Coincident molecular auxeticity and negative order parameter in a liquid crystal elastomer

D Mistry,S L Mickthwaite,P B Morgan,S D Connell,J H Clamp,H F Gleeson

doi:10.1038/s41467-018-07587-y

D Mistry, S L Mickthwaite + Show 4 more

Open Access

https://doi.org/10.1038/s41467-018-07587-y

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Abstract

Auxetic materials have negative Poisson’s ratios and so expand rather than contract in one or several direction(s) perpendicular to applied extensions. The auxetics community has long sought synthetic molecular auxetics – non-porous, inherently auxetic materials which are simple to fabricate and avoid porosity-related weakening. Here, we report, synthetic molecular auxeticity for a non-porous liquid crystal elastomer. For strains above ~0.8 applied perpendicular to the liquid crystal director, the liquid crystal elastomer becomes auxetic with the maximum negative Poisson’s ratio measured to date being -0.74 ± 0.03 – larger than most values seen in naturally occurring molecular auxetics. The emergence of auxeticity coincides with the liquid crystal elastomer backbone adopting a negative order parameter, QB = -0.41 ± 0.01 – further implying negative liquid crystal ordering. The reported behaviours consistently agree with theoretical predictions from Warner and Terentjev liquid crystal elastomer theory. Our results open the door for the design of synthetic molecular auxetics.

Highlights

Auxetic materials have negative Poisson’s ratios and so expand rather than contract in one or several direction(s) perpendicular to applied extensions
We propose that this demonstration of a synthetic molecular auxetic represents the origin of an approach to producing molecular auxetics which, in combination with the responsiveness of liquid crystal elastomer (LCE), could lead to exciting materials with multi-functional behaviours
The films of monodomain side-chain LCE were composed of 6-(4-cyano-biphenyl4’-yloxy)hexyl acrylate (A6OCB, 34 mol%), 2-ethylhexyl acrylate (EHA, 49 mol%) and 1,4-bis-[4-(6-acryloyloxyhex-yloxy)benzoyloxy]-2-methylbenzene (RM82, 17 mol%) (Fig. 1a) produced as described in ref. 30, further details are given in the methods and Supplementary Fig. 1

Summary

Introduction

Auxetic materials have negative Poisson’s ratios and so expand rather than contract in one or several direction(s) perpendicular to applied extensions. Polymeric liquid crystalline materials are the most promising material for displaying molecular auxeticity[26,27,28] He et al.[26,27] studied a series of polydomain main-chain liquid crystal polymers and observed a strain-induced increase in the polymer inter-chain separation via X-ray diffraction. This result was taken to suggest the potential for auxetic behaviour, to date only positive values for Poisson’s ratio have been recorded in such materials[26,27]

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Results

Conclusion