4D Printed Liquid Crystal Elastomer And Its Thermally Induced Deformation Properties

Liquid crystal elastomer（LCE）is a kind of intelligent material that can realize reversible deformation under external stimulation.With the progress of synthesis technology,LCE has been applied to the field of 4D printing.Direct ink writing（DIW）technology can induce the orientation of LCE mesogens during the printing process,which is a main procedure of 4D printing LCE materials.At present,the DIW technology for LCE has insufficient printing accuracy and can not obtain materials with high orientation.As a result,LCE is mostly limited to in-situ deformation and it is difficult to realize controllable movement.Even if the motion behavior is realized through special structure or stimulus setting,it is still hard to obtain intelligence,which limits the further application of LCE as soft robot and intelligent sensing material.To solve these problems,in this paper,LCE materials with rich thermal deformation properties were obtained by using accurate DIW technology.Several stimulus responsing performances are realized by adjusting the printing path to form different mesomorphic orientation arrays.LCE materials with regular morphology and highly oriented mesogens were obtained by using an extrusion needle with an outer diameter of 0.25 mm and corresponding printing parameters.Calculating from 2D wide-angle X-ray scattering test,the order parameter of printed LCE is up to0.50.The highly oriented single LCE fiber achieved more than 40%reversible shrinkage along the printing direction when heated.By adjusting the geometry and printing path,a variety of thermally deformable materials are obtained,and the thermally driven responses such as bending,bulge and helix are realized.Based on the thermal deformation,the thermal motion behavior of printed LCE materials after deformation is further studied.It is found that due to the concentrated release of thermal stress,the LCE printed through concentric path can realize thermally induced driving performance such as thermally induced flea jumping and thermal pressure springing.The cross path printed LCE can realize untethered autonomous thermal induced rolling after forming a helical cylinder on a 180°C heating surface.It should be noted that these driving properties are obtained by one-time 4D printing without any subsequent processing or programming process.While the thermally induced rolling process is a completely spontaneous process.The rolling direction and speed are controlled by the curvature direction and size of LCE cylinder.The maximum rolling speed of LCE cylinder with a length of 10 cm can reach 48 cm min^-1.The autonomous rolling structure shows bionic tactile strain ability,and can climb over or turn back according to the height of obstacles it encountered.Its spiral cylinder shape provides load space and can pull goods more than dozens of times its own weight.Based on the driving performance of the above single component LCE materials,carbon-based materials such as carbon nanotubes and graphene quantum dots are introduced as photothermal agents to further realize the remote control of 4D printing LCE,through adding light-driving actuations in addition to thermal derived actuation.The light-response behaviors such as light driven deformation and light controlled rolling steering are realized through shape design of LCE composite.Generally,this discovery makes LCE materials no longer limited to simple actuators,but as a soft robot showing intelligent bionic behavior,which lays a foundation for realizing intelligent bionics,transportation,exploring unknown environment and other applications of LCEs.