Preparation And Performance Research Of Multi-responsive PNIPAM Hydrogel Actuator

In recent years,as a member of the family of intelligent hydrogels,poly（N-isopropylacrylamide）（PNIPAM）-based hydrogels have attracted wide attention because of their temperature-responsiveness.Among them,hydrogel actuator is an important application field of temperature-responsive PNIPAM based hydrogels.However,at present,most PNIPAM-based hydrogel actuators still have some problems,such as few response types,slow response speed,poor mechanical properties and narrow application range.Based on the above issuses,the purpose of this work was to construct multi-responsive PNIPAM-based hydrogel actuators.The composite nanomaterials were introduced into PNIPAM hydrogel by one-pot method,and the asymmetric structure of hydrogel actuator was designed by the assistance of gravity and magnetic force.Finally,the chemical structure,micro-morphology,mechanical properties,actuating effect and practical application were tested and analyzed.The details are as follows:（1）In this chapter,a Au@PDA/PNIPAM hydrogel actuator was constructed with with temperature and near-infrared（NIR）resonpsiveness,which was composed of the photothermal conversion material gold/polydopamine nanoparticles（Au@PDA NPs）and PNIPAM.Au NPs were loaded onto the surface of PDA nanoparticles,resulting in the successful preparation of Au@PDA NPs.Au@PDA NPs can endow PNIPAM-based hydrogels with NIR-responsiveness,and the natural sedimentation process of Au@PDA NPs made Au@PDA/PNIPAM hydrogels have asymmetric structure.Compared with pure PNIPAM hydrogel,the elongation at break of Au@PDA/PNIPAM hydrogel increased to 186%,and the tensile strength also increased to 18 k Pa.In deionized water at 55℃,the Au@PDA/PNIPAM hydrogel actuator could reach its maximum bending angle of 689°in 14 s.When irradiated with 1.6 W/cm² NIR light,the Au@PDA/PNIPAM hydrogel actuator could reach the maximum bending angle of 311°in 20 s.The Au@PDA/PNIPAM hydrogel actuator exhibited reversible and stable temperature and NIR light responsiveness under the repeated stimulation of NIR-20℃and 55℃-20℃.It has been successfully applied to circuit switches,bionic flowers and soft manipulators,and which verified the great practical potentials.（2）In this chapter,a novel PNIPAM based hydrogel actuator was constructed,which simultaneously had temperature-NIR-magnetic triple responsiveness.Firstly,magnetic graphite oxide（GO/Fe₃O₄,m GO）nanomaterials were prepared,which have both magnetic responsiveness and photothermal conversion ability.Then m GO was introduced into PNIPAM hydrogel with the aid of magnetic field in the process of polymerization to form the gradient distribution of m GO.Therefore,the asymmetric structure of m GO/PNIPAM hydrogel could be divided into m GO-rich layer and m GO-free layer.The m GO/PNIPAM hydrogel actuator bent 647°within 30 s in deionized water at 55℃,293°in 45 s under 1.6 W/cm² NIR light,and 74°within 3 s in magnetic field.The cyclic bending behavior in temperature,NIR irradiation and magnetic field also confirmed that the m GO/PNIPAM hydrogel actuation had good stability and reversibility.The m GO/PNIPAM hydrogel actuator was successfully used for NIR circuit switching,simulating petal closing,starfish movement and bionic octopus hunting.（3）In this chapter,silver@graphite oxide（Ag@GO）nanoparticles and polyacrylic acid（PAA）were introduced into temperature-responsive PNIPAM hydrogels to make them have quadruple responsiveness.PNIPAM hydrogels have temperature responsive.Ag@GO nanoparticles were introduced to endow PNIPAM hydrogel with NIR-responsiveness,and PAA was introduced to endow the hydrogel with salt-responsiveness and p H-responsiveness.The hydrogel actuator bent to 189°within 60 s in deionized water at 55℃;The hydrogel actuator bent to 192°in 25 s under the NIR light of 808 nm 1.6 W/cm²;In 1 mol/L Na Cl solution,the hydrogel actuator bent to the maximum bending angle of 125°within 60 s;In 1 mol/L HCl solution,the hydrogel actuator bent to the maximum bending angle of 175°within 15 s;In 1 mol/L Na OH solution,the hydrogel actuator bent to 140°in 180 s.Because the hydrogel actuator had quadruple responsiveness,it was successfully used for circuit switching and simulating petal closing,palm grasping and releasing.（4）It is reported that PDA has good adhesion.Therefore,in this chapter,PDA was modified on the surface of MXene to obtain the PDA@MXene composite nanomaterials.It was then added into the PNIPAM hydrogel to prepare the PDA@MXene/PNIPAM hydrogel.Finally,the segmented hydrogel actuators were assembled with adhesive tape,filter membranes,and hydrogels.Through the NIR light heating test and mechanical properties test,it was found that the photothermal conversion ability of PDA@MXene was enhanced,and the mechanical properties of PDA@MXene/PNIPAM hydrogel were also enhanced.The segmented hydrogel actuator bent to470°within 1.75 s in 55℃deionized water and 203°within 15 s under irradiation of 808 nm,6W/cm² NIR light.Moreover,the cyclic bending behavior under 55℃and NIR light irradiation also confirmed the good actuation stability of the segmented hydrogel actuator.The segmented hydrogel actuators could be programmed to achieve unidirectional bending,bidirectional bending and rolling.The segmented hydrogel actuator could also simulate finger bending,petal closure,words and Tai Chi movements.Based on the excellent temperature and NIR responsiveness of segmented hydrogel actuators,they had been successfully applied to smart devices including hydrogel hooks,circuit switches,and fluid valves.