4D Printing of a Liquid Crystal Elastomer with a Controllable Orientation Gradient.

Liquid crystal elastomers (LCEs), a class of soft materials capable of a large and reversible change in the shape under the trigger of external stimuli, can be fabricated into diverse architectures with complicated deformation modes through four-dimensional (4D) printing. However, the printable LCE ink is only in the form of monomeric precursors and the deformation mode is limited to contraction/extension deformation. Herein, we report a novel approach to break through these limitations. We achieved 4D printing of a single-component liquid crystal polymer ink in its isotropy state through direct ink writing (DIW) technology. The drawing force imposed by the movement of nozzle in the extruded printing process was able to align the mesogen units along the specific printing path. An orientation gradient perpendicular to the printing direction was obtained due to the existence of a temperature gradient between the two sides of printed samples and could be further fixed by post-photo-cross-linking treatment through the dimerizable groups in the LCE, realizing a new actuation mode in the field of extrusion-based printing of LCE actuators. The printed film was able to change reversibly from a strip to a tightly hollow cylinder and could reversibly lift up an object with roughly 600 times its own weight. The orientation gradient can be patterned through liquid-assistant printing or programmed structure design to integrate both bending and contraction actuation modes on the same printed sample, leading to complex deformation and two-dimensional (2D) planar porous structure to three-dimensional (3D) porous cylinder transition. This study opens up a new prospect to directly print a wide variety of LCE actuators with versatile actuation modes.