Programmable Liquid Crystal Elastomers Via Magnetic Field Assisted Oligomerization

Abstract

Liquid crystal elastomers (LCEs) are highly promising soft actuators due to their ability to undergo significant, rapid, and programmable shape transformations. Achieving this actuation requires precise mesogen alignment before crosslinking. Magnetic field alignment offers an advantage over other techniques as it enables volumetric mesogen alignment. However, magnetic alignment has to be performed in the state where mesogens exhibit anisotropic diamagnetic properties suitable for alignment, typically in the nematic phase. Therefore, it is difficult to obtain a stable nematic liquid crystalline mixture before crosslinking, while preventing mesogen recrystallization. To address this challenge, a method using a magnetic field assisted thio-Michael polyaddition followed by the photo-crosslinking of liquid crystal oligomers (LCOs) is presented. This process is straightforward and allows the synthesis of magnetically aligned high molecular weight LCOs, which maintain a nematic phase before crosslinking. By applying different magnetic field orientations during the polyaddition, three distinct LCE networks are synthesized, each demonstrating unique reversible deformation behaviors, including radial contraction and expansion, thickness contraction, and bending. The potential of this approach is further demonstrated with LCE-based four-leaf clovers exhibiting selective and reversible large-angle bending deformation, despite the high thickness of the material. This simple approach should help researchers in various fields looking for a practical and versatile method for designing robust LCE actuators.