Functional Supramolecular Hydrogels Mediated by Host-Guest Chemistry

Abstract

Hydrogels mediated by host-guest interactions represent a compelling class of materials with a myriad of promising applications including medical devices, wearable electronics and even for use as a strong adhesive. Several high-impact applications involving cyclodextrin (CD) mediated hydrogels have been reported; however, there are still a lot of structure-property relationships that need to be understood to gain a full understanding of these materials. Within the younger field of hydrogels crosslinked by cucurbit[n]urils (CB[n]) the recently established ‘complex first’ approach to prepare materials has unlocked a range of exciting possibilities which were not achievable with previous methods. The first chapter of this thesis introduces the core concepts that underpin the science performed herein. Host-guest chemistry is introduced, with a specific focus on both CB[n] and CD and the driving forces behind their formation of inclusion complexes. Polymeric hydrogels are then discussed with regards to both their classification and characterisation techniques. Next, hostguest mediated hydrogels are examined and several CD- and CB[n]-based materials are highlighted. Furthermore, the concept of a ‘polymers first’ vs a ‘complex first’ approach for the preparation of these hydrogels is discussed. Finally, stimuli-responsive hydrogels are explored, and several stimuli and response mechanisms are discussed. The second chapter describes the preparation of host-guest mediated hydrogels crosslinked by pseduorotaxane interactions between alpha-cyclodextrin and stoppered alkyl chains. For the first time, pseudorotaxane ‘stoppers’ will be employed that combine both electronic repulsion and increasing steric bulk. The resultant hydrogels are characterised by rheology, tensile testing, nuclear magnetic resonance spectroscopy, small-angle x-ray scattering and differential scanning calorimetry across a range of water contents. A theory that describes the different hydrogel behaviour considering both the steric bulk of the stopper and the water content of the material is then proposed. The next chapter establishes a facile, generic, column-free synthetic strategy to prepare alkylpyridinium acrylamide monomers. The resulting monomers make excellent CB[8] guests that form strong, polymerisable homoternary complexes. Copolymerising these ‘supramonomers’ with acrylamide affords a range of tough hydrogels with storage moduli more than an order of magnitude higher than a 100% acrylamide control hydrogel. Furthermore, by selecting pyridine derivatives that are intrinsically fluorescent, bulk fluorescence is endowed to the hydrogels without the need for any additional dyes. The final results chapter builds on the idea of introducing new properties to a hydrogel through the rational choice of CB[8] guests. Here, a redox-mediated hydrogel is prepared, which, when in the oxidised state, is highly ductile but when reduced becomes stronger, tougher, more elastic and less ductile with a five-fold increase in storage modulus and 50-fold increase in Young’s Modulus. It is demonstrated that this redox-promoted mechanical enhancement can be repeated over several cycles and that the changes in mechanical properties are drastically greater than those reported for existing materials. Finally, Chapter five draws overall conclusions from all of the results discussed in this thesis and with a broad outlook to the future discusses the potential prospects of the work described herein.