DNMR measurements of an asymmetric odd liquid crystal dimer: determination of the intramolecular angle and the degree of order of the two rigid cores.

In this work, we present a Deuteron Nuclear Magnetic Resonance (DNMR) study of the non-symmetric odd liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py), formed by a pro-mesogenic cyanobiphenyl unit and a bulky pyrene-containing unit, linked via alkoxy flexible chain. We have synthesized two partially deuterated samples: one with the deuterium atoms in the cyanobiphenyl moiety (dCBO7O.Py) and the other one with the deuterium atoms in the pyrenimine-benzylidene unit (CBO7O.dPy). We have performed angular distribution analysis in the SmA glassy state, obtaining the degree of order of both rigid cores and an estimation of the internal molecular angle between both structures. With the results from the angular study, we have been able to determine the degree of order of both rigid units in either the N phase and the SmA phase, far enough from the glass transition. Both rigid cores have the same degree of order close to the nematic-isotropic phase transition, but as the compound is cooled down, the degree of order of the cyanobiphenyl moiety is clearly higher than that of the pyrene-containing unit. The critical behaviour of the order parameter of the pyrene-containing moiety is consistent with the fact that, for CBO7O.Py, the N-I phase transition is tricritical, which seems to indicate that the uniaxial order parameter of the dimer is dominated by the degree of order of the pyrene-containing core.

Deuteron NMR investigation on orientational order parameter in polymer dispersed liquid crystal elastomers.

Polymer-dispersed liquid crystal elastomers have been recently introduced as a thermomechanically active composite material, consisting of magnetically oriented liquid crystal elastomer particles incorporated in a cured polymer matrix. Their thermomechanical properties are largely governed by the degree of imprinted particle alignment, which can be assessed by means of deuterium perturbed 2H-NMR. Spectra of samples with various degrees of imprinted particle alignment were recorded and the results simulated using the discrete reorientational exchange model developed for determining the dispersion of liquid crystal elastomer's domain orientational distribution. We show that the model can be applied to measure the orientational distribution of embedded liquid crystal microparticles and successfully determine the orientational order parameter in the composite system. Thermomechanical measurements correlate well with the obtained results, thus additionally confirming the validity of the applied method.

Order parameters and time evolution of mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF by DNMR.

Uniaxial order parameters of the nematic and columnar mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF have been determined from deuteron nuclear magnetic resonance, where random confinement of the system by the dispersion of aerosil nanoparticles has been performed to help obtain the angular dependent spectra. The long-time evolution study of the order parameters shows that the system requires tens of hours to stabilize after a deep change in temperature, in contrast with the very fast assembly process of the aggregates. Finally, the degree of order of the water molecules, forced by the uniaxial environment, has been determined.

Brief overview on 2H NMR studies of polysiloxane-based side-chain nematic elastomers.

This is a brief overview on recent studies on liquid crystalline elastomers (LCEs) based on polysiloxane chain, in the form of monodomain films, selectively (2)H-labeled in different parts of the LCE samples, i.e. on the crosslinker or mesogenic units. (2)H NMR spectroscopic techniques were used to measure the temperature dependence of the quadrupolar splittings, line widths and relaxation times, T(1) and T(2). From these data, several information about the orientational order parameter, S, of various LCE fragments, thermodynamic features of the isotropic-nematic transition and main motional processes could be generalized for this type of elastomers.

Polymer-dispersed liquid crystal elastomers as moldable shape-programmable material.

The current development of soft shape-memory materials often results in materials that are typically limited to the synthesis of thin-walled specimens and usually rely on complex, low-yield manufacturing techniques to fabricate macro-sized, solid three-dimensional objects. However, such geometrical limitations and slow production rates can significantly hinder their practical implementation. In this work, we demonstrate a shape-memory composite material that can be effortlessly molded into arbitrary shapes or sizes. The composite material is made from main-chain liquid crystal elastomer (MC-LCE) microparticles dispersed in a silicone polymer matrix. Shape-programmability is achieved via low-temperature induced glassiness and hardening of MC-LCE inclusions, which effectively freezes-in any mechanically instilled deformations. Once thermally reset, the composite returns to its initial shape and can be shape-programmed again. Magnetically aligning MC-LCE microparticles prior to curing allows the shape-programmed artefacts to be additionally thermomechanically functionalized. Therefore, our material enables efficient morphing among the virgin, thermally-programmed, and thermomechanically-controlled shapes.

