title: Controlling the self-assembly and aggregation of pre-gelled low molecular weight gelators
Creators: name: Ginesi, Rebecca Elaine
Publisher: name: University of Glasgow
Year: 2024

description: Hydrogels from low molecular weight gelators (LMWGs) continue to attract notable interest, with many potential applications. However, there are still significant gaps in our understanding of these systems and the correlation between the pre-gel and final gel states. The kinetics of the gelation process play a crucial role in the bulk properties of the hydrogel, presenting an opportunity to fine-tune these systems to meet the requirements of the chosen application. Therefore, it is possible to use a single gelator for multiple applications. In this thesis, we report on the ability to modify the pre-gel structures before triggering gelation to develop materials with a range of interesting properties to suit multiple potential applications.

First, we show how a well-studied amino acid-appended perylene bisimide (PBI) can form diverse hydrogels with distinct mechanical and optical properties. These differences are achieved by adjusting the solubility and aggregate structure, influenced by the initial pH and the history of the solution before gelling. Through the utilisation of small-angle neutron scattering, rheology, 1H NMR spectroscopy, and absorption spectroscopy, we exemplify the impact of initial pH on the gelation kinetics and final gel properties.

We then expand upon previous work, which used heat-cool cycles to alter the properties of LMWG solutions to modulate the self-assembly of three amino acid-appended PBIs. We exemplify the use of heat-cool cycles as a tool for modulating the self-assembly behaviour of amino acid-appended PBIs using small-angle neutron scattering, rheology, and absorption spectroscopy. We determine the impact on the resulting hydrogels and thin films using rheology, nanoindentation, and voltammetry. We find that heating and cooling both influence the aggregation, which consequently changes the resistivity of the resulting films. This work highlights the importance of controlling the temperature of solutions but also opens up more aggregation states, allowing the same molecule to have many different uses.

Finally, we utilise a non-gelling polymer additive to create hydrogels tailored for 3D printing applications. Using rheology and small-angle neutron scattering, we aim to gain a deeper understanding of how printing affects the overall properties of the hydrogels. We demonstrate that upon drying, the resultant xerogels exhibit alignment attributed to the shear force exerted during printing, leading to enhanced film uniformity. Furthermore, these materials display mechanoresponsive behaviour, with an increased photoresponse upon bending. Consequently, these hydrogels show potential for diverse applications, including wearable sensors and electronics.

Overall, we demonstrate that LMWG systems display a variety of structures in both the solution and gel states, which significantly influences their resulting properties. This work provides a greater understanding of the relationship between gelator structures and the material’s bulk properties. Moreover, it challenges the notion that the discovery of new gelator molecules is essential for attaining novel or specific properties.