Mechanical Enhancement And Stimuli-responsiveness Of Smart Nanocomposite Hydrogel

Responsive polymer hydrogels are promising materials for the fabrication of smart soft sensors and devices in areas of environmental monitoring,biomedical materials etc.,which requires better performance and functionality of hydrogels.Conventional crosslinking hydrogels show poor mechanical property and slow response rate and the intent to improve mechanical properties leads to slower responsiveness and vice versa.In contrast,organic/inorganic nanocomposite hydrogels demonstrate to maintain strong and toughness with capacity to fast response owing to their unique network structures.However,the instability of nanoclay in solutions of salts or organic ionic monomers may result in aggregation of clay sheets,thus limit the functionality and responsiveness of nanocomposite hydrogels(NC hydrogel).The responsive NC hydrogels now are mainly temperature-sensitive,lacking diversity of functionality and responsiveness.Further exploration of NC hydrogel functions may enhance smart hydrogels' applications as sensors and actuators.In this thesis,responsive NC hydrogels are prepared and their responsiveness for pH,ionic strength,and even actuation are studied.The mainly results are as follows:(1)Responsive nanocomposite polyelectrolyte hydrogels were synthesized by in-situ copolymerization of ionic monomers and acrylamide(AAm)upon clay platelets in the presence of tetrasodium pyrophosphate decahydrate(TSPP),without using any chemical crosslinkers.TSPP was added to avoid severe aggregation of clay dispersions,thus ensured the clay sheets' role as multifunctional crosslinkers.NC hydrogels with negative charges were obtained by copolymerization of acrylamide(AAm)and 2-acrylamido-2-methylpropanesulfonic acid(AMPS),while positively charged NC gels were obtained by using AAm and dimethylaminoethyl methacrylate methylchloride(DMC).The responsive behaviors of these NC polyelectrolyte hydrogels in pH and ionic strength buffer solutions have been investigated.The hydrogels were able to actuate in electric field,where actuating rate were depended on the electric field strength and ionic monomer content of the hydrogels.These hydrogels showed outstanding fatigue resistance against cyclic compression loadings,and high tensile strength and toughness.After tensile tests,the gels were able to recover at room temperature.Finally,the oppositely charged hydrogels are assembled,with the interface strength increasing with the concentration of ionic monomers,which suggests a critical role played by electrostatic attraction at the interface of oppositely charged NC hydrogels.(2)The reversible printing techniques were employed in a nanocomposite hydrogel system containing poly(vinyl amine-co-acrylamide)to create a hydrogel actuator.PAAm/MMT NC hydrogels were synthesized by free radical polymerization of acrylamide(AAm)monomer with exfoliated NaMMT and KPS as initiator.Part of the amide groups were converted into primary amine groups through Hofmann rearrangement in order to entitle the capability to chelate metal ions,after which ionopring techniques were applied to introduce metal ions to change local crosslinking through amine-metal ions complexation.The imprinted hydrogel showed differential swelling and shrinking in water and organic solvent,thus a cross-like actuator was designed to manipulate small objects out of water.Gel clippers that underwent folding and unfolding through redox reactions of metal ions by changing AC current directions were also designed to pick up light objects in the air.The resulted hydrogel actuator could manipulate small objects both in air and in liquid solutions in minutes and in seconds,respectively,and looked for further device processing.