Syntheses And Characterizations Of Multi-Responsive And Multiple Shape Memory Hydrogels

As a kind of the intelligent polymer which can restore the temporary shape to the original shape under the external stimulus(e.g.pH,heat,light),the shape memory hydrogel has been paid more and more attention in recent years.However,the current shape memory hydrogels have two disadvantages that a single stimulus response and a limited amount of the fixed temporary shape,which will confine its application.In view of these two aspects,we have prepared the multi-responsive hydrogel and multiple shape memory hydrogel respectively.The main contents of this paper are as follow:(1)A multi-responsive shape memory hydrogel.The chitosan(CS),oxidize dextran(Odex)and acrylamide(AAm)were mixed and the first network(PAAm)was formed by AAm polymerization as a permanent network to maintain the original shape,and the second network was formed by Schiff base bonds or CS-metal coordination as the temporary cross-links to realize the shape memory effect of the hydrogel.Firstly,the temporary shape of the hydrogel can be fixed because of the amino groups of CS and the aldehydes of Odex could form the Schiff base bonds under the basic conditions and the temporary shape recover to the original shape in acid,alanine or vitamin B6 solutions,respectively.Moreover,the CS can coordinate with a variety of transition metal ions.The CS-Ag+ hydrogel was chosen as the model system and subjected to systematical study.The hydrogel deformed temporary shape fixed by Ag+ respectively immersed in the EDTA·2Na,halide ions,oxalate or H2SO4 solutions,the recovery process could be observed.Finally,taking advantage of the two reversible interactions,triple shape memory effect could be successfully achieved by programmable changing of the external stimulus,and the triple shape recovery could be accomplished step by step or in one step.(2)A multiple shape memory hydrogel.The chitosan(CS)and acrylamide(AAm)were mixed and the PAAm as a permanent network to maintain the original shape.As CS is able to self-assemble into microcrystalline and chain entanglement by simple soaking in alkaline and saline solutions,respectively,the two physical interactions could be applied to stabilize temporary shapes,and endow the hydrogel dual shape memory ability.Moreover,taking advantage of the diffused transition mechanism,multiple shape memory effect could be successfully accomplished.In addition,by combining the two physical interactions,programmable triple shape memory effect could be achieved.