Influences Of Phase-separated Structure Of Hydrogels On Their Mechanical And Electronic Behaviors

Gels have been widely used in tissue engineering,bioelectronics,and soft robotics due to their excellent biocompatibility,structural diversity,and stimulative responses.Gels are frequently required to have great mechanical strength and toughness in these applications.One strategy to enhance the high stretchability,strength and toughness of gels is to introduce the non-covalent bonds as sacrificial bonds into the polymer network.These hydrogels frequently exhibit structural heterogeneities and phase separation at the microscopical scale due to sophisticated physical interactions and the presence of chemical cross-linkers.Most research to date has been on the production of tough hydrogels or ionogels based on the principle of phase separation,whereas structural analysis and the impact of structural inhomogeneity on mechanical performance and conductivity remain unknown.In this paper,we investigated the structural formation and the influence of structural inhomogeneity on the mechanical performance and conductivity of H-bonded hydrogels and ionogels made from poly(N,N-dimethylacrylamide-co-methacrylic acid)(P(DMAA-co-MAAc)matrix.The mechanical behavior,swelling behavior and conductively of gels was effectively regulated by their chemical compositions.The structural characterization of hydrogels was carried out using various methods,and the relationship between deformation at the microscopic and macroscopic scales was established.In addition,we systematically explore the relationship between the structure,the mechanical property,and conductivity mechanism of ionogels.The following is a summary of the main works:(1)The effect of micro-phase separation on mechanical behavior of“dual cross-linked gels”(physically and chemically crosslinked)P(DMAA-co-MAAc)based on the H-bonds.Swollen and shrunk hydrogels with considerably varied mechanical properties were observed after systematically changing the chemical composition.The swollen hydrogels(Q_v>1)are soft and weak,with low fracture stress,elongation,Young’s modulus,and fracture energy,indicating typical neutral hydrogel behavior;however,the shrinking hydrogels(Q_v<1)have high elongation,tensile stress,Young’s modulus,and fracture energy,indicating strong and tough mechanical properties.The substantial Qv dependency of mechanical behaviors is suggested by the quantitative link between swelling and mechanical properties.Static SAXS results revealed that both swollen and shrunk hydrogel regions are noted to demonstrate structural heterogeneity with a non-uniform distribution of the polymer phase(from the nano-to submicron-scale).Dynamic H-bonds acting as reversible sacrificial bonds have been shown to have outstanding viscoelasticity and self-healing properties in uniaxial tensile and loading-unloading cycle testing.(2)Structural analysis of hydrogels P(DMAA-co-MAAc)based on small-angle X-ray scattering(SAXS)and small-angle neutron scattering(SANS)technology.It is noted that SANS contrast matching results further prove a highly heterogeneous microstructure for P(DMAA-co-MAAc)hydrogels.Combined with the contrast variation utilizing SANS,scattering invariant INV,and the filler-reinforced matrix model,the structure parameters,including the polymer volume fraction of the dense and sparse regions,volume fraction occupied in the space,and average correlation length of the long-range and short-range heterogeneous structures,were explored using a scaling model based on a two-phase system composed of the densely and sparsely cross-linked regions.Furthermore,by fitting the SANS profiles,it is concluded that both shrunk and swollen hydrogels have similar thermodynamic fluctuationsand distinct frozen correlation lengths.Furthermore,in situ SAXS analysis revealed that shrunk and robust hydrogels exhibited affine deformation,while swollen and neutral hydrogels followed the nonaffine deformation.The observed deformation modes resulted in the correspondingly distinct mechanical behaviors.(3)Synthesis of ionically conductive gels,and their structure,and conductivity analysis.The ionogels with excellent mechanical performances,promising electrical conductivity and good adaptability were prepared from the random copolymerization of DMAA and MAAc in presence of ionic liquid 1-ethyl-3-methylimidazole trifluoromethanesulfonate.The DMAA content and mass fraction of the ionic liquid were systematically adjusted in order to regulate its stretchability,mechanical strength,and conductivity.Ionically gels are attractive possibilities in the realm of flexible electronic devices due to their tunable mechanical qualities and conductive properties.SAXS results revealed a heterogeneous microstructure,which is most likely connected to the mechanical properties.The conduction mechanism and mobility of polymer segments of these ionogels at high and low temperature circumstances were explored using a combination of electrochemical impedance spectrum,rheology,and dielectric relaxation spectrum.The findings demonstrated that ionic liquid motion and polymer segment relaxation behaviors are intertwined and have a considerable temperature dependence.Furthermore,it is shown that these ionically conductive gels have strong adherence to a variety of substrates,making them ideal for making a strain sensor with highly sensitive resistance signals when deformed.This research is thought to open up new possibilities for advanced soft electronic devices.