Director reorientation in nematic-liquid-single-crystal elastomers by external mechanical stress

Abstract

As has been shown recently by Kundler and Finkelmann, a sample of nematicliquid-single-crystal elastomers subject to a mechanical stress perpendicular to the initial director orientation shows a reorientation of the director reminiscent of that observed in lowmolecular-weight nematic liquid crystals in a magnetic field. Here we present a simple model, which captures all the essential features. We calculate the threshold stress and show that the director reorientation occurs over the entire sample. A weakly nonlinear analysis gives a forward bifurcation in agreement with the experimental results. We also discuss the origin of the domain walls in the director observed experimentally and give an expression for the thickness of these walls.

Material instabilities in nematic elastomers

“…For what concerns the issue of semi-soft behavior, i.e., the fact that an initial threshold stress needs to be overcome, and then increasing stresses are required to drive the system along the path (3.1), several competing methods of analysis are already available (see [9,19]). A consensus on the microscopic mechanisms explaining semi-softness, and on a macroscopic model taking them into account, has yet to be reached.…”

Multiscale Modeling of Materials — the Role of Analysis

“…This observation led to a theoretical search for corrections to (3.5), the main candidates being higher gradient energies (penalizing sharp changes in the director direction n) and disorder in the cross-linking configuration. Various corrections to W BTW have been proposed in the literature [59,62,29,60] and will be discussed in Section 3.5 below. The last expression in (3.5) emphasizes the high symmetry of the problem which can be obtained by choosing as reference configuration the stress-free state of the ideal isotropic "high-temperature" phase, which differs from the actual cross-linking configuration by a uniaxial stretch U n 0 .…”

Phase Transitions in Liquid Crystalline Elastomers: A Fundamental Aspect of LCEs as Artificial Muscles

“…A quasi-isotropic model for LSCEs (equation (15.1)) has been developed by Weilepp & Brand (1996) (the WB model) to account for the appearance of a threshold stress, CTe (equation (15.2)) , for reorientation of n (by an angle 'P) that appears when CT is applied to an LSCE at an arbitrary angle to n (cf. Kundler & Finkelmann (1995)).…”

“…In the example presented here, the analysis of the WB model finds the observed threshold field as a forward bifurcation with applied stress, CT, the control parameter (cf. Weilepp & Brand (1996)). …”

“…Weilepp & Brand (1996) and the data of Kundler & Finkelmann (1995) who measured the reorientation angle, cp, as a function of A.…”