Smart Polymer Hydrogel Based On Supromolecular And Dynamic Covalent Bonds

Smart polymer hydrogel materials are one of the important research directions in the field of materials.The introduction of supramolecular and dynamic covalent bonds into smart polymer hydrogel materials has also attracted great attention.In this paper,a novel shape memory hydrogel with adjustable mechanical properties and multi-shape memory effects was prepared by a simple one-pot method.The prepared hydrogel contains three reversible interactions,including the coordination of the carboxyl group in the sodium alginate with the Fe3+,the formation of dynamic borate ester bond between the borax and the PVA,and the physical crystallization formed by freezing of the PVA,these three reversible interactions can be used to memorized the temporary shape and impart multi-shape memory behavior to the hydrogel.Furthermore,the mechanical properties of the prepared hydrogel can be adjusted by changing the concentration of iron ions,borax and the number of freezing/thawing.In order to impact the shape memory hydrogel multi-responsive in shape recovery,we prepared a multi-shape memory hydrogel by combining oxidation sodium alginate,PVA and acrylamide.The hydrogel forms a Schiff base in alkaline solution to fix the temporary shape,and the Schiff base bond will be destroyed under acid solution or solution with a lot of amino or aldehyde groups,the temporary shape will recovery in the end.The carboxyl group in the oxidized sodium alginate can be chelated with calcium ions to fix the shape of the hydrogel,and the fixed shape of hydrogel can be recovered by immersing the hydrogel in sulfate or carbonate solution.The polyvinyl alcohol in the hydrogel can form a dynamic borate bond with the borax to fix the hydrogel shape,and the borate bond can be destroyed under acid solution or in glucose solution to restore the shape.Based on the dynamic covalent bond,we introduce it into a temperature-driven smart bilayer hydrogel,in which the lower is shape memory layer which contains oxidation sodium alginate and polyacrylamide,and the upper layer is actuation layer which contains poly-N-isopropyl acrylamide.When the chemically cross-linked poly-N-isopropylacrylamide layer hydrogel was immersed in a salt solution at 45 ?,the volume suddenly contracted,and a large amount of water was discharged,and it was bent toward the poly-N-isopropylacrylamide side.When the curved bilayer hydrogel is immersed in a sodium hydroxide solution at 15 ?,the upper hydrogel will absorb a large amount of water,the bent hydrogel will recover,and the hydrogel will not recover when recovered to a certain extent,and then the hydrogel will be recovered again when soaking in acid solution at 15 ?.This hydrogel does not require an external force to fix the hydrogel shape compared to conventional hydrogels.