title: Controlling the formulation structures and properties of Low Molecular Weight Gelators
Creators: name: Thomson, Lisa
Publisher: name: University of Glasgow
Year: 2022

description: Low Molecular Weight Gelators (LMWGs) are a versatile class of material which, when an appropriate stimulus is applied, can form organogels or hydrogels by self-assembly. In general, to form LMWG hydrogels, the LMWG is first suspended in water at high pH to form a precursor solution. This precursor solution can then be used to form hydrogels by various trigger methods including, but not limited to, a pH change, addition of metal ions or by temperature change. Structures are formed by the gelator molecules in both the solution and gel phases. In this Thesis, we report on the ability to control these structures in order to develop materials with a range of interesting properties. We describe a preparation method for the creation of the LMWG precursor solutions. To do this, we examine the effects of stirring, pH and addition of salt during the solution preparation stage. We show that all three factors investigated have some effect on the solutions which contain “living” worm-like micellar structures. The preparation method is suitable for various concentrations of solution and gives reproducibly similar looking solutions as well as reproducible rheology and images under cross-polarised light. We then use this preparation method to examine the LMWG solution structures of a single gelator in more detail. To do this, we utilise rheology, polarised optical microscopy and small-angle X-ray scattering. We show that different structures are present in the solutions when the concentration of gelator and the counterions present in the solutions are altered. This in turn alters the overall properties of the solutions. When applying different stimuli to these solutions (temperature and magnetic field) the different structures respond in different ways. We show examples of how we can utilise these different solution properties with specific applications in mind. Finally, we control the LMWG structures in the gel phase to produce gradient stiffness hydrogels. We do this by incorporating a photoacid generator to the gelator precursor solution and irradiate with ultraviolet light. To examine the overall bulk properties of these gels we use traditional oscillatory rheology and to prove the existence of different stiffnesses within a single sample, we use cavitation rheology. Cavitation rheology is able to detect the subtle differences in stiffness within the gels which traditional bulk rheology cannot. The gradients in stiffness are temporary due to the reversible nature of these materials. The gradient gels have potential for use in tissue engineering where the different gel stiffnesses mimic the different stiffnesses of tissue in the body. Overall, we show LMWG systems exhibit diverse structures in both the solution and gel phases. This transfers to the overall properties of both phases. This work provides a greater understanding of the relationship between gelator structures and bulk properties of the materials; and challenges the conception that new gelator molecules must be discovered in order to achieve new or specific properties.