Functional understanding of structural components in polypseudorotaxane gels
Biomedical devices, wearable interfaces, and soft robots require compliant soft materials with reliable mechanical performance, yet the molecular principles needed to design such materials are not always fully understood. Thus, there is an urgent need to understand how to design novel stretchable and soft hydrogels with enhanced toughness, i.e. resistance to breakage. Slide-ring gels (SRGs) are a class of unconventional soft materials formed by polyrotaxanes (PR) building blocks whereby sliding cyclodextrins (CD) replace traditional chemical and physical crosslinks and in turn enhance toughness and stretchability by several orders of magnitude. The rational design of SRGs re- mains challenging due to a lack of a mechanistic understanding of the role played by individual components. In this work, we implement simple synthetic perturbations that individually target polymer-CD and CD-CD interactions in unstoppered pseudopolyrotaxanes (pPR) solutions and measure the consequent changes in viscoelastic properties. Weakening the intermolecular interaction between CDs abolishes crystallisation and significantly weakens the gel. At the same time, thermodynamically favouring threading by using azobenzene-modified PEG chains favours the formation of stable and long-lived networks even at remarkably low CD stoichiometries. We rationalise these observations with coarse-grained Molecular Dynamics simulations which clarify the structure-function relationship of the components forming these materials. Our work contributes to guiding the rational design and development of enhanced SRG hydrogels and elastomers.