Martin Chambers

Liquid crystal elastomer-nanoparticle systems for actuation

Liquid crystal elastomers (LCE) are currently of great interest due to conjoining of mesogenic ordering and rubber elasticity, exhibited in their large spontaneous thermally stimulated changes in shape. It has been shown that nanoparticles (nanotubes, photo-isomerisable dyes, magnetic nanoparticles…) can be incorporated into these LCE networks to create a more sensitive network to external stimuli (i.e. strain or stress, optical, electrical, electro-thermal, magnetic…). Here, we briefly summarise the current state of LCE–nanoparticle systems and explain in detail one system utilising carbon nanoparticles integrated at surfaces that may be used for electro-thermal heating of LCE systems.

Giant flexoelectricity in bent-core nematic liquid crystal elastomers

Recently ferroelectric ceramic and bent-core nematic liquid crystals have demonstrated flexoelectricity (coupling between curvature strains to electric polarization) up to 104 times larger than the previous standards. This may allow for usable electromechanical devices. However, ceramics are too rigid to withstand large bending and bent-core nematic fluids must be physically supported—their technological applicability is still limited. In this paper, we show that novel side-chain bent-core nematic elastomers not only produce giant flexoelectricity but are also robust and flexible enough for microscale parasitic power generation.

Actuation of liquid crystal elastomers reprocessed with carbon nanoparticles

Liquid crystal elastomers are currently of great interest due to their large thermally stimulated changes in shape. Here the authors show that by using an existing network and conducting carbon nanoparticles dispersed in a solvent with high swelling capability, a surface integrated layer can be created. This layer allows the effective resistivity to be reduced from highly insulating to usable values for electrical actuation and withstands large changes in geometrical shape both in contraction and expansion. Utilizing a resistive “Joule” heating effect, the reprocessed system shows a 150% length change and can be cycled beyond 10kcycles.

Investigations on an integrated conducting nanoparticle–liquid crystal elastomer layer

A process is outlined in which an existing liquid crystal elastomer (LCE) can be reprocessed from an insulating network to create an effectively conducting network. This is performed through the LCE volume expansion in a suitable solution containing conducting nanoparticles. Subsequent volume compression results in the formation of a conducting layer at the LCE surfaces. The swelling behaviour of the LCE is illustrated. Elemental composition analysis and electron imagining techniques show that the conducting layer is composed of conducting nanoparticles and LCE material. It was found that the integrated layer thickness and resistivity can be controlled through the LCE surface expansion ratio and conducting nanoparticle concentration, respectively.

Properties of non-symmetric bent-core liquid crystals with variable flexible chain length

New non-symmetrical bent-core mesogens with a variable flexible chain length have been synthesised. The compounds were derived from 3-hydroxybenzoic acid, which is the central unit of the molecules. The tails on both ends were connected by ester functionality, one tail contains a terminal double bond and the other is saturated. The compounds represent novel non-symmetric molecules both in their core and tails. The main features of differential scanning calorimetry, polarising optical microscopy, electro-optical and polarisation current measurements for all studied materials indicate a transition between a single tilted synclinic ferroelectric and an antiferroelectric smectic phase.

Extraordinary properties of nematic phases of bent-core liquid crystals

We briefly review systematic and comprehensive studies on several chlorine-substituted bent-core liquid crystal materials in their nematic phases. The results, in comparison to rod-shaped molecules, are both extraordinary and technologically significant. Specifically: a) Electrohydrodynamic instabilities provide unique patterns including well defined, periodic stripes and optically isotropic structures. b) Rheological measurements using different probe techniques (dynamic light scattering, pulsed magnetic field, electrorotation) reveal that the ratio of the flow and rotational viscosities are over two orders of magnitudes larger in bentcore than in calamitic materials which proves that the molecule shape and not its size is responsible for this behaviour. c) Giant flexoelectric response, as measured by dynamic light scattering and by directly probing the induced current when the material is subject to oscillatory bend deformation, turns out to be more than three orders of magnitude larger than in calamitics and 50 times larger than molecular shape considerations alone would predict. The magnitude of this effect renders these materials as promising candidates for efficient conversion between mechanical and electrical energy. d) The converse of this effect when the bent-core material sandwiched between plastic substrates 4 times thicker than the liquid crystal material provided displacements in the range of 100nm that is sensitive to the polarity of the applied field thus suggesting applications as beam steering and precision motion controls.

Some Advances in Liquid Crystal Elastomers: From Crosslinks Affected Ordering to Carbon Nanoparticles Enabled Actuation

Liquid crystal elastomers (LCE) exhibit a combination of elasticity and mesogenic ordering, yielding large thermally stimulated changes in shape. These LCE systems although well characterised, still yield open questions in the nature of how the crosslinking affects the LCE phase transition. Therefore calorimetry and deuteron-nuclear magnetic resonance were used to study the isotropic-nematic phase transition of uniformly ordered LCE. We observed that the density of crosslinkers strongly affects the nematic-isotropic phase transition. The observed spread critical transitions are explained with a dispersion of local mechanical fields that yields a weakly disordered orientational state composed of regions that exhibit temperature profiles of the nematic order parameter ranging from first order to supercritical. On increasing crosslinking density, the predominantly first order thermodynamic response transforms into a predominantly supercritical one. Additionally, to illustrate the response of these actuating systems, it was demonstrated that a LCE can be electrically heated. The insulating LCE network was reprocessed using conducting nanoparticles dispersed in a solvent with high LCE swelling capability. This results in a low electrical resistivity surface layer of LCE network with a high concentration of conducting nanoparticles. The reprocessing allows the effective resistivity of a LCE film to be reduced from highly insulating values to values useable for electrical actuation. This layer in addition withstands large changes in geometrical shape both in contraction and expansion. Utilizing a resistive “Joule” heating effect, the reprocessed system exhibits an indirect electromechanical effect characterised by a 150% length change that can be cycled for more than 10, 000 times.