylene agreed with the equations of Fickian diffusion and the classical permeation model, where the diffusion and permeation coefficients decreased in general with increasing feed pressure.

The second type of induced-ageing experiments comprised of systematic autoclave experiments using supercritical CO₂ at constant 90 ℃ and 100 barg and various polyethylene specimens of different grades. The aim of this experiment was to compare the material properties of different polyethylene grades before and after prolonged exposure to supercritical CO₂ with destructive tests (e.g., differential scanning calorimetry, dynamic mechanical analysis, tensile tests), and non-destructive tests (e.g., visual inspection, density, ultrasound broadband spectroscopy). The collective analysis of the experimental outcomes showed subtle differences in the degree of crystallinity between aged and unaged specimens, and a similar profile for the melt temperatures. Comparison of first and second melt cycles revealed permanent structural alterations in some specimens. Elastic moduli calculated from lamellar thickness, static tensile tests, dynamic mechanical analysis, and acoustic spectroscopy suggested that a difference could be drawn between aged and unaged specimens, in which elastic moduli generally increased with exposure to ageing conditions. Finally, empirical elastic modulus of various polyethylene grades fitted well with degree of crystallinity and the findings agree with literature.

Furthermore, given the promising results of acoustic spectroscopy, a bespoke piezoelectric micromachined ultrasound transducer was designed and fabricated with a view towards developing an in-situ assessment of polymer ageing. The design and fabrication processes are thoroughly presented. Initial probing of various materials with the micro-transducers showed distinctions in signatures between polymeric and metallic specimens due to the differences in material properties. This demonstrates potential for the micro-transducers to be used for age-monitoring of semi-crystalline polymers in the future, even though initial probing of the aged versus unaged specimens did not reveal distinctive relationships using the current air-coupled design.

The carried work contributes to the understanding of ageing of semi-crystalline polymers and provides significant contributions towards their deployment in high pressure applications. It also presents a novel outlook on the feasibility of material resonance-based analysis methods, such as piezoelectric micromachined ultrasound transducers, to be used for non-destructive evaluation and age-monitoring of solid specimens in general, and semi-crystalline polymers in particular.