Investigation of Tribological and Mechanical Properties at the Nanoscale of Modified Polymer Thin Films through Ion Irradiation

  • Michael James Davis

Student thesis: Doctoral Thesis


Polymers have a wide range of applications and are growing in popularity in a wide range of industries. These polymers however, do not always have the mechanical properties needed for specific industrial applications and so surface modification is employed in order to improve the materials mechanical properties. For this thesis, low energy argon ion beam irradiation was utilised to irradiate different molecular weight samples of polystyrene and poly(methyl methacrylate) in order to investigate their chemical and mechanical properties as a result of this irradiation. Chemical changes will be analysed in this thesis with the use of X-ray photoelectron spectroscopy and Raman spectroscopy. Mechanical changes will be recorded with atomic force microscopy, utilising methods such as contact mode lateral force microscopy and indentation.
This thesis has investigated the chemical and mechanical changes of polymers after low energy ion beam irradiation to understand how a material which predominantly forms cross- links after irradiation and a polymer which predominantly forms chain scissions after irradiation, will structurally and mechanically change. Emphasis shall be put on the types of structural change, the correlations between the irradiation and mechanical changes and the benefits that these changes will have on the materials.
The chemical analysis in this thesis has found that both polystyrene and poly(methyl methacrylate) have formed new chemical structures which are carbon-rich as a result of the irradiation. Evidence of amorphous carbon structures found on both surfaces provides interesting new possibilities for the samples and their chemical and mechanical properties. XPS experimental data has shown an increase in sp2 and sp3 bonds for polystyrene. The bond concentration value doubles with these bonds for lower molecular weight samples and an increase is also observed poly(methyl methacrylate) CxHy bonds by approximately 5%. The sp2, sp3 and CxHy bonds show good evidence for carbon-rich surfaces being created as a result of the surface irradiation. Raman data has also been investigated, and both polystyrene and poly(methyl methacrylate) show increased levels of amorphous carbon with the increasing irradiation.
Indentation experiments have discovered that low energy argon ion beam irradiation has had an effect on the surface mechanical properties of the polymers, with the Young’s modulus reducing it to approximately 80% of the original value in polystyrene but shows inconsistent results for poly(methyl methacrylate). The results recorded from the indentation experiments also found that for cross-linking samples, the hardness was found to increase to approximately 120% of the untreated samples whereas chain scissioning samples fluctuated with the irradiation. Both samples have indicated a greater resistance to wear from the hardness and Young’s modulus ratio.
The mechanical responses of the polymers were further investigated in terms of the friction response, coefficient of friction and resistance to wear. All samples predominantly decreased in friction response and in coefficient of friction, with the cross-linking samples reducing by up to a third of the coefficient of friction value of the untreated sample and the chain scission samples reducing to less than half of the untreated sample’s coefficient of friction. The samples also experienced a greater resistance to the wear forces against them. This is evidenced by the untreated samples displaying surface ploughing and deformation, whereas surface irradiation caused samples to resist any deformation with the increasing applied lateral force.
From this thesis, links can be made between irradiation causing carbon-rich surfaces, and how that improves upon polymer surface mechanical properties. The phenomenon of surface irradiation and molecular ejection and which part dominates throughout the irradiation process will be mapped in chemical analysis and mechanical responses. The experimental setup will combine an in-depth chemical and mechanical analysis of both cross-linking and chain scission polymers when subjected to increasing amounts of low energy ion beam irradiation.
Low energy surface irradiation is something which has received little attention in its potential to improve upon a polymer’s surface property; most works on irradiation will focus on higher energy irradiation and how that degrades a polymer over time. When looking into polymer irradiation, some focus has been made on cross-linking and how that changes surface chemistry, however very little work exists into the mechanics behind predominantly chain scission samples after irradiation and specifically a comparison between the two polymer types and their preference to either cross-link or form chain scissions. The combination of chemical and mechanical analysis that this thesis goes into has previously not been investigated. Combining the two methods of analysis and how the chemical changes can be connected to and indeed have an effect on the surface mechanical properties of these irradiated polymers will reveal new and valuable information about the behaviour of polymer irradiation and nanomechanics.
Date of Award14 Oct 2023
Original languageEnglish
SupervisorJurgita Zekonyte (Supervisor), Aleksander Ryszard Krupski (Supervisor) & Jovana Radulovic (Supervisor)

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