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

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.

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

“…Since the form of eq 3 does not allow it to replicate the apparent initial increase in modulus between θ i = 0° and 10°, it is not surprising that the best‐fitted value for E 1 lies outside the range (17 ± 2 MPa) measured from the 2° sample. As we have previously deduced that, for the present LCE, ν 21 ≅ 0, the above fitting allows us to deduce an approximate value for G 12 of 1.7 MPa …”

“…To a first approximation, considering the true stresses removes the strain dependency of the tensile load curves. We have previously shown that when strained perpendicular to the director, the present LCE conforms well to the shear‐free volume conserving condition of λ x λ y λ z = 1 (where λ i is the deformation along the i th principle axis)—even at strains of 150% . While director rotations within the plane of deformation may give rise to shear contributions, λ xy and λ yx , these are neglected in this work for simplicity.…”

“…We see that the model curve for θ i = 89° lies much closer to the experimental data points than the model curve for θ i = 88°. This slight disagreement is not too surprising given the present LCE is known to have a strain‐dependent order parameter when stretched perpendicular to the director, while eq 5 assumes a constant order parameter …”

“…While director rotations within the plane of deformation may give rise to shear contributions, λ xy and λ yx , these are neglected in this work for simplicity. Thus, in all cases, we calculate the true stress (which accounts for the strain‐dependent sample cross‐sectional area) by multiplying the engineering stress (force divided by initial sample cross‐sectional area) by the longitudinal deformation, λ x = ɛ x + 1, where ɛ x is the longitudinal strain …”

“…Developing such materials would enable and increase the functionality of next‐generation technologies such as soft robotics and biomedical devices . 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 . 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 .…”