Colour changing electro active polymer systems
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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.
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