Investigation of the detailed structure of ion tracks in polymer foils
Creators: name: Wang, Xue
Publisher: name: The Australian National University
Year: 2022

description: When energetic heavy ions penetrating solids, they release sufficient energy by electronic excitations and ionization of target atoms to form parallel and narrow damage trails called ion tracks. In MeV-GeV range, the continuous and uniform ion tracks only form when the electronic energy loss exceeds a material-dependent threshold. Polymers are important to track research due to the high irradiation-sensitivity and applications of track-based nanopores such as filtration and molecular sensing. However, in contrast to the intensive investigation in inorganic materials, limitations in characterization using electron microscopes and track formation simulations result in an incomprehensive understanding of the track structure and the formation mechanism in polymers. With complex chain structures, the track formation involves bond breaking, cross-linking, degassing, and carbonization, leading to detailed track structures. The most suitable characterization methods such as small angle X-ray scattering (SAXS) with the Hard Cylinder Model cannot provide detailed information on the track structure. In this thesis, by combining the SAXS technique with FTIR and UV-Vis spectroscopies, new methods for characterizing detailed track structures in polymers were proposed to better understand the irradiation effects on track structure. Two new SAXS form factor models termed the Soft Cylinder Model and the Core Transition Model were developed based on the knowledge of track formation in polymers. The Soft Cylinder Model describes a core region of constant low electron density and a transition region by a gradual increase in density from the core to the zone without density change. To enable a computationally less expensive and faster fitting process of SAXS data with sufficient precision, the Core Transition Model was developed by assuming a linear density increase in the transition region instead of a gradual change. After considering the angular and size distributions, these two models can better fit the SAXS data in various polymers like PET, PI, PC and PMMA, and well analyze uniform track structures with an overlap of less than 14% in polymer foils of different thicknesses. Furthermore, the Core Transition Model was combined with FTIR and UV-Vis spectra to quantitatively yield a detailed track structure in PET. The SAXS results show that for PET foils irradiated by 3-185 MeV Au ions, both core and transition sizes decrease with decreasing energy loss. The ion track may tend to a continuous but non-uniform structure when the energy loss is reduced to 9-10 keV/nm. In addition, the quantitative analysis of FTIR spectra can provide rich structural information regarding the radial distribution of bond-breaking cross sections. Results show that the broken bonds are distributed in the core and transition regions and can thus yield a core-transition track structure. The sizes of the core and transition increase approximately linearly with the energy loss. Moreover, by quantitative analysis of UV-Vis spectra, the obtained damage cross section of carbonization increases with increasing energy loss and is inferred to be mainly located inside the core region. The velocity effect observed in PET by SAXS shows that when energy losses are comparable, the size of the tracks formed by low-velocity ions is approximately twice more than that of the tracks formed by high-velocity ions. The estimated dependence between the irradiation parameters and the resulting track sizes showed that the total track radius measured by SAXS and FTIR is comparable to the traditional track core size estimated from the etching curve. In summary, this thesis presents new characterization methods for detailed track structures in polymers by combining SAXS, FTIR and UV-Vis techniques. This systematic study helps enhance the understanding of the track formation regarding the radial distribution of electron density, bond-breaking efficiency as well as localized carbonization.