Structure Design And Characterization Of Novel Rapid Response Clay/poly (n-isopropylacrylamide) Nanocomposite Hydrogels

Poly(N-isopropylacrylamide)(PNIPAAm)hydrogels have attracted much attention as the advanced soft materials because of thermo-sensitivity,resulting from their phase transition at their lower critical solution temperature(LCST≈32℃).However,conventional chemically cross-linking hydrogels(OR gels)are always brittle and exhibit weak mechanical properties,leading to limit the applications of hydrogels.Therefore,it is necessary to improve both the mechanical properties and response behavior of polymer hydrogels for wider applications.As reported previously,we have successfully prepared high clay content nanocomposite hydrogels by using Laponite XLS,one kind of clay modified by tetrasodium pyrophosphate(Na₂P₂O₇)(i.e.ionic dispersant).S-N gels have shown excellent mechanical properties, compared to OR gels.It is found that with increasing clay content, mechanical properties of S-N gels become better,but the response behavior of those are gradually decreased.A new temperature-sensitive clay/PNIPAAm hydrogels with rapid responsive rate would be preferable.Because the network structure of S-N gels is non-covalent,essentially different from the covalent network structure of OR gels,the structure and properties of S-N gels should be affected by posttreatment,such as solvent,heat,or light,etc.Generally, several methods have been used to improve the response dynamics,such as freeze-drying for forming porous structure,and preparation with phenol aqueous solution.With respect to the above considerations,this paper is focus on the superb enhancement of response behavior with the basis of certain mechanical property.The results are as follows:(1)A kind of rapid responsive nanocomposite hydrogels(S-N-H gels) was obtained by posttreated in HCl solution.The swelling/deswelling behavior of S-N-H gels has been systematically investigated by modulating posttreatment time and the concentration of HCl solution. Furthermore,it was surprisingly found that both the swelling ratio and deswelling rate of S-N gel were enhanced via the posttreatment.(2)Clay/PNIPAAm nanocomposite hydrogels with modified Ni(OH)₂ nanowires(S-N-Ni gels)were prepared by in-situ ultraviolet(UV) initiated polymerization.Since high surfactant energy,the dispersion performance of Ni(OH)₂ nanowires with different contents of PEG6000 and CTAB in the aqueous solution was discussed.The effects of these two surfactants on mechanical properties and response behavior of S-N-Ni gels have been investigated in detail.(3)As Ni(OH)₂ soluble in acid solution easily,S-N gels with porous micro-structure were successfully achieved in the method of S-N-Ni gels posttreated by HC1 solution.The equilibrium swelling ratio of the hydrogels could be greatly improved by the formation of porous micro-structure and the effects of HC1 solution posttreatment.In addition,we preliminary studied mechanical properties and swelling/deswelling dynamics of P-S-N gels,and analyzed the different response behavior with previous non-porous S-N gels.The ultrahigh swelling P-S-N gels with certain tensile strength and break elongation will widen the application of hydrogels,such as drug delivery systems and recyclable absorbents,etc.