Abstract
A mechanical stretch applied to a monodomain twisted nematic elastomer thin film untwists the helical director distribution into a uniaxially aligned state. If the film thickness is less than the helical pitch, untwisting behavior is stretching direction dependent in the plane normal to the helix axis. We investigate untwisting of twisted nematic elastomers by optical, mechanical, and x-ray scattering methods. We adapt a model for cholesteric elastomers to our observations and find that the material exhibits characteristics with the combined properties of both a smectic and twisted nematic configuration.
Highlights
Advances in chemistry and fabrication methods have greatly expanded the possibilities for technologies utilizing liquid crystal elastomers (LCEs) as well as studying fundamental properties of LCE materials
We investigate untwisting of twisted nematic elastomers by optical, mechanical, and x-ray scattering methods
When p is the same wavelength as an incident light beam, the cholesteric LCE will selectively reflect the light of the matching wavelength
Summary
If the film thickness is less than the helical pitch, untwisting behavior is stretching direction dependent in the plane normal to the helix axis. In a cholesteric LCE, the nematic director twists about a helix axis with a material intrinsic pitch p. The stretching behavior will exhibit a critical phenomenon only if the portion of the helix phase present contains a director perpendicular to the stretching direction. For portions of the helix phase with a director component perpendicular to the stretching direction, there will be a critical stretch where the perpendicular director component must select an untwisting direction This effect has previously been called director pinning in cholesteric LCEs [10]. For long-pitch twisted nematic LCEs, the film geometry and portion of the helix phase present will affect the thermal actuation characteristics [12].
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