Studies On Preparation And Properties Of Double Physical Crosslinked Self-healing Hydrogels

Hydrogels composed of reversible dynamic bonds have the characteristics of self-repairing for extending their service life.However,the mechanical strength and repair efficiency of the self-repairing hydrogels are still low.Changing the structure of the crosslinked network in the self-repairing gels is an effective way to improve their performances.In this study,two kinds of interactions are introduced to prepare the gels with double cross-linked networks.The hydrophobic association is as a first crosslinking point,and the interactions of the adsorption of sodium montmorillonite?MMT?or the ionic bond of iron ions(Fe³⁺)are as the second cross-linking point to prepare three kinds of double-physical cross-linked hydrogels.Their swelling behavior,viscoelastic behavior,mechanical properties,self-repairing properties and self-healing performances were investigated in detail.First,a typic surfactant self-assembled in the water to form micelles.Then,one monomer,octadecyl acrylate entered into the micelle,drived by the force of hydrophobic interaction.Poly?methacrylic acid-octadecyl ester acrylamide?/montmorillonite double crosslinked hydrogel?HPAAm/MMT DC?were prepared by subsequently adding another hydrophilic monomer acrylamide and MMT into the micelle solution,based on the hydrophobic association and MMT physical adsorption in situ micellar copolymerization.The structure of the gel was confirmed by fourier transform infrared spectrum.The microstructure of the gel was controlled by changing the amount of MMT.Scanning electron microscopy?SEM?showed that the pores of the gel with addition of MMT became smaller and more uniform,and the pore wall became thicker.The swelling rate test showed that the equilibrium swelling rate of gel firstly increased and then decreased with swelling time.MMT caused the double crosslinking of the gel to increase the interaction among the gel networks.At the same time,it also increased the dissipation of the gel during the stretching process.Rheological and tensile tests showed that storage modulus,tensile modulus and tensile strength of the gel were improved.HPAAm/MMT DC gel could be self-repaired,based on the reversibility of physical adsorption and hydrophobic interaction,and the repairing efficiency was 70%.The gel has excellent self-recovery and anti-fatigue properties,which are automatically restored within 20 min after deformation at room temperature.After repeated 6 times,the self-recovery efficiency still kept without change.In order to further study the effect of the interactions of the double cross-linked network on the gel properties.On the basis of hydrophobic cross-linking,adding Fe ions are known to form ionic bond with carboxyl groups.Therefore,acrylic acid was used to fabricated double cross-linked hydrogel.After polymerization and addition of Fe³⁺,poly?acrylic acid?/Fe³⁺ double crosslinked(HPAA-Fe³⁺ DC)gel was prepared.The addition of Fe³⁺ caused the equilibrium swelling of the gel to increase and then decrease.Cyclic tensile test results showed that HPAA-Fe³⁺ DC gels could effectively dissipate mechanical energy during the stretching process compared to single crosslinked gels.The results of tensile test and rheological test showed that storage modulus,tensile modulus and tensile strength of the gel could be improved by adjusting the cross-linking degree of the hydrophobic interaction and the ionic bond ineraction.The hydrogel showed good tensile properties,and the optimum tensile strength of HPAA-Fe³⁺ DC gel achieved to 0.73 MPa.The repairing efficiency of gel was 95% at 24 h,and the self-recovery of gel was 91% at 40 min,indicating excellent self-repairing and self-recovery performances of the gel.The introduction of strong ionic bond interaction into the gel network could indeed further improve the comprehensive performance of self-repairing hydrogel.In the double cross-linked gel,the hydrophobic association cross-linking point is affected by the structure of the surfactant.In order to investigate the effect of surfactant structure on the properties of double cross-linked gel,polyvinyl alcohol?PVA?macromolecule was used as surfactants to prepare poly?acrylic acid?/Fe³⁺ double crosslinked(HPAA-Fe³⁺@PVA DC)hydrogel based on the hydrophobic association.The tensile-retraction test showed that the addition of PVA resulted in an increase in dissipation energy during gel deformation.The maximum tensile strength of HPAA-Fe³⁺@PVA DC gel could reach 0.93 MPa,which increased the tensile modulus and tensile strength by adjusting the content of Fe³⁺.The entanglement of PVA macromolecules increased the mechanical strength of the gel.HPAA-Fe³⁺@PVA DC gel was repaired at room temperature for 24 h,and the self-healing efficiency was above 70%.The gel recovery rate was 89% after 70 min at room temperature.The double-physical crosslinking hydrogel could be fabricated by the interactions of the ionic bond,physical adsorption and the hydrophobic association.The change of the network structure can improve the rheological properties,mechanical properties,self-repairing performances and self-recovery performances of the gels.