A photosensitive liquid crystal studied by 14N NMR, 2H NMR, and DFT calculations.

An azobenzene derivative, namely diheptylazobenzene, showing the nematic and smectic A liquid crystalline phases, was investigated by means of a combined approach based on NMR and DFT calculations. (14)N NMR quadrupole- and chemical-shift-perturbed spectra were acquired in the whole mesophasic range, providing both experimental quadrupolar splittings and chemical shift anisotropy values. On the same mesogen, deuterium labelled at the α-position of the hydrocarbon chain, (2)H NMR quadrupole-perturbed spectra were recorded. The analysis of these NMR data was performed with the help of ab initio calculations, in vacuo and by taking into account the effect of the anisotropic environment typical of liquid crystals, by using the IEF-PCM model. The geometry optimizations of the azomesogen in the trans and cis configurations were performed by DFT calculations employing the combination of B3LYP functional with the 6-311G(d) basis set. The analysis of experimental NMR data was performed by considering the trans configuration as the most populated one and the corresponding quadrupolar tensors and chemical shielding tensors were determined at the DFT level of theory. The main result of this work is the determination of a relatively high and temperature-dependent molecular biaxiality of the trans state of this azomesogen.

Analysis of Extra Virgin Olive Oils from Two Italian Regions by Means of Proton Nuclear Magnetic Resonance Relaxation and Relaxometry Measurements.

The interest in development of new non-destructive methods for characterization of extra virgin olive oils (EVOOs) has been increasing in the recent years. Among different experimental techniques, nuclear magnetic resonance (NMR) relaxation measurements are very promising in the field of food characterization and authentication. In this study, we focused on relaxation times T1 and T2 measured at different magnetic field strengths (namely, 2, 100, and 400 MHz) and 1H NMR T1 relaxometry dispersions directly on olive oil samples without any chemical/physical treatments. A large set of EVOO samples produced in two regions of Italy, Tuscany and Apulia, were investigated by means of 1H NMR relaxation techniques. The relaxation studies reported here show several common features between the two sets of EVOO samples, thus indicating that relaxation properties, namely, the ranges of values of T1 and T2 at 2 and 100 MHz, are characteristic of EVOOs, independently from the cultivars, climate, and geographic origin. This is a promising result in view of quality control and monitoring.

Sensitivity of Proton NMR Relaxation and Proton NMR Diffusion Measurements to Olive Oil Adulterations with Vegetable Oils.

Olive oils and, in particular, extra-virgin olive oils (EVOOs) are one of the most frauded food. Among the different adulterations of EVOOs, the mixture of high-quality olive oils with vegetable oils is one of the most common in the market. The need for fast and cheap techniques able to detect extra-virgin olive oil adulterations was the main motivation for the present research work based on 1H NMR relaxation and diffusion measurements. In particular, the 1H NMR relaxation times, T1 and T2, measured at 2 and 100 MHz on about 60 EVOO samples produced in Italy are compared with those measured on four different vegetable oils, produced from macadamia nuts, linseeds, sunflower seeds, and soybeans. Self-diffusion coefficients on this set of olive oils and vegetable oil samples were measured by means of the 1H NMR diffusion ordered spectroscopy (DOSY) technique, showing that, except for the macadamia oil, other vegetable oils are characterized by an average diffusion coefficient sensibly different from extra-virgin olive oils. Preliminary tests based on both NMR relaxation and diffusometry methods indicate that eventual adulterations of EVOO with linseed oil and macadamia oil are the easiest and the most difficult frauds to be detected, respectively.

7Li NMR investigation of Li-Li pair ordering in the paraelectric phase of weakly substitutionally disordered K(1-x)Li(x)TaO3.

Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed 7Li NMR at temperatures (150-400 K) far above the nominal glass transition temperature (≈50 K for Li concentration x=0.03). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.

Phase separation in nematic microemulsions probed by one-dimensional spectroscopic deuteron magnetic resonance microimaging.

We present a study of a phase-transition-driven separation in microemulsions of nanosized lyotropic inverse micelles and thermotropic liquid crystal pentylcyanobiphenyl (5CB) with 5%, 8%, and 15% micelle concentration. Using deuteron nuclear magnetic resonance (DNMR) microimaging in combination with conventional microscopy as well as ac calorimetry, we demonstrate a phase separation scenario in which micelles are expelled from the nematic phase during the I-N conversion. Due to a difference in density the micelle-rich isotropiclike phase spatially separates from the micelle-free nematic phase. A relatively sharp interface, formed between the two phases, can be controllably shifted by temperature-induced conversion between the phases. Once expelled, micelles do not remix into the nematic phase, whereas in the isotropic state their remixing takes place over several days. Temperature dependence of the linewidth of isotropic spectral component has been analyzed in terms of molecular reorientations mediated by translational displacements, assuming isotropically distributed directors of nanosized nematic domains. With our results, the existence of the proposed transparent nematic state cannot be completely ruled out. However, if present, the nematic order in such a phase must be extremely low.

