Auxetic materials exhibit a negative Poisson’s ratio, i.e., they become thicker rather than thinner in at least one dimension when strained. Recently, a nematic liquid crystal elastomer (LCE) was shown to be the first synthetic auxetic material at a molecular level. Understanding the mechanism of the auxetic response in LCEs is clearly important, and it has been suggested through detailed Raman scattering studies that it is related to the reduction of uniaxial order and emergence of biaxial order on strain. In this paper, we demonstrate direct observation of the biaxial order in an auxetic LCE under strain. We fabricated ~100 μm thick LCE strips with complementary geometries, exhibiting either planar or homeotropic alignment, in which the auxetic response is seen in the thickness or width of the sample, respectively. Polarized Raman scattering measurements on the planar sample show directly the reduction in the uniaxial order parameters on strain and suggest the emergence of biaxial order to mediate the auxetic response in the sample thickness. The homeotropic sample is studied via conoscopy, allowing direct observation of both the auxetic response in the width of the sample and increasing biaxiality in the LCE as it is strained. We verified that the mechanism of the auxetic response in auxetic LCEs is due to the emergence of the biaxial order and conclude such materials can be added to the small number of biaxial nematic systems that have been observed. Importantly, we also show that the mechanical Frèedericksz transition seen in some LCEs is consistent with a strain-induced transition from an optically positive to an optically negative biaxial system under strain, rather than a director rotation in a uniaxial system.

Elastomeric pressure‐sensitive adhesives (PSAs) form adhesive bonds under light pressure. Liquid crystal elastomers (LCEs) are exciting PSA candidates as they can impart both anisotropy and temperature‐dependence to adhesion, but the full potential of their anisotropic adhesion is unexplored. Here, identical side‐chain LCEs, produced as transparent isotropic or nematic films, are investigated; the latter aligned in homeotropic or planar geometries. Their room‐temperature adhesion, determined through a 90˚ peel test, is consistent with theoretical predictions and strongest in a planar geometry (peeled parallel to the director) with adhesive force per unit length of 0.067 × 10− ³ Nm ⁻¹ . In contrast, adhesion of the planar perpendicular, isotropic, and homeotropic films is 62.5%, 38.5%, and 23.0% lower, respectively. The surface contribution to adhesion is identical for all films, confirming that the variation in adhesion is determined solely by the bulk LCE alignment controlled during film preparation. A temperature‐dependent adhesion factor is determined from 0˚C to 80˚C using dynamic mechanical analysis, and found to be in excellent agreement with the peel data at room temperature. Molecular relaxations active above the glass transition temperature are dominant in determining LCE adhesion. The results show that side‐chain LCEs can function as transparent, tunable, broad‐temperature smart PSAs.

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.