Functional Tough Hydrogel Materials Based On Polyvinyl Alcohol

Hydrogel bears the advantages of high water content,good biocompatibility and large porosity,so it has potential applications in biomedicine and soft electronics.In order to use hydrogels as structural matrix materials?such as load-bearing materials?,the mechanical properties of hydrogels have been intensively studied.At present,several strong and tough hydrogel systems have been convincingly demonstrated.However,different application scenarios have specific requirements on the mechanical response behavior of hydrogels.Therefore,rational design of the hydrogel's mechanical response behavior is of practical and scientific significance.Secondly,in practical applications,besides the mechanical properties of hydrogels,it is also very important to impart specific functionality in hydrogel.As soft electronics and sensors,hydrogels are required to be conductive,while as biomedical materials and actuators,hydrogels are required to respond to different stimuli,with programmable shape-morphing behavior.Therefore,Design and construct functional and strong hydrogels has always been the focus of research in the field of hydrogels.At present,most hydrogels cannot comprehensively integrate mechanical properties with functionalities such as conductivity or shape memory effect.For example,for conductive hydrogels,the mechanical strength of existing conductive hydrogels is relatively low.The current strategy for improving mechanical properties is mainly to use non-conductive flexible substrates.However,the use of non-conductive polymer matrix may adversely affect the conductive performance.Also,the fabrication processes are often complicated.For thermally induced shape memory hydrogels,most of them exhibit dual shape memory effects,but specific applications often require customized shapes or even secondary shaping,so multiple shape memory hydrogels are rare yet desirable.At present,the preparation of thermally induced multiple shape memory hydrogels is mainly achieved through the combination of two crystalline hydrophobic groups,as demonstrated in a dual-melting point hydrogel system,so that the hydrogel has a triple shape memory effect.There are shortcomings such as poor mechanical properties and complex synthesis.In order to address above-mentioned problems,this thesis conducts the research content as follows:First,we synthesis a series of octyl modified PVA derivatives,PVA-C8-DS polymers with prescribed PVA backbone and side chains but various DS values?0.1-0.5?.These polymers are converted into as-prepared hydrogels?as-C8-DS?via solvent-non-solvent exchange.We study the volume shrinkage behavior of the as-C8-DS series by being immersed in hot water at various temperatures.Under the optimized hydrothermal treatment,a series of strong and tough C8-DS hydrogels are obtained.We measure the tensile behaviors of both as-C8-DS and C8-DS series at varied initial strain rates.By analyzing the first-order differential on the tensile curves of as-C8-DS and C8-DS series,we quantitatively decouple the mechanical contribution of strong and weak hydrophobic clusters to unveil the structure of dispersed hydrophobic clusters.In combination with this structure information,we discuss the effect of hydrothermal treatment and DS on the mechanical properties,including Young's modulus,fracture stress and fracture strain.This work sheds light on the correlationship between structural parameter of hydrogel and dispersed hydrophobic clusters.Secondly,by optimizing the length of the alkyl side chain on the polyvinyl alcohol derivative and DS,we selected a type of thermally programmable and malleable multiple-shape hydrogel.After undergoing water vapor exchange and hydrothermal treatment,the hydrophobic alkyl side chains of the polymer can form hydrophobic clusters with different strengths as"dispersible"physical crosslinking net-points.Due to its dynamics and reversibility,multiple temporary shapes and permanent shapes can be edited under external force to achieve a well-designed and complex shape change.This hydrogel system can be used as an intelligent device.The hydrogel performs thermally induced bidirectional twining around a one-dimensional substrate.In organizational applications,it is expected to achieve self-mounting and self-detaching behavior of soft devices on tissues with minimal invasionFinally,we adopted a simple and effective immersion treatment is employed to concurrently enhance mechanical properties and electrical conductivity of the hydrogel.By controlling the soaking time,the mechanical properties and electrical conductivity can be adjusted simply and effectively.The significant improvement in mechanical properties and electrical conductivity is mainly attributed to multiple non-covalent interactions.By controlling the soaking time,the best sample obtained showed high water content?75 wt.%?,high tensile stress?about 2.5 MPa?,high elongation?>600%?,reasonable electrical conductivity(about 25 m S cm^-1),and the characteristics of rapid self-healing with the help of hot water,and demonstrated that the conductive hydrogel can be used as a strain sensor.This method is not only applicable to conductive hydrogels,but can also be applied to other functional hydrogels with weaker mechanical properties.