Nanostructured Composites Based on Liquid-Crystalline Elastomers.

Liquid-crystalline elastomers (LCEs) are the object of many research investigations due to their reversible and controllable shape deformations, and their high potential for use in the field of soft robots and artificial muscles. This review focuses on recent studies about polymer composites based on LCEs and nanomaterials having different chemistry and morphology, with the aim of instilling new physical properties into LCEs. The synthesis, physico-chemical characterization, actuation properties, and applications of LCE-based composites reported in the literature are reviewed. Several cases are discussed: (1) the addition of various carbon nanomaterials to LCEs, from carbon black to carbon nanotubes, to the recent attempts to include graphene layers to enhance the thermo-mechanic properties of LCEs; (2) the use of various types of nanoparticles, such as ferroelectric ceramics, gold nanoparticles, conductive molybdenum-oxide nanowires, and magnetic iron-oxide nanoparticles, to induce electro-actuation, magnetic-actuation, or photo-actuation into the LCE-based composites; (3) the deposition on LCE surfaces of thin layers of conductive materials (i.e., conductive polymers and gold nanolayers) to produce bending actuation by applying on/off voltage cycles or surface-wrinkling phenomena in view of tunable optical applications. Some future perspectives of this field of soft materials conclude the review.

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.

Electrocaloric and elastocaloric effects in soft materials.

Materials with large caloric effect have the promise of realizing solid-state refrigeration which has potential to be more efficient and environmentally friendly compared with current cooling technologies. Recently, the focus of caloric effects investigations has shifted towards soft materials. An overview of recent direct measurements of the large electrocaloric effect (ECE) in a composite mixture of a liquid crystal and nanoparticles (NPs) and large elastocaloric (eC) effect in main-chain liquid crystal elastomers is given. In mixtures of 12CB liquid crystal with functionalized CdSSe NPs, an ECE exceeding 5 K was found in the vicinity of the isotropic to smectic A phase transition. It is shown that the NPs smear the isotropic to smectic coexistence range in which a large ECE is observed due to latent heat enhancement. NPs acting as traps for ions reduce the moving-ion density and consequently the Joule heating. Direct eC measurements indicate that the significant eC response can be found in main-chain liquid crystalline elastomers, but at a fraction of the stress field in contrast to other eC materials. Both soft materials could play a significant role as active cooling elements or parts of thermal diodes in development of new cooling devices.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Unusual dynamic behavior in the isotropic phase of banana mesogens detected by 2H NMR line width and T2 measurements.

In this work the first experimental observation of a peculiar behavior in the isotropic phase of liquid crystals by means of 2H NMR is reported. In particular, two five-ring banana-shaped mesogens, the 1,3-phenylenebis{4,4'-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), selectively deuterium labeled on their central rings, are the subject of our investigation. The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line width and spin-spin relaxation time (T2) analysis. The results obtained reveal that the unusual line broadening observed in the 2H NMR spectra in the isotropic phase, even far above the isotropic phase-mesophase transition, has a homogeneous nature, thus indicating the presence of reorientational motions much slower than in conventional isotropic liquid-crystalline phases.

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.

Behavior of mesogenic molecules deposited at the alumina-air interface: a deuteron NMR study.

