Frustrated smectic liquid crystal elastomers as multifunctional mechanical metamaterials

Auxetics are materials that show a negative Poisson's ratio, i.e. thickening when under tensile strain. The discovery of nematic liquid crystal elastomers (LCEs) as molecular auxetic materials was a paradigm-shift in the design of mechanical metamaterials as they are tuneable, transparent, scalable, non-porous and facile to fabricate. Here, we report that smectic LCEs extend known synthetic molecular auxetics to a second family. The LCEs are acrylate-based, lightly crosslinked ( ~ 8 mol%) and polymerised in the nematic phase of a precursor mixture. The resulting smectic-A LCEs exhibit correlation lengths of a few molecular lengths, suggesting layer frustration, which we confirm occurs via the mechanism predicted by de Gennes in 1975. Three of the LCEs described exhibit negative Poisson's ratios at strains above ~40%, thereby marking the observation of auxetic behaviour in smectic LCEs. Shear oscillatory measurements on planar and homeotropic LCEs demonstrate highly anisotropic moduli, enabling elucidation of highly anisotropic adhesive properties, with differences as large as 20x suggested between geometries. Room temperature tack measurements reveal a 31% difference in adhesion between geometries. Furthermore, we find that in the homeotropic case, shear moduli are comparable in both nematic and smectic A LCEs, validating theoretical expectations for such systems.