Correlation Between The Internal-stress And Actuation Behavior Of Reversible Shape-shifting Polymeric Materials

Reversible shape-shifting polymers(RSSPs)are a class of intelligent materials whose shapes can shift reversibly under external stimuli,including responsive hydrogels,dielectric elastomers(DE),two-way shape memory polymers(2W-SMP),and liquid crystal elastomers(LCE).For RSSPs,the potentially achieved functions are directedly determined by the actuation modes that they can perform.The correlation between stress and strain,reveals that the actuation originates from the reversible generation/release of internal stress within the materials.Thus,the key to expanding its applications by enriching the actuation modes of RSSPs is to achieve precise control of their internal stress.For a pre-stretched elastic polymer,both a phase transition and bond exchange process can realize the regulation of the conformational entropy of the polymer chain to engine internal stress.The above two processes are also the molecular basis of shape memory polymers and dynamic covalent networks,respectively.Accordingly,we selected the 2W-SMP,DE,and LCE as the matrix materials,and rationally utilized the shape memory effects(physical strategy)and dynamic covalent networks(chemical strategy)to engine their internal stress.For 2W-SMP,we embedded light-and thermal-responsive reversible covalent bonds into the network simultaneously.The material could reconfigure its permanent shape via stress relaxation from the thermo-activated bond exchange and realize desired reversible shape memory performance via stress fixation based on the light-initiated crosslinking in a spatial-selective manner.The overall result is fabrication of a soft robot with a 3D complex structure and desired deformation behavior is successfully achieved.For DE,we also introduced dynamic covalent bonds into the elastomer network and relaxed the internal stress generated during the fixation of a favored 3D structure through the bond exchange reaction,thereby an external frame-free 3D structural DE was finally demonstrated.Moreover,the prepared DE device can reconstruct its actuation mode via repeatable permanent shape reconfiguration.For LCE,we introduced solvent during the preparation process to achieve a solvent-responsive shape memory effect in LCE.The reversible fixing/releasing of internal stress via this shape memory effect can realize the reprogrammable orientation of liquid crystal mesogens.Benefited from DLP printing method,the prepared LCE can undergo reversible actuation between its unlimited deformed temporary shapes and complex 3D printed permanent shapes.