Thin molecular depositions of 4(')-pentyl-4-cyonobiphenyl (5CB) mesogenic molecules are investigated via quadrupole-perturbed deuteron nuclear magnetic resonance (DNMR) spectroscopy. Uniform and controlled thickness molecular surface depositions are prepared on the inner cylindrical surfaces of Anopore membranes by the solvent-evaporation technique. As a result, 5CB molecules are found in two different configurations: a bulklike one with parallel axial arrangement, and a surface one with planar radial arrangement. If the 5CB surface coverage exceeds c approximately 0.35, only the bulk state is present. In the coverage range between 0.015 and 0.35, the bulklike state and the surface layer coexist, conforming to a typical dewetting scenario. Below c approximately 0.015, only the surface layer is present. The dilution of the surface deposition with decreasing coverage is manifested as an increase in the DNMR doublet frequency splitting. The surface orientational order parameter Q, the surface biaxiality eta, and the diffusion coefficient D(S) are determined from the DNMR spectral patterns obtained at different sample orientations in the external magnetic field. These angular patterns prove that in highly diluted surface depositions the molecules lie flat on the surface. However, they are not frozen and their molecular axes rapidly reorient on the DNMR measurement time scale, typically 10(-4) s, while remaining confined to the surface. Simultaneously, molecules diffuse over the surface with a surface diffusion constant on the order of 10(-11) m(2) s(-1). Such molecular diffusion is responsible for an effective biaxiality on the DNMR time scale. However, an inherent biaxiality cannot be completely ruled out and thus may play a minor role. The surface phase has a two-dimensional (2D) gas character with some (possible) indicators of 2D-liquid properties.

Deuteron NMR study of molecular ordering in a holographic-polymer-dispersed liquid crystal.

Using deuteron nuclear magnetic resonance (NMR) and dynamic light scattering, we study the orientational order and dynamics of a BL038-5CB liquid-crystal mixture in a holographic polymer dispersed liquid-crystal material (HPDLC) as used for switchable diffractive optical elements. At high temperatures, where the liquid crystal is predominantly in the isotropic phase, the HPDLC deuteron NMR linewidth and transverse spin-relaxation rate T-12 are two orders of magnitude larger than in the bulk. The analysis shows that the surface-induced order parameter in HPDLC is significantly larger than in similar confining systems and that translational diffusion of molecules in the surface layer is at least two orders of magnitude slower than in the rest of the cavity. The unusual temperature dependence of T-12 upon cooling suggests the possibility of a partial separation of the 5CB component in the liquid-crystal mixture. The onset of the nematic phase in HPDLC occurs at considerably lower temperature than in the bulk and takes place gradually due to different sizes and different content of non-liquid-crystalline ingredients in droplets. Parts of the droplets are found isotropic even at room temperature and the structure of the nematic director field in the droplets is only slightly anisotropic. We point out the capability of NMR to detect the actual state of liquid-crystalline order in HPDLCs and to contribute in this way to the improvement of the switching efficiency of diffraction gratings.

Depth profile of optically recorded patterns in light-sensitive liquid-crystal elastomers.

We investigated nonlinear absorption and photobleaching processes in a liquid-crystal elastomer doped with light-sensitive azobenzene moiety. A conventional one-dimensional holographic grating was recorded in the material with the use of two crossed UV laser beams and the angular dependence of the diffraction efficiency in the vicinity of the Bragg peak was analyzed. These measurements gave information on the depth to which trans to cis isomerization had progressed into the sample as a function of the UV irradiation time. Using a numerical model that takes into account the propagation of writing beams and rate equations for the local concentration of the absorbing trans conformer, we computed the expected spatial distribution of the trans and cis conformers and the shape of the corresponding Bragg diffraction peak for different irradiation doses. Due to residual absorption of the cis conformers the depth of the recording progresses logarithmically with time and is limited by the thermal relaxation from the cis to trans conformation.

Micropatterning of light-sensitive liquid-crystal elastomers.

We demonstrate that photoisomerizable liquid-crystal elastomer soft films can be used as tunable holographic gratings. Optomechanical mechanism of imprinting one-dimensional grating structure into the soft matrix by two-beam uv laser interference can be clearly resolved from the time dependence of the reading beam diffraction patterns. We analyze the observed response in terms of cis-trans isomerization-controlled modulation of the grating profile. The grating period can be tuned reversibly by stretching or contraction of the specimen, either thermomechanically or by applying external stress. Temperature-induced modifications of the grating parameters in the vicinity of the nematic-paranematic phase transition are also examined.

Piezoresistivity and electro-thermomechanical degradation of a conducting layer of nanoparticles integrated at the liquid crystal elastomer surface.

When a liquid crystal elastomer (LCE) is reprocessed with conducting nanosized particles a conducting layer can be formed at the LCE surfaces. Here, two different LCE materials and two different conducting carbon particles were used. These four reprocessed LCEs were investigated when subject to a thermal phase transition and mechanical extension. Here it is shown that the resistance change with strain ('piezoresistivity') for these reprocessed LCEs can be described through lattice percolation and geometrical changes in the LCE shape. The mechanisms and rate of degradation are also described for the conducting layer as a function of the number of electro-thermomechanical strain cycles performed.