New insights into the nature of semi-soft elasticity and “mechanical-Fréedericksz transitions” in liquid crystal elastomers

The mechanical properties of an all-acrylate Liquid Crystal Elastomer (LCE) with a glass transition of 14±1°C are reported. The highly nonlinear load curve has a characteristic shape associated with semi-soft elasticity (SSE). Conversely, measurements of the director orientation throughout tensile loading instead indicate a “mechanical-Fréedericksz” transition (MFT). Values of the step length anisotropy, r, are independently calculated from the theories of SSE (r= 3.2±0.4), MFT (9.3<r<30.0) and thermally-induced length change (r=3.8±0.5). From simultaneously recorded polarising microscopy textures, the consequences of the above discrepancies are considered. Further, a mechanically-induced negative order parameter is observed. Results show the tensile load curve shape cannot solely be used to determine the underlying physics. Consequently, the LCE properties cannot be fully described by theories of SSE or MFTs alone. This suggests that the theory of LCEs is not yet complete. The conclusions suggest that both the LC order parameter and r must be functions of the mechanical deformation.