Abstract/Summary

The design of hydrogen bonding motifs is an incredibly attractive proposition for a wide range of applications; these motifs can be integrated into polymeric materials to produce stronger, tougher, stiffer, or more elastic materials that exhibit thermo-reversible behaviour. This thesis describes the design of hydrogen bonding motifs and their successful incorporation in supramolecular materials through the formation of low molecular weight ‘super’ hydrogelators and elastomeric materials. Hydrogelators were chosen as they offer a unique insight into supramolecular assembly, and they remove the influence of the polymeric backbone which is often responsible for the mechanical properties of polymer networks. The objective was to understand the structure to property relationships for low molecular weight substrates and how that translated to functional materials such as hydrogels. With an appreciation of the mode of assembly of these units and an understanding of how to tailor the assembly of the hydrogen bonding motif, these motifs could then be applied to polymeric systems to generate novel supramolecular elastomers. Chapter 1. provides a summary of molecular recognition phenomena, the application of weak non-covalent interactions and self-assembly processes in polymeric systems to create functional supramolecular materials. Chapter 2. details the synthesis of eight low molecular weight bis-aromatic ureas which contain carboxylic acid groups such that, upon pH switching form extended networks through carboxylate-carboxylate interactions. The supramolecular assembly was tailored by introducing methyl substituents on the aromatic rings, which attenuated the assembly by providing steric bulk and forcing the urea and phenyl substituents into a non-planar arrangement, thus promoting urea-urea stacking interactions. The expectation was that forcing the urea-phenyl into a non-planar arrangement would strengthen the urea-urea interaction and, by extension improve the robustness of the hydrogelators. The strengthening of the urea-urea interaction was evident, as rather than form stable gels, the bis-ureas with the strongest urea-urea interactions were found to undergo gel to crystal transitions. Chapter 3. outlines the synthesis and characterisation of thirteen telechelic poly(butadiene)- based elastomers whose properties were modulated by changing the assembly motif at the end of the polymer chain. The effect of the end-group was stark upon the elastomers’ properties; in the absence of a nitro substituent to act as a hydrogen bonding acceptor, the polyurethanes would form exceptionally phase-separated materials that were unsuitable for extensive mechanical testing as they presented behaviour comparable to as crosslinked rubbers. Extensive mechanical property assessment revealed that in the presence of a nitro moiety, the resultant polyurethanes would behave as thermoreversible elastomers that demonstrated self-healing and reversible adherence to a range of different substrates. Chapter 4. describes the shift in approach to supramolecular polyurethanes, in that, rather than end functionalise telechelic apolar polymer backbones, an apolar backbone, namely PTMG, was chain extended with a bis aromatic urea assembly motif to form chain extended polyurethanes. The synthesised polyurethanes exhibited a significant increase in their mechanical properties because of the addition of a supramolecular assembly within the repeat unit of the resultant polyurethanes. Additionally, the resultant supramolecular elastomers demonstrated self-healing capabilities. The General and Specific Experimental Procedures are given at the end of each Chapter — as are the References relevant to a specific Chapter. The thesis also features an Appendix section that contains the analytical data for Chapters 2–4.