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We investigate light-induced patterning of a monodomain side-chain liquid crystal elastomer (SC-LCE) doped with light-sensitive azobenzene moiety in the temperature region close to the nematic-paranematic phase transition. We show that a strongly nonlinear relationship between the concentration of the cis isomers of the azomesogens and the refractive index modification of the material, which is characteristic for the phase transition region, results in nonmonotonous time dependence of the diffraction efficiency of a probe beam. From this effect we determine the sensitivity of the nematic transition temperature on the molar fraction of the cis isomers. The relation between the cis isomer molar fraction and nematic order also provides a possibility for recording hidden holograms, which can be made visible by cooling the sample from the paranematic to the nematic phase.

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.

Several side-chain liquid single crystalline elastomers (LSCEs) were investigated by means of ²H NMR spectroscopy focusing on the temperature dependence of the spectral line width and spectral shape, as well as the T-dependence of spin-lattice and spin-spin relaxation times (T1 and T2). All measurements were performed at the Larmor frequency of 76.753 MHz for ²H. The LSCEs were prepared in the form of monodomain films by using either ²H-labelled monomers (or co-monomers), ²H-labelled cross-linkers or ²H-labelled molecular probes free to diffuse in the LSCE matrix. In all cases, deuterons are on the phenyl moiety. The main features of the measured ²H NMR properties (static and dynamic ones) in these different cases were discussed and comparedwith those of low-molecular-weightmesogens (i.e. the same molecules used as monomers or co-monomers in the side-chain LSCEs) in the nematic and smectic A phases. Among all investigated LSCE samples, those ²H-labelled on the cross-linker sites present distinct properties either on the ²H NMR spectral line width and spectral shape in the paranematic and in the nematic phases, which can be interpreted in terms of director reorientational dynamics in the intermediate-slow motional regime, taking into account the inherent local director misalignment of LSCEs. [ABSTRACT FROM AUTHOR]

An appealing property of optical diffractive structures from elastomeric materials is a possibility to regulate their optical patterns and consequently also their diffractive features with mechanical straining. We investigated the effect of strain on diffraction characteristics of holographic gratings recorded in a monodomain side-chain liquid crystalline elastomer. The strain was imposed either parallel or perpendicular to the initial alignment direction of the material. At temperatures far below the nematic–paranematic phase transition, straining along the initial alignment affects mainly the diffraction pattern, while the diffraction efficiency remains almost constant. In contrast, at temperatures close to the nematic–paranematic phase transition, the diffraction efficiency is also significantly affected. Straining in the direction perpendicular to the initial alignment strongly and diversely influences both the diffraction pattern and the diffraction efficiency. The difference between the two cases is attributed to shear–stripe domains, which form only during straining perpendicular to the initial alignment and cause optical diffraction that competes with the diffraction from the holographic grating structure. [ABSTRACT FROM AUTHOR]

We present the characterization of new binary mixtures made of two rod-like molecules: a photosensitive azobenzene derivative (calledJ7) and a smectogen (calledM11). The interest in these two specific compounds is related to their use as basic constituents of photo-active liquid crystalline elastomers and their opto-mechanic applications. Differential scanning calorimetry and polarized optical microscopy were used to characterize the phase diagram of the binary systemJ7-M11.2H NMR spectroscopy of the2H-selectively labelledM11, either alone or as a probe in binary mixtures containing different ratiosJ7/M11, was applied to study the orientational ordering properties. [ABSTRACT FROM PUBLISHER]

This is a brief overview on recent studies on liquid crystalline elastomers (LCEs) based on polysiloxane chain, in the form of monodomain films, selectively 2H-labeled in different parts of the LCE samples, i.e. on the crosslinker or mesogenic units. 2H NMR spectroscopic techniques were used to measure the temperature dependence of the quadrupolar splittings, line widths and relaxation times, T1 and T2. 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. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Recent progresses of NMR spectroscopy to the study of liquid crystals and related ordered systems are surveyed. Particular attention will be devoted to review suitably tailored NMR experiments and their applications on those systems (e.g. biaxial nematics, chiral smectics, V shaped mesogens, liquid crystalline elastomers) which are presently subject of active and innovative research. [Copyright &y& Elsevier]

