Construction And Enhancement Mechanism Of Dual Nanocomposite Hydrogel With Inorganic Hybrid Crosslinking

The nanocomposite hydrogels with Laponite as a multifunctional crosslinker have excellent mechanical and optical properties.However,only with Laponite as crosslinker,the rigidity of the nanocomposite hydrogel can not be enhanced effectively while low toughness may be observed in SiO₂-based gel due to poor compatibility of SiO₂ nanoparticles,thus hardly to enhance the mechanical properties of the hydrogels further.The sol-gel method can synthesize organic/inorganic hybrid materials with uniform size under mild reaction conditions.The prepared organic/inorganic nano-silicon hybrid hydrogels integrate the stability of inorganic silicon and functionality of organic polymers together.Based on the reactivity of silane with the hydroxyl groups of clay,in this work,Laponite/SiO₂-based dual nanocomposite hydrogels were synthesized through a simultaneous sol-gel method and in-situ free radical copolymerization.The enhancement of dual nanocomposite mechanism was studied,and the strengthen mechanism of covalent bonds and non-covalent bonds between SiO₂ and polymers of dual nanocomposite was systematically investigated.The binding interaction of the monomer/Laponite/SiO₂ aqueous mixtures were studied through size distribution,zeta-potential,and viscosity measurements;The covalent bond between Laponite and SiO₂ was confirmed by 29 Si solid NMR spectra;The dispersion of Laponite and SiO₂ were observed by TEM and XRD;The dynamic mechanical properties of dual nanocomposite were measured using rheology technology;The mechanical properties of the dual nanocomposite gels were investigated by uniaxial tensile and compression tests.Finally,essences of mechanical enhancement are revealed by correlating microstructure and rheological behavior of the dual nanocomposite hydrogel.The main contents and results are as follows:?1?A series of tough poly?acrylamide-co-methacrylic acid?P?AM-co-MAA?/Laponite/SiO₂ dual nanocomposite hydrogels with improved compressive strength were fabricated through sol-gel technique and free radical polymerization of AM and MAA in the presence of Laponite and SiO₂ precursor ?-methacryloxypropyltrimethoxysilane?MPTMS?in an aqueous system.Covalently linked Laponite and SiO₂ worked as co-crosslinker.Due to the multi-strengthening mechanism of the dual nanocomposite,the hydrogel was super tough and strong with a compressive stress of 32.00 MPa without rupture even at 100% strain,while it exhibited excellent fatigue resistant properties.?2?Stretchable hydrogels have been developed by fabricating the dual nanocomposite structure with hydrophilic monomer and SiO₂ precursor tetraethyl orthosilicate?TEOS?in the presence of Laponite.The mechanical enhancement of this hydrogel was investigated in detail and it was markedly influenced by the monomer structure and contents of TEOS and Laponite.The strong interaction among NIPAM and inorganic hybrid crosslinker is accounted for the excellent mechanical properties of PNIPAM/Laponite/SiO₂ dual nanocomposite hydrogel.Uniaxial tensile tests showed high tensile property?fracture elongation: 1845%;fracture strength: 271.41 kPa?and elastic modulus of the hydrogel significantly increased with TEOS contents.Compression strength could reach 7.06 MPa and the corresponding fracture energy was 1185.53 J·m^-2,meanwhile it showed remarkable fatigue resistance.The mechanical reinforcement of this hydrogel is heavily relied on the covalent bonding of SiO₂ with Laponite and physical interaction of polymer chains with inorganic hybrid crosslinker.?3?The dual nanocomposite gel PNIPAM/Laponite/SiO₂ exhibited rapid phase transition upon shrinkage/reswelling processes in water-polar solvents mixtures.More impressively,the phase transition behavior and time could be controlled through versatile organic solvents.The attractive competitive effects in forming hydrogel-water hydrogen bonds and hydrogel-polar solvents hydrogen bonds were believed to be essential to the unconventional phase transition phenomenon.?4?In order to prove the enhancement mechanism of dual nanocomposite,the effect of direct addtion SiO₂ nanoparticles and the SiO₂ nanoparticles obtained from TEOS sol-gel method on the stability of the NIPAM/Laponite dispersion,as well as the influence of mechanical properties of PNIPAM/Laponite/SiO₂?SiO₂?nanocomposite hydrogel and PNIPAM/Laponite/SiO₂?TEOS?dual nanocomposite hydrogel were studied.Combined with the results of dynamic frequency sweep,creep-recovery test,and stress relaxation,it revealed that in the dual nanocomposite hydrogels,covalently bonded Laponite and SiO₂ worked as multifunctional inorganic hybrid crosslinker.The toughness-enhancement mechanism was not just the integration of polymer chain fracture/high-functionality crosslinker Laponite and SiO₂ plus meso-/macro-scale composites.More importantly,Laponite and SiO₂ were connected by covalent bonds to enhance compatibility among gel matrix and form hierarchical structure which could well maintain high elasticity under deformation.?5?The nanoparticles?eg.clay nanolayer: bentonite;carbon materials: GO,C-dot;metal oxide: TiO₂;polymer nanoparticle: CNCs?with abundant hydroxyl groups on the surface were selected.Made them compound with silane?MPTMS or TEOS?and worked as multi-function co-crossliker together to prepare dual nanocomposite hydrogel.Based on the results of compression test,it showed that the addition of SiO₂ could increase the strength and toughness of the gel.Therefore,under the premise of choosing appropriate silane,nanoparticles and pH value,the dual nanocomposite mechanism can be used as a universal enhancement mechanism for the application of tough gel materials.