Design And Characterization Of Shape Memory Hydrogels Based On Different Interactions

At present,the research of intelligent materials is a popular discipline,the shape memory material is a kind of intelligent material.Shape memory materials can fix a temporary shape in a certain or more stimulating environment（such as pH,light,redox,electricity,magnetic field,temperature）with an external force,and can restore to the original permanent shape when stimulated in certain environment.Shape memory materials developed from the initial Ti-Ni shape memory alloys to various plastic and rubber shape memory polymers,and then to the new shape memory hydrogels.From the development trend of shape memory materials,it can be seen that shape memory hydrogels combine various advantages of shape memory polymers and smart hydrogels,created new areas of research,excavated the huge application potentials in the molecular components,micro-robot,biomedical and other high-tech fields.However,most of the current researches on shape-memory smart hydrogels are almost confined to the thermal response based on the hydrophobic interaction.Therefore,developing a new type of thermally responsive shape memory hydrogel and other variety of responsive hydrogels,as well as the applications of shape memory hydrogel are the trend in the future.Otherwise,the shape of the most shape memory hydrogels are fixed by the chemical crosslinking,and the shape memory function is realized by the physical crosslinking,as a result,the hydrogels can not be reused and being the pollution of the environment.In this paper,a simple strategy is proposed to design and fabricate high-performance shape memory hydrogels by combining the hydrophobic interactions between hydrophobic segments and the coordination interactions between the metal ions and some groups and the host-guest interactions.The main innovative contribution of this thesis can be divided into the following three sections:（1）A novel ferric-phosphate induced shape memory（SM）hydrogel is prepared by the one-step copolymerization of isopropenyl phosphonic acid（IPPA）and acrylamide（AM）in the presence of a crosslinker polyethylene glycol diacrylate（PEGDA）.Different from the traditional SM hydrogels,our SM hydrogel can be processed into various shapes as needed and recovers to its original form in ’multiconditions?such as in the presence of a reducing agent or in the presence of a competitive complexing agent.This unique feature is attributed to the fact that the oxidized ferric ions show a high complexation ability with phosphate groups of IPPA,which acts as a physical crosslinker to form the secondary networks within the hydrogels to induce the shape memory effect.The memory behavior was totally reversible,owing to Fe³⁺ that can be reduced to Fe2+ and extracted by the complexing agent such as EDTA.2Na.（2）A novel thermally sensitive shape memory（SM）hydrogel is prepared by block copolymerization of a cationic surfactant monomer,dimethylhexadecyl[2-（dimethylamino）ethylmethacrylate]ammoniumbromide（C16DMAEMA）,and acrylamide（AM）in the presence of a-cyclodextrin（a-CD）using N,N’-methylenebisacrylamide（MBA）as a crosslinker.XRD,solid state ¹³C NMR,and DSC measurements show that the crystalline domains,induced by the hydrogen bonds between a-CDs threaded on the hydrophobic units of the polymer chains through the host-guest approach,can reversibly melt and crystallize at different temperatures.Rheological measurements show that both the elastic modulus G’ and viscous modulus G" drastically change due to the formation and dissolution of the crystalline domains.These thermo-sensitive crystalline domains serve as reversible physical crosslinks,endowing the hydrogel with excellent SM properties.（3）A novel dual physically crosslinked shape memory hydrogel is prepared from simple ternary copolymerization of acrylamide（AM）,acrylic acid（AA）and n-octadecyl acrylate（C18）,in the absence of chemical crosslinkers.Through either ionic/complex binding of carboxyl groups via trivalent cations and hydrophobic association,the hydrogel can memorize a temporary shape successfully,which recovers to its permanent form in the presence of a reducing agent or other complexing angent,correspondingly.This kind of physically crosslinked hydrogel is in strong contrast to the conventional chemically cross-linked shape memory hydrogels.This fascinating feature undoubtedly enriches the shape memory hydrogel systems.Thus,we believe that the facile strategy could provide new opportunities with regard to the design and practical application of stimulus-responsive hydrogel systems.