Photochromic Polymers as Solar Thermal Fuels

Evans, Ethan and Gortz, Verena and Griffin, John (2022) Photochromic Polymers as Solar Thermal Fuels. Masters thesis, Lancaster University.

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Abstract

Solar thermal fuels could provide a move away from open cycle storage, being a renewable resource that utilises photochromic molecules such as azobenzene, to achieve solar energy harvesting, storage and release in a single material. The synthesis of an azobenzene based monomer and mildly syndiotactic polymer, alongside the characterisation of energy storage, energy release, thermal properties and structural properties is presented. The tailoring of azobenzene based polymers to achieve certain levels of crystallinity and how that could increase the energy density of a solar thermal fuel is discussed. It was found that free-radical polymerisation could achieve a polymer of mild syndiotacticity however, this tacticity did not appear to produce any form of crystallinity in the polymer when characterised by differential scanning calorimetry, with no additional recrystallisation exotherm present. This result was contradicted by wide angle X-ray scanning measurements, which showed a small level of crystallinity in the unirradiated form of the polymer, indicated by an additional peak at the q value of 0.33 Å-1, corresponding to a dspacing of 19.04 Å, attributed to the ordered spacing between polymer layers. The synthesised polymer was irradiated with 365 nm light, to explore its energy release properties, with a comparatively high energy density of 134 J g-1 being recorded. The effect of solvents on these energy release properties was also explored however, P1 samples dried from DCM and THF were recorded as 83.64 J g-1 (cis percentage: 61.8 %) and -90.96 J g-1 (cis percentage: 63.1 %) respectively, indicating that there was little to no difference between the use of the two solvents. This result was not conclusive due to not fully optimal drying conditions in samples dried from THF. No evidence was found to show any contribution of crystallinity in the synthesised polymer,to an increased overall energy density of the polymer solar thermal fuel system. Solid-state charging of the polymer via drop casting and irradiation at 365 nm was unsuccessful with the polymer only reaching a low cis percentage of around 19.5 %. 13 C CPMAS experiments were carried out to assess the sensitivity of the experiment to differences in P1 isomers, overall, it appeared that SSNMR is not very sensitive to changes in P1 isomers, however, there were some small differences present.