Amine-Acrylate Liquid Single Crystal Elastomers Reinforced by Hydrogen Bonding

Abstract

Liquid crystalline elastomers (LCEs) have been considered one of the most promising material concepts for artificial muscles. However, accomplishing actuation of LCEs requires macroscopic alignment of the liquid-crystalline orientation in the rubbery network, which imposes challenges in the materials chemistry and processing. A two-stage curing strategy has been the dominating approach during last three decades. Despite its many successes, the method is difficult in practice and requires delicate experiential skills, dealing with intrinsic fragility of intermediate gels after the first crosslinking stage. Here, a robust fabrication method for monodomain LCE based on the amine-acrylate aza-Michael addition is developed, involving two readily commercially available components with no catalyst. The method is based on the large kinetic difference of hydrogen addition in primary amines to acrylates, which offers a sufficient gap separating two stages of curing and enabling versatile mechanical alignment techniques for manufacturing monodomain LCE in both liquid and gel states. Importantly, the mechanically robust network, helping processability at a partial-crosslinking stage, is facilitated by the chemically generated hydrogen bonding all through the process, as a by-product of hydrogen addition. Such a facile two-component kit-like fabrication should aid researchers from various fields in the search for a practical and reliable process of making soft actuators.