Synthesis And Characterization Of High-strength Hydrogels(Ionogels)

Hydrogels（ionogels）are stretchable soft materials composed of polymers and solvents（water or ionic liquids）with good flexibility,designable mechanical properties,rich variable functional groups,customizable chemical structures,self-healing properties,electrical conductivity and multiple stimulus responsiveness,etc.Hydrogels（ionogels）have shown good application prospects in ionic skins,smart wear systems,health monitoring,interface adhesion,flexible energy devices,soft robotics,drug delivery and impact resistance,providing a material basis for the development of frontier technologies.However,the practical applications of hydrogels（ionogels）are limited by their short service life and poor reversibility due to the crack propagation sensitivity,the irreversibility of covalent crosslinked networks,and the unstable mechanical properties of reversible crosslinked networks under long-term cycling.The rational design of the topological configuration and the introduction of energy dissipation and enhancement mechanisms to substantially improve the mechanical properties of gels are urgent needs and existing challenges.This thesis focuses on the preparation of high-strength,stretchable,low-hysteresis,crack propagation insensitive,and fatigue-resistant tough hydrogels（ionogels）by constructing novel network topologies and comprehensively investigating their conformational relationships with the extreme mechanical properties.The main research results are as follows:1.Tough hydrogels with isotropic crack propagation resistanceA universal strategy for developing hydrogels with unprecedented isotropic crack propagation resistance only depending on the interpenetrating entanglements of polymer chains（PAAM or PAMPS）in deformable polymeric microspheres（PAMPS or PAAM）was proposed.The deformable interpenetrating network in microspheres can transform the hydrogel from isotropic to anisotropic instantaneously in any load direction,and effectively alleviate the stress concentration at the crack tip,dissipate energy,and eliminate notch sensitivity.Our best isotropic hydrogel displays an ultimate strain of 5300%,toughness of18.9 MJ m^-3,fracture energy of 157 k J m^-2,and fatigue threshold of 4.2 k J m^-2.Furthermore,the mechanical strength of hydrogels can be simply tuned by solvent replacement.The strategy presented here could be expanded to prepare other isotropic hydrogels with super tear-resistant and anti-fatigue properties,based on a wide variety of deformable microspheres and matrix polymers.2.Recyclable,healable,and tough ionogels insensitive to crack propagationPhysical crosslinking is different from irreversible covalent crosslinking and can effectively alleviate irreversible fatigue damage and crack propagation caused by long-term cyclic loading.Reversible bonds help to promote reversible mechanical interlocking and polymer segment reorganization to realize the recycling and reuse of ionogels.Here,we replace covalent crosslinking of host materials with entanglement.The entangled microdomains were used as physical crosslinking while introducing reversible bond interactions.The interpenetrating,entangled,and elastic microdomains of linear segments and covalent-network microspheres provide mechanical stability（20,000 cycles of compression）,eliminate stress concentration at the crack tip under load（crack propagation strain:5800%）,and achieve unprecedented tear and fatigue resistance of ionogels in any load direction.Moreover,reversible entanglements and non-covalent interactions can be disentangled and recombined to achieve recycling and mechanical regeneration,and the recyclability of covalent-network microdomains are realized.3.Hysteresis-free and tough gels with crack propagation insensitivity under large deformationsHysteresis-free hydrogels within a limited deformation range（ε<11）have been created by strategies such as nanocomposites,polyprotein crosslinking,stretching induced periodic crystallization,and network reconstruction.However,large deformation（ε>20）and high elasticity are inherently contradictory attributes.Herein,we present a nanoconfined polymerization（NCP）strategy for producing tough and hysteresis-free（or near-zero hysteresis）gels under large deformations（ε=0-80）.Gels with such a rapid self-reinforcement characteristic were prepared through in situ polymerization in covalent organic frameworks（COFs）or molecular sieves（MSs）with the mass fraction as low as 0.03wt.%COFs（or MSs）with unique nanochannel structure and strong hydrogen-bonding interactions with polymer segments were crucial for achieving this unusual mechanical property.Moreover,the rigid COFs（or MSs）effectively relieved stress concentration at the crack tips and prevented crack propagation,which significantly enhanced the ultimate fracture strain（17580±308%）,toughness(87.7±2.3 MJ m^-3),and crack propagation strain（5800%）of the gels.This strategy is generalizable based on a series of COFs（or MSs）and polymer combinations.4.Supramolecular ionogels tougher than metalsHigh-strength ionogels through the synergy of force-induced crystallization and halometallate ionic liquid created supramolecular ionic networks were developed.The prepared polyvinyl alcohol/halometallate ionic liquid ionogels show the excellent mechanical properties,including ultimate fracture stress（63.1±2.1 MPa）,strain（5248±113%）,and unprecedented toughness(1947±52 MJ m^-3)which is much higher than that of most metals and alloys.Furthermore,the ionogels can achieve reversibility by water to realize green recovery and restoration of damaged mechanical properties.