B. Zalar

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.

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.

How do banana-shaped molecules get oriented (if they do) in the magnetic field?

In this work the orientation of banana-shaped molecules in the magnetic field is investigated. A new and original hypothesis of the peculiar aggregation of banana-shaped molecules in the nematic phase in the presence of a magnetic field is here reported. Deuterium NMR measurements on two selectively deuterium labelled mesogens and preliminary calculations of the magnetic susceptibility anisotropy of the aromatic core of these bent molecules are reported and discussed to support our hypothesis.

Polymer-dispersed liquid crystal elastomers.

The need for mechanical manipulation during the curing of conventional liquid crystal elastomers diminishes their applicability in the field of shape-programmable soft materials and future applications in additive manufacturing. Here we report on polymer-dispersed liquid crystal elastomers, novel composite materials that eliminate this difficulty. Their thermal shape memory anisotropy is imprinted by curing in external magnetic field, providing for conventional moulding of macroscopically sized soft, thermomechanically active elastic objects of general shapes. The binary soft-soft composition of isotropic elastomer matrix, filled with freeze-fracture-fabricated, oriented liquid crystal elastomer microparticles as colloidal inclusions, allows for fine-tuning of thermal morphing behaviour. This is accomplished by adjusting the concentration, spatial distribution and orientation of microparticles or using blends of microparticles with different thermomechanical characteristics. We demonstrate that any Gaussian thermomechanical deformation mode (bend, cup, saddle, left and right twist) of a planar sample, as well as beat-like actuation, is attainable with bilayer microparticle configurations.

NMR and the spherical random bond–random field model of relaxor ferroelectrics

To determine the nature of the relaxor state and, in particular, to discriminate between a ferroelectric state broken up into nanodomains under the constraint of quenched random fields and a glassy state we have measured the temperature dependences of the nonlinear dielectric susceptibility in zero field as well as the temperature dependence of the local polarization distribution function and the Edwards–Anderson order parameter in PMN single crystal via 93Nb NMR. The experimental results can be quantitatively described by the newly proposed Spherical Random Bond–Random Field model of relaxor ferroelectrics.

Paranematic–nematic phase transition in liquid crystalline elastomers: a 2H-NMR study

In this study, selectively deuterium-labelled monodomain liquid single crystalline elastomers have been investigated by means of 2H-NMR spectroscopy. The analysis of the spectral features as well as the study of the orientational properties in the paranematic and nematic phases confirmed the supercriticality of the paranematic–nematic transition as an intrinsic property of these systems. Moreover, the combined study of the spin–spin relaxation times (T 2) and the 2H-NMR spectral linewidth measured in the paranematic phase indicates the presence of slow dynamic motions.

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.

Effect of co-monomers' relative concentration on self-assembling behaviour of side-chain liquid crystalline elastomers

This work deals with the design and characterization of a new series of liquid crystalline elastomers in the form of monodomain films, showing self-assembling behaviour, namely the nematic and the orthogonal smectic A phases. The procedure for the design and preparation of monodomain and polydomain polysiloxane-based side-chain liquid crystalline elastomers containing different concentrations of two mesogenic monomers and a constant density (about 15 mol%) of the crosslinker is reported. The phase diagram and mesomorphic behaviour of the new resulting liquid crystalline elastomers were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and especially X-ray diffraction studies, which helped to clearly identify the smectic A phase. Among new liquid crystalline elastomer films, a specific concentration of co-mesogens gives an unconventional and fascinating system with a direct transition from the isotropic to smectic A phase. Results of the thermo-mechanic studies confirmed the shape-memory properties of these films, which have elastic properties optimal for applications as thermo-mechanic actuators.

2H NMR orientational study of a probe dissolved in nematic solution and, used as crosslinker, in a liquid crystalline elastomer

The orientational order parameters, Szz and Biax, of UB-d4 [1,4-bis(undec-10-en-1-yloxy)benzene-d4 ] in the ZLI1167 nematic mixture are studied by means of 2H NMR at variable temperature. The Szz(T) trend fairly follows the Maier–Saupe theoretical curve; therefore, UB-d4 can be considered an excellent probe to monitor the ordering of the nematic phase itself, provided that Biax is not neglected and the complete analysis of quadrupolar and dipolar splittings is performed. UB-d4 is a typical crosslinker in liquid crystal elastomers (LCEs): the order parameters here collected help us to discuss the peculiarities of the crosslinker as a probe for the study of internal order in LCEs. From the analysis of our data and of data in the literature, we infer that the order parameter Biax of the crosslinker in the LCE matrix is low enough to be neglected without severely perturbing the determination of . The crosslinker UB-d4 is a reliable probe to monitor the nematic order degree inside the LSCE-UB-d4 sample.

Polar nanoclusters in relaxors

The central problem in the physics of relaxors is the nature of the polar nanoclusters. Whereas relaxors are homogeneous at high enough temperatures, polar nanoregions immersed in a neutral matrix are formed below a certain temperature Tb. This should lead to a two component system. Here we present direct microscopic evidence for the two component nature of relaxors. We show that the chemical shift perturbed 207 Pb NMR spectra of these systems consist of an isotropic component corresponding to a spherical glassy matrix which does not respond to an applied electric field, and an anisotropic component, corresponding to frozen out polar nanoclusters which order in a strong enough electric field, forming a ferroelectric phase. This is as well reflected in the dynamic properties where the relaxation time distribution function starts to become asymmetric with decreasing temperature and a second maximum-which is never seen in dipolar glasses and is obviously due to polar clusters-appears on further cooling. We also show that the basic difference between dipolar glasses and relaxors is the fact that polar nanoclusters can be oriented in a strong enough electric field and a ferroelectric phase can be induced. This is not the case in dipolar glasses where the response is due to single dipoles which can not be ordered by applied electric fields.