Auxetic Liquid Crystal Vitrimers with Adjustable Poisson's Ratios Enabled by Topological Rearrangements of Polymer Network

Abstract

Metamaterials feature extraordinary physical properties that break the cognitive limitations of human beings on traditional materials. Auxetic materials and liquid crystal elastomers (LCEs) are representative of typical mechanical and thermal metamaterials. Their combination may introduce some unconventional and counterintuitive performances. Nevertheless, studies on LCEs with negative Poisson's ratio (v) are still rare. Herein, a liquid crystal vitrimer (Poly‐LCE) is developed that is a polydomain main‐chain LCE containing dynamic ester bonds. Its orientation process to monodomain (Mono‐LCE) is greatly simplified by transesterification reaction‐induced topological network rearrangement under mechanical alignment. By optimizing geometric parameters of re‐entrant (R) structures and orientation of liquid crystal units, all samples of R‐Poly‐LCE, R‐Mono‐LCE (//), and R‐Mono‐LCE (⊥) show negative Poisson's ratio (NPR) below 2% elongation (v = −0.22–0 for R‐Poly‐LCE, v = −0.12–0 for R‐Mono‐LCE (//) and v = −0.16–0 for R‐Mono‐LCE (⊥)). Interestingly, R‐Poly‐LCE presents v > 0 within 2%–10% axial elongation, while R‐Mono‐LCE (//) and R‐Mono‐LCE (⊥) exhibit v ≈ 0 under the same elongation. Materials with negative and zero Poisson's ratios are interesting in niche applications. This work develops a simple method to prepare these materials by liquid crystal vitrimers.