Investigation On Formation And Properties Of Supramolecular Hydrogels

Supramolecular hydrogels are a kind of soft materials constructed by a three-dimensional network,immobilizing a large number of water molecules.The complex networks are formed by multiple noncovalent interactions,such as hydrogen bonding,electrostatic interaction,metal coordination,π-π stacking and host-guest interactions.Because of the dynamic crosslinks,hydrogels exhibit stimuli-responsiveness and dynamic behavior,attracting extensive attention for applications in diverse fields including self-healing materials,shape memory materials,biomedicine.Supramolecular hydrogels with different mechanical properties are constructed to meet different application requirements by molecule desgin.Supramolecular hydrogels based on small molecule gelators or polymer molecules exhibit different formation mechanism and properties.In this thesis,different types of hydrogels have been constructed via hydrogen bonding,electrostatic interactions and host-guest interaction of cucurbiturils.These hydrogels were formed by small molecules and polymers,including sodium deoxycholate,sodium guanosine monophosphate,polylysine and polyethylene glycol.The influence of molecular structure and construction strategy on macroscopic properties was discussed,promoting in-depth understanding of the mechanism and properties of supramolecular hydrogels,and expanding the functional applications of supramolecular hydrogels.In Chapter 1,we introduced the concept and classification of supramolecular hydrogels and various non-covalent interactions in supramolecular hydrogels,especially the host-guest interaction based on cucurbituril.The properties,characteristics and applications of supramolecular hydrogels were summarized,and the contents and significance of this thesis were outlined.In Chapter 2,supramolecular hydrogels based on deoxycholate were constructed by hydrogen bonding and electrostatic interactions.The addition of inorganic salts and the regulation of pH can control the main driving force.At higher pH,hydrogels were constructed by electrostatic interaction between deoxycholate and additional Na+.When pH was reduced,deoxycholate was protonated,and the hydrogels was constructed by hydrogen bonding between deoxycholic acid and deoxycholate.Properties of hydrogels were related to the main driving forces.The combination of two interactions can significantly improve the gelation ability.In Chapter 3,supramolecular hydrogels composed of GMP-quadruplexes were prepared by addition of lanthanide ions or cationic polymers.In the first system,hydrogels were formed by balancing the cation-dipole and electrostatic interaction.G-quadruplexes were formed by cation-dipole interaction between lanthanum ions and G-quartets,while electrostatic interaction of phosphate groups was weakened by adjusting pH.A significant fluorescence enhancement of thioflavin T was observed by fixing chromophore in G-quadruplexes structures.In the second system,hyperbranched poly(ethylenimine)(PEI)was introduced to GMP system.G-quadruplexes were formed by the cation-dipole between Na+and G-quartets and electrostatic interaction phosphate groups of GMP and-NH2 of PEI.The hydrogels exhibited good adsorption of picric acid.In Chapter 4,a dynamic supramolecular hydrogel based on tetra-arm PEG was formed by the host-guest interactions of cucurbit[71]uril and carboxylate terminal groups.The kinetics of host-guest crosslinking can be controlled by pH,resulting in the modification of macroscopic properties with dynamic properties spanning over 4 orders of magnitude.With the pH increased,stable and self-supporting hydrogels with slow relaxation were formed,attributed to the slow dynamics of host-guest crosslinks.The mechanical properties of hydrogels are determined by dissociation rate constant.The quantitative relationship between hydrogels properties and the kinetics and mechanisms was observed.We hope it helpful to tailor mechanical properties of materials from molecular design to meet different application requirements.In Chapter 5,dynamic dual-crosslinked hydrogels via host-guest interactions and dynamic imine bonds were constructed by using polylysine modified phenylalanine,polyethylene glycol with terminal benzaldehyde and cucurbit[8]uril.The hydrogels exhibited enhanced mechanical strength,rapid self-healing and responsive properties.Imine bonds with slow dynamics provided sufficient mechanical strength and rigidity for hydrogels,while CB[8]crosslinks increased the viscosity of the polymer solution.Compared with single crosslinked hydrogels,dual crosslinked hydrogels showed higher mechanical strength and toughness and faster self-healing rate.Dual crosslinked hydrogels also inherited the stimuli responsiveness of CB[8]and imine bonds.In Chapter 6,double network hydrogels based on cucurbit[7]uril and cucurbit[8]uril crosslinks were constructed.We synthesized two supramolecular crosslinks based on the host-guest interaction of cucurbit[7]uril-cyclohexane and cucurbit[8]uril-imidazole.CB[8]crosslinked networks with high crosslinking density can break to dissipate energy.The double network hydrogels showed good mechanical properties,which enriched the construction of the supramolecular hydrogels with double networks.We explored the relationship between assembly mechanism and properties in different supramolecular hydrogels.It was found that the combination of small molecules and polymers and multiple interactions could enhance gel properties,such as enhanced gelation ability,excellent mechanical strength and multiple stimulus responsiveness.We designed a model system to explore the quantitative relationship between the mechanical properties of gels and the dissociation rate of crosslinking bonds.These researches can improve our understanding of the property-structure relationships in supramolecular systems and their associated influence on the macroscopic behavior of dynamically crosslinked materials.