New bilayered composite systems with tunable and temperature-dependent formation of periodical wrinkles on the surface are the object of this report. The samples were prepared by spin-coating deposition of a thin film of the conducting polymer poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on the surface of standard monodomain liquid crystal elastomer (LCE) films. Several bilayered materials were prepared by changing the thickness of PEDOT:PSS nanofilms. Basic characterization showed very good stability and adhesion between the two components also after performing multiple heat cycles around nematic-to-isotropic transition temperature of the LCE. Interestingly, formation of uniaxially aligned microwrinkles was observed, with most of the wrinkles aligned along perpendicular direction with respect to the nematic director, due to reversible elongation/compression of the LCE during thermal cycles. [ABSTRACT FROM PUBLISHER]

The optical mechanism for imprinting one-dimensional grating structures into thin films of a light-sensitive monodomain liquid crystal elastomer is investigated by analyzing the time dependence of optical diffraction properties. The recording kinetics shows an irregular oscillatory behavior, which is most expressed at small grating spacings and at temperatures close to the nematic-isotropic phase transition. The oscillations are attributed to the opto-mechanical response of the film, i.e., to contraction of the film during the recording process. At temperatures far below the nematic-isotropic phase transition, the spontaneous erasure kinetics exhibits exponential relaxation with relaxation time following the Arrhenius activation law. However, at temperatures close to the nematic-isotropic phase transition, the erasure process shows an interesting nonmonotonic behavior that we attribute to the non-linear relation between the concentration of the photo-transformed chemical groups and the nematic order parameter. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: For the first time new type of liquid single crystal elastomers forming the chiral smectic A* (SmA*) phase has been prepared using chiral lactic acid derivative as a co-monomer. The synthesis and the basic characterization of the ferroelectric liquid crystalline co-monomer based on differential scanning calorimetry and small/wide-angle X-ray scattering are reported and discussed. The monomer possesses the paraelectric SmA* and the ferroelectric SmC* phases over a broad temperature range. The preparation of new smectic liquid crystalline elastomers together with characterization of their mesomorphic and structural properties is also reported. New elastomers possess the orthogonal paraelectric SmA* phase over 50 K broad temperature range. [Copyright &y& Elsevier]

In this study, we report the preparation, structure characterization, and application of new MoO nanowires, promising candidates for lithium intercalation, hydrogen sensing, and smart windows due to their photochromic property. These nanowires are a mixture of MoO and conductive MoO phase and have been used to prepare composite films based on liquid single crystal elastomers (LSCE). The structure, morphology, and thermomechanic behavior of these films are discussed. In particular, we show that the particular combination of molybdenum-based nanowires and LSCEs allows for doping of liquid single crystal elastomers, preserving the pristine mechanical and optical properties of the host matrix. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

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 (T2) and the 2H-NMR spectral linewidth measured in the paranematic phase indicates the presence of slow dynamic motions. [ABSTRACT FROM AUTHOR]

Abstract: Photoactive materials based on azobenzene derivatives exhibit interesting properties related to the reversible photo-isomerisation between the trans and cis isomers of the azo-compound. In this work we report the preparation and physical–chemical characterization of Liquid Single Crystal Elastomers (LSCEs) containing azobenzene derivative side-chain mesogens as co-monomer. The interplay between the orientational ordering and mechanical response in the nematic phase is investigated by thermomechanical measurements as well as by deuteron NMR spectroscopy. We demonstrate that local nematic order can be reliably probed in azo-LSCEs using low molar mass deuterated cyanobiphenyl nematogen in low concentrations. No phase separation of the dopant is observed. The nematoelastic coupling between the effective sample deformation and average nematic order parameter is present over a wide temperature range even in the systems with relatively large heterogeneity of the nematic order and misaligned nematic domains. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]