School of Chemistry - Theses
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ItemNew techniques for elucidating the degradation of modern paints using polyvinyl acetate as a case study( 2017)Conservation of modern art poses many challenges, which include the use and take up of new materials with untested ageing mechanisms. While recent work has established several analytical tools and techniques that can identify and characterise modern materials, further work is required to ascertain fundamental chemical degradation pathways of a range of artists’ materials. Polyvinyl acetate (PVAc) is one such material and is commonly used in the creation and conservation of art. Understanding how PVAc degrades over time is a problem of both fundamental and practical importance. This study explores tools available in polymer chemistry and applies these to conservation research in order to gain further insight into the degradation of PVAc. Computational chemistry techniques using ab initio and Density Functional Theory methods were applied to explore key degradation reactions of PVAc materials. First, benchmarking studies were performed to identify the most appropriate level of theory to use in this investigation. BHandHLYP/cc-pVDZ was identified as being the most suitable as it offered excellent agreement of results calculated with the high-level benchmark method, G3(MP2)- RAD, at a fraction of the computer costs. Second, hydrogen abstraction reactions using dimeric and trimeric units of PVAc with hydroxyl, peroxyl and t-butoxyl radicals were modelled. The results from this study indicated that hydrogen abstraction was very likely to occur at room temperature with energy barriers for abstraction by hydroxyl and t-butoxyl radicals being below 24 kJ mol-1 and 53 kJ mol-1 respectively. Main chain and side chain scission reactions were also modelled, however the energy barriers for these reactions were all in excess of 100 kJ mol-1 indicating that these reactions are unlikely to contribute greatly to the degradation of PVAc at room temperature. Accelerated ageing studies were applied to PVAc films so as to provide insight into the longterm behaviour of PVAc when subjected to heat and light. Profluorescent nitroxide techniques were trailed to monitor the free radical mediated degradation as many commonly employed analytical techniques lack the molecular sensitivity to monitor the onset of degradation within paint films. A perylene based profluorescent nitroxide was identified as most suitable for this application and was synthesized and used to monitor the degradation of solvent borne PVAc resins and two commercial PVAc dispersion products when exposed to four accelerated ageing conditions. While the first nitroxide with the water-based dispersion products presented issues of incompatibility, a novel water-soluble profluorescent nitroxide was synthesized and initial accelerated ageing studies were conducted using one of the commercial dispersion products. To elucidate further information regarding the chemical reactions that occur during degradation, 13C NMR spectroscopy was used to analyse selected PVAc resin and dispersion samples exposed to various accelerated ageing conditions, including samples aged for over five years. The results of this study demonstrate that both the computational chemistry and profluorescent nitroxide techniques are useful tools for degradation studies within conservation research. This study also indicates that PVAc is highly susceptible to free radical induced degradation. This finding provides a secure direction for much needed ongoing research to better help determine practical treatments required to ameliorate the impact of free radical species if important cultural material is to be saved for future generations.