Self-Assembly of Azobenzene Photosurfactants and the Relationship with Macroscopic Properties

Abstract

Azobenzene photosurfactants (AzoPS) demonstrate the combined ability to change their shape upon irradiation with light and to self-organise into polymolecular assemblies. Photoisomerisation changes the polarity and shape of the surfactant on demand, which has led to the exploration of these materials in a diverse range of applications. The interplay between these properties also affects the concentration-dependent self-assembly of azobenzene photosurfactant into micelles. This thesis focuses on four non-ionic azobenzene photosurfactants, which have been systematically varied in terms of their structure to study their self-assembly behaviour as a function of molecular structure, isomeric form, concentration, temperature and applied shear. The reciprocal effect of self-assembly behaviour on macroscopic properties such as viscosity, viscoelasticity, optical anisotropy and soft-templating ability is also investigated, emphasising how control over self-assembled structure can be used to modulate key properties and applications. In general, non-ionic photosurfactants have been less studied compared to their ionic counterparts and there are very few reports of their behaviour under shear, formation of lyotropic liquid crystal phases and use as templating agents. In this work, small-angle scattering is used extensively to probe the shape and dimensions of the self-assembled surfactant aggregates in solution. It will become apparent this is a key technique in the characterisation of dynamic and soft matter systems. Chapter 3 of this thesis will focus on the dynamic self-assembly behaviour of two AzoPS under photoisomerisation, with an initial study on the flow behaviour. Chapter 4 will take a more detailed look into the relationship between the self-assembled structure and flow behaviour of an AzoPS, using combined rheology and small-angle scattering measurements. In Chapter 5, the formation of lyotropic liquid crystal phases by all four AzoPS as a function of concentration, temperature and isomeric form, will be investigated, and binary concentration-temperature phase diagrams for each AzoPS constructed. In Chapter 6, the ability of AzoPS to act as soft-templating agents to form porous titania nanoparticles will be proven, along with an investigation on the effect of template structure and irradiation conditions on the photocatalytic ability of the product titania. Finally, conclusions will be drawn on the basis of the above results, along with a discussion of the evolution of the field during the time this work took place, with signposting for potential future work.