Effect of stretching angle on the stress plateau behavior of main-chain liquid crystal elastomers.
basic_science · Level V
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- Record sourced from PubMed, PMID 33599677.
- Also identified by DOI 10.1039/d0sm02244f.
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Abstract
The equilibrium nonlinear stress-stretch relationships for a monodomain main-chain nematic elastomer (MNE) are investigated by varying the angle between the stretching and initial director axes (θ<sub>0</sub>). Angle θ<sub>0</sub> has pronounced effects on the ultimate elongation as well as on the width of the low stress plateau regime (Λ<sub>p</sub>) during director rotation, whereas θ<sub>0</sub> has no appreciable effect on the plateau stress (σ<sub>p</sub>). In the stretching normal to the initial director (θ<sub>0</sub> = 90°), the plateau end exceeds 200% strain. At oblique angles of 90° > θ<sub>0</sub>≥ 35°, Λ<sub>p</sub> decreases with decreasing θ<sub>0</sub>, whereas the definite plateau regime vanishes at θ<sub>0</sub> < 24°. Wide-angle X-ray scattering and polarized optical microscopy measurements reveal that the director rotates uniformly in the biased direction for the MNE of θ<sub>0</sub>°≪ 90°, whereas directors rotating clockwise and counterclockwise are coexistent for θ<sub>0</sub> = 90°. Over the entire plateau regime, the MNEs exhibit pure shear deformation characterized by a Poisson's ratio of zero in the direction of the rotation axis. The Λ<sub>p</sub> for the corresponding polydomain NE (PNE) undergoing a transition to the monodomain alignment is smaller than that of the MNE of θ<sub>0</sub> = 90°, while the σ<sub>p</sub> values for both NEs are almost similar. The semi-soft elasticity concept satisfactorily explains the effects of θ<sub>0</sub> on Λ<sub>p</sub>, and the Λ<sub>p</sub> value of the PNE, using a single anisotropy parameter which is evaluated from the degree of thermally induced deformation of MNEs.