Colour changing electro active polymer systems
Abstract
Dielectric elastomers are electroactive polymers, which change size and shape in response
to an electrical field. Dielectric elastomer actuators (DEAs) are highly promising new
technologies in optical applications such as tuneable optical lenses, diffraction gratings
and active camouflage. This thesis aims to develop a new approach to create a strain
actuated compliant colour changing device that is controlled using DEAs as they offer
stretchability, low weight, high efficiency, low cost and the possibility for miniaturisation.
Conventional DEAs use transparent elastomeric materials with no significant colour
change with strain. Conversely, liquid crystal materials are known to display dynamic
colour changing behaviour, thereby making them good candidate materials. The thesis
examines both the potential for colour changing soft actuators and the upcoming
challenges in this field as well as the key concepts around liquid crystals that exhibit
colour change.
An initial approach was aimed at creating colour changes using dielectric elastomer
actuators that drove a masked positioner. This method showed colour change since the
mask changes the colour visualisation.
The second approach used polymer dispersed liquid crystals, such as a nematic liquid
crystal within a reactive silicone resin. The immiscibility of these compounds resulted in
a dispersion of the liquid crystal droplets in the silicone matrix. However, the optical
properties could not be controlled through mechanical deformation alone and the
alignment of resulting LC droplets in the PDLC films was sensitive to the substrate used
to perform the actuation.
The next approach used reactive cholesteric liquid crystals (CLC) instead. A thin film
coating process was preferred to carefully control the film’s thickness by stretching. In
free standing films a planar cholesteric alignment was obtained with mesogens aligned
parallel to the substrate and colour was achieved based on the selective reflection of light.
A transfer print technique was introduced to combine CLC coatings with elastomeric
substrates that can be stretched. However, no colour change was achieved in response to
mechanical deformation primarily due to the modulus and strength mismatch between the
thin film and the elastomeric susbstrate material.
Finally, lightly crosslinked liquid crystal elastomers using a combination of reactive and
non-reactive liquid crystals were produced that were compatible with elastomer substrate
materials. In free standing films planar cholesteric alignment was obtained with mesogens
aligned parallel to the substrate. Successfully a reversible colour change based on
selective reflection of light was achieved in response to a mechanical deformation.
Authors
Hediyeh, ZahabiCollections
- Theses [4121]