Investigation Of 4-arm Polyethylene Glycol Shape Memory Hydrogels

Shape memory hydrogels?SMHs?are one class of smart hydrogels,which can memorize a temporary shape and recover to the permanent shape under external stimuli.Among various stimulation methods,temperature is the most classical and common stimulation method,which has the advantages of low cost,simple operation and fast response.Therefore,temperature-responsive shape memory hydrogels have realistic research significance and wide application prospect.However,in the actual operation of shape memory,direct heating is unsafe and unrealistic.Also,operating temperature that is much higher than body temperature can be extremely harmful.These inherent defects of temperature-responsive hydrogels have greatly promoted the development of body temperature-responsive SMHs and essentially non-thermally induced SMHs.Traditional hydrogels have poor mechanical properties.However,hydrogels with uniform network structure can be easily prepared by using 4-arm polyethylene glycol?4-arm PEG?.In this way,the synergy of hydrogel network under stress is increased and the mechanical properties of hydrogel are improved.Based on the above background,in the first three chapters of this paper,4-arm PEG with good crystallinity was selected as the gel skeleton to prepare a body temperature and water-induced shape memory hydrogel with excellent mechanical properties.In the second chapter,the 4-arm PEG terminal was grafted with ethyl bromoacetate,and the ester bond was hydrolyzed to obtain carboxylate of 4-arm PEG?4-arm PEG-COOH?.Then,4-arm PEG-DA polymer was prepared by modifying dopa?DA?to 4-arm PEG terminal through amide reaction.Based on the oxidation mechanism of DA,the NaIO₄ was utilized to convert dopamine into quinone.Finally,the chemically crosslinked hydrogels were prepared by oxidation coupling between quinone.The gel has uniform structure and excellent mechanical properties.Tensile fracture strength can reach megapascal level ranging from 2.68to 13.02 MPa,and elongation at break can reach 2501%.In the third chapter,we investigated the body temperature and water-induced shape memory properties of 4-arm PEG hydrogel.Firstly,the effect of temperature and water on the crystallization phase was investigated by X-ray Diffraction?XRD?,and the effect of water on the melting point of crystallization phase was investigated by Differential Scanning Calorimetry?DSC?.The results show that both temperature and water can cause the reversible formation and destruction of the crystalline phase,and the melting point of the crystalline phase decreased as the water content increased.Then,bending experiment was used to investigate body temperature and water-induced shape memory properties of 4-arm PEG hydrogel.The results showed that the gel had excellent recovery performance under the stimulation of body temperature or water.Furthermore,under the synergistic effect of body temperature and water,the gel could recover faster and better,with a recovery time of 1 min14 s and a recovery rate of 98.9%.In the fourth chapter,two kinds of precursor polymers with the application prospect of shape memory function were prepared.Based on the 4-arm PEG-COOH synthesized in chapter 2,the?-Cyclodextrin??-CD?and azobenzene?AZO?were respectively grafted to the4-arm PEG terminal by amide reaction.¹HNMR was used to characterize chemical structures of 4-arm PEG-CD and 4-arm PEG-AZO.Based on the host-guest inclusion mechanism of?-CD and AZO,and photoisomerization of AZO,the 4-arm PEG-CD and 4-arm PEG-AZO are expected to form an optical responsive 4-arm PEG network.It can be used as a reversible phase,and the chemically cross-linked 4-arm PEG-DA network can be used as a fixing phase to prepare an optical responsive shape memory hydrogel with dual network structure.