Synergistic Regulation Of Shape Memory And Humidity-responsive Deformation Of EAA Films

Based on the deformation behavior of animals and plants,shape changing polymer(SCP)is one of the current research hotspots.It has the ability to respond to external environment and reversibly change shape.Humidity,as one of the important environmental parameters,plays an important role in all areas of daily life.However,compared with heat,light,electric field,and magnetic field responsive shape changing polymers,there are few studies on polymer materials with humidity-responsive deformation.The key to the control of humidity-responsive deformation lies in the anisotropic structure of the material.Existing research focuses on the local control of the anisotropic structure to increase the complexity of the humidity-responsive deformation behavior.However,due to the material design and molding process,the shape and structure of the existing humidity-responsive polymer materials are difficult to change once they are formed,so that they can only follow a fixed path which provides a new strategy to achieve a strategic combination of a planar two-dimensional shape transformation(in the dry state)and a specific three-dimensional shape change(in the wet state).The main challenge to further improve its deformation ability is the control of its shape and deformation path.On the other hand,shape memory polymer(SMP)can realize the responsive change of shape through the storage and release of strain.SMP with a single initial shape can store different strains through shape programming,and deform into different temporary shapes.The adjustability of process mentioned above is excellent.However,SCP and SMP achieve deformation based on the different materials,structures,and mechanisms,leading to current efforts are being made on a strategy of the combination of double-layer composites,and the drawback of these work lies in that the preparations are cumbersome and the deformation behaviors restrict each other.Herein we demonstrated the combination of respective advantage of SCP and SMP can be a efficient path to achieve tunable and reversible stimulated deformation.The development of new combining strategies is still an important challenge for related research.We noted that a new strategy of introducing humidity-responsive capability into the SMP matrix through surface treatment.Taking polyethylene-acrylic acid copolymer(EAA)as an example,a series of thicknesses of EAA film are prepared by solution molding,and surface treatment makes its humidity-responsive deformation more remarkable.Combined with shape memory property,EAA film realizes the flexible and adjustable humidity-responsive deformation behavior of the shape and reversible path.The specific research work is summarized as follows:(1)The surface treatment of EAA film by acid and base solution induces the reversible conversion of carboxyl groups on the molecular chain to carboxylates,so as to obviously control the humidity absorption capacity of the material.Due to the gradient penetration of the solution,the surface treatment progresses a gradient transformation mentioned above which realizes the reversible deformation of the humidity response.Further combining with the shape memory property,through repeated shape programming and surface treatment,the film is given flexible and adjustable humidity-responsive and reversible deformation behavior.In addition,the water molecules absorbed by the film can be used as a conductive path,giving the film the potential of sensing breathing and deformating to realize a smart switch of the conductive path.(2)Through the surface treatment of base solution and ferric chloride aqueous solution,iron ions are introduced to form an iron ion-carboxylate gradient coordination cross-linking network on the surface of the EAA film.The coordination cross-linking is irradiated with ultraviolet and visible light to reduce the iron ion to ferrous ion and dissociate,and the formed carboxylate has significantly enhanced hygroscopicity.The significant change in hygroscopicity makes the material a photo-programmable humidity-responsive deformation behavior.Furthermore,combining with the shape memory property of the EAA film,the flexible control of the shape and humidity-responsive deformation path will realize.In summary,this research can provide a new strategy for realizing the flexible adjustment of humidity in response to SCP deformation behavior and the strategic combination of SCP and SMP.