Mechanical deformations of a liquid crystal elastomer at director angles between 0° and 90°: Deducing an empirical model encompassing anisotropic nonlinearity

Devesh Mistry,Helen F Gleeson

doi:10.1002/polb.24879

Abstract

ABSTRACTDespite the wealth of studies reporting mechanical properties of liquid crystal elastomers (LCEs), no theory can currently describe their complete mechanical anisotropy and nonlinearity. Here, we present the first comprehensive study of mechanical anisotropy in an all‐acrylate LCE via tensile tests that simultaneously track liquid crystal (LC) director rotation. We then use an empirical approach to gain a deeper insight into the LCE's mechanical responses at values of strain, up to 1.5, for initial director orientations between 0° and 90°. Using a method analogous to time–temperature superposition, we create master curves for the LCE's mechanical response and use these to deduce a model that accurately predicts the load curve of the LCE for stresses applied at angles between 15° and 70° relative to the initial LC director. This LCE has been shown to exhibit auxetic behavior for deformations perpendicular to the director. Interestingly, our empirical model predicts that the LCE will further demonstrate auxetic behavior when stressed at angles between 54° and 90° to the director. Our approach could be extended to any LCE; so it represents a significant step forward toward models that would aid the further development of LCE theory and the design and modeling of LCE‐based technologies. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1367–1377

Highlights

While much research has investigated the mechanical behaviors of the socalled monodomain Liquid crystal elastomers (LCEs), the vast majority of studies performed to date focus solely on the case of stresses applied perpendicular to the liquid crystal (LC) director
We have reported, for the first time, experimental results for the tensile load and director rotation behavior of an LCE stressed at a variety of different angles relative to the LC director
While the anisotropy in elastic moduli broadly agrees with expectations for uniaxial anisotropic materials, the additional complexities reported show the mechanical richness of LCEs continues to grow

Summary

Introduction

A current trend in materials science is to develop soft materials, which can mimic the structure, anisotropy, and functionality of materials and tissues found in nature.[1,2,3,4,5,6] Developing such materials would enable and increase the functionality of next-generation technologies such as soft robotics and biomedical devices.[7,8,9] Liquid crystal elastomers (LCEs), which incorporate liquid crystal (LC) order into a lightly crosslinked polymer network, are one such class of bio-similar materials—celebrated for their unique mechanical behaviors and their remarkable shape responsivity.[4,10,11,12,13,14,15,16,17,18,19,20,21]While the majority of LCE research typically focuses on the development and application of their shape actuation behavior, there is an increasing body of research studying the use of LCEs as mechanical and structural materials in fields such as flexible electronics and biomedical devices.[22,23,24,25] In these fields, an LCE’s mechanical anisotropy and programmability, shape programmability, and shock dissipation offer the prospect of bio-inspired devices with enhanced functionality and robustness over existing devices.Currently, LCE devices are limited to laboratory prototypes, partly because the full structure–property relationships of LCEs are yet to be understood, and so real-world devices cannot yet be designed and developed. While the majority of LCE research typically focuses on the development and application of their shape actuation behavior, there is an increasing body of research studying the use of LCEs as mechanical and structural materials in fields such as flexible electronics and biomedical devices.[22,23,24,25] In these fields, an LCE’s mechanical anisotropy and programmability, shape programmability, and shock dissipation offer the prospect of bio-inspired devices with enhanced functionality and robustness over existing devices. While much research has investigated the mechanical behaviors of the socalled monodomain LCEs (in which the average molecular orientation—or LC director—is aligned over macroscopic length scales), the vast majority of studies performed to date focus solely on the case of stresses applied perpendicular to the LC director

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