Preparation And Application Of Laponite-based Nanocomposite Hydrogels

Laponite nanosheets have been widely used to prepare tough nanocomposite hydrogels.However,due to the electrostatic interaction between the nanosheets,"house of cards"structures are inclined to form in aqueous media,leading to inhomongeneous dispersion which thereby limits the further improvement of the mechanical properties of the nanocomposite hydrogels.In this paper,a simple supramolecular assembly strategy is proposed to significantly improve the dispersibility of Laponite nanosheets in aqueous media.As a result,the mechanical properties of the Laponite-based nanocomposite hydrogels are significantly improved.The prepared hydrogels exhibit stable and efficient adsorption performance for cationic dyes.The detailed research content is as follows:1.Based on the electrostatic properties of Laponite nanosheets in aqueous media,a method to improve the dispersion of Laponite nanosheets in aqueous media is proposed.Upon addition of anionic poly(acrylic acid)(PAA),the cationic edges of Laponite nanosheets were effectively screened due to the adsorption of PAA driven by electrostatic interactions,which can effectively inhibit the formation of the "house of cards" structures between the Laponite nanosheets,thereby significantly improving the dispersibility of the Laponite nanosheets in the aqueous medium.As such,Laponite/PAA nanocomposite dispersion with high concentration but low viscosity can be obtained.The formation of Laponite/PAA composite nanosheets were confirmed by proton nuclear magnetic resonance spectroscopy;the influences of the molecular weight of PAA and pH value on the dispersion of Laponite nanosheets were revealed by rheological tests.2.On the basis of the above work,nanocomposite hydrogels bearing excellent mechanical performances were prepared by adding a high concentration of Laponite/PAA composite nanosheets because of the good dispersibility of Laponite/PAA composite nanosheets in aqueous medium.A series of PAAm nanocomposite hydrogels containing different contents of Laponite nanosheets and crosslinking moities were prepared through the insitu polymerization and crosslinking of acrylamide(AAm)in the presence of different concentrations of Laponite/PAA composite nanosheets and the crosslinking agents.The prepared hydrogels showed high stretchability and toughness,with a tensile elongation of 2500%and a tensile strength of 1.2 MPa.The microstructures of the prepared nanocomposite hydrogels were systematically characterized by scanning electron microscope(SEM)and high resolution transmission electron microscope(HRTEM);the tensile tests were used to investigate the influences of Laponite concentrations and cross-linking agent concentrations on the mechanical properties of the nanocomposite hydrogels,and the underlying mechanism for the improvement of mechanical properties of the prepared nanocomposite hydrogels was revealed.3.Based on the electrostatic interaction between the negatively charged surfaces of Laponite in the prepared hydrogel and the cationic dyes,a study on the adsorption of cationic dyes by the hydrogels in an aqueous environment was carried out.The effects of adsorption time,salt concentration,pH value,and initial dye concentration on the dye adsorption ratio were explored.The results showed that the adsorption ratio of the crystal violet(CV)and rhodamine B(RhB)by the prepared nanocomposite hydrogels can reach 82.78 and 85.50%within 2 hours,respectively.Subsequently,four adsorption kinetics models(pseudo-first-order kinetics model,pseudo-second-order kinetics model,intra-particle diffusion model,Elovich model)and two adsorption isothermal models(Langmuir model,Freundlich model)were used to numerically fit the adsorption process with the aim to reval the adsorption mechanism.The fitting results demonstrated that the adsorptions of CV and RhB by Laponite/PAA/PAAm hydrogels were achieved through a electron sharing or exchange process,and it seemed that the adsorption process is a homogeneous monolayer adsorption.