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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.

In this work the 2H NMR spin-lattice relaxation times, T1Z and T1Q, and the spin-spin relaxation times measured by means of the quadrupolar echo sequence, T2, have been analyzed in order to study the dynamic processes of a liquid crystalline dendrimer labelled on the aromatic core of the lateral mesogenic units in the SmA phase. Several theoretical models describing both internal and overall molecular diffusion motions have been used to reproduce the temperature dependence of the spectral densities J(0)(0), J1(omega0) and J2(2omega0) experimentally determined at the Larmor frequency omega0 of 61.3 MHz. This analysis identifies in the internal rotation of the deuterated phenyl ring around its para axis the main contribution to the longitudinal relaxation. Moreover, the best fitting of the minimum in the trend of both T1Z and T1Q was obtained by introducing a distribution of activation energies for such internal motion, thus confirming previous hypothesis of dynamic heterogeneity. A much slower motion, affecting the sole transverse relaxation and ascribable to the reorientation of the whole macromolecular dendrimers, was modelled through the so called "slowly relaxing local structure" approach.

A liquid crystal (HZL 7/*) containing an (S)-2-methylbutyl-(S)-lactate unit in the chiral chain, is investigated by means of (2)H and (13)C NMR spectroscopy in order to obtain information on its orientational order, its molecular structure and the effect of external magnetic fields on the supramolecular structure of its phases. This mesogen presents very peculiar mesomorphic properties and exhibits frustrated TGBA* and TGBC* phases in a wide temperature range up to 60 degrees C, as well as an additional phase transition from TGBC(1)* to TGBC(2)*. (2)H NMR measurements show, for the first time, a peculiar magnetic field effect in unwinding the supramolecular structure of both the TGBA* and TGBC* phases. This effect is particularly evident at higher magnetic fields, while different behaviour is observed at lower magnetic fields. This indicates that the supramolecular structure is very sensitive to magnetic fields of the order of 1 Tesla. Moreover, the analysis of the (2)H and (13)C NMR spectra of HZL 7/* allow us to obtain several structural properties, such as the tilt angle of the TGBC* phases and the local orientational order parameters referred to the phenyl and biphenyl fragments. This is the first structural characterization of the frustrated phases of these complexes by means of NMR.

Side-chain liquid crystal elastomers (SC-LCEs) have been designed by using a new smectic crosslinker. Two types of monodomain films were prepared based on polysiloxane chains, with a different relative concentration of both crosslinker and mesogenic comonomers. The mesomorphic behavior of the two SC-LCE systems was investigated by differential scanning calorimetry and polarized optical microscopy showing a different mesomorphic behavior: in one case, we obtained a nematic SC-LCE film, in the other case, a Smectic A SC-LCE film. In both systems, the mesophases were stable in a wide temperature range. Moreover, the SC-LCE films possess a relatively high orientation at room temperature. The physical-chemical properties, such as the local orientational ordering, structural organization, and dynamics of SC-LCEs' constituents were studied by means of static and dynamic 2H NMR experiments, small-angle X-ray, and wide-angle X-ray diffractions. The relevant physical properties, such as the thermo-elastic and thermo-mechanic behaviors, are reported and discussed in view of the practical applications. [ABSTRACT FROM AUTHOR]

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]

Public engagement becomes increasingly important for scientists. One reason is the demand of the taxpayer to know what her or his money is being spent on, and why. The other one is that in a world that increasingly relies on technology, student engagement even at a very young age becomes a target to assure the needed supply of well-educated and especially motivated scientists for the decades to come. And it falls on the older generation of current researchers to leave the comfort of their lab once in a while, to awaken the interest for science among the population. Many people may know that there is a ‘liquid crystal’ in their mobile (cell) phone display, but when prompted, no one really knows what that liquid crystal actually is, let alone how the display they use many times every day, actually works in principle. It is part of our job to change this. In this contribution Valentina Domenici and Ingo Dierking would like to report on two recent Science Festival events in which they took part, one held in Genoa, Italy, and the other in Manchester, UK. [ABSTRACT FROM AUTHOR]

In this work, we report a first detailed study of the mesophasic behaviour of the calamitic liquid crystal, a (4,4′-bis(4″-carboxybelzyloxy)biphenyl derivate, labelled 9/2 RS/RS, showing an isotropic-isotropic-(columnar)-cubic phase behaviour on cooling.1H NMR diffusometry allowed to distinguish between the two isotropic phases in terms of diffusion characterized by different Arrhenius activation energy, while13C NMR static measurements and2H NMR investigations were of help in understanding the orientational ordering properties of the metastable columnar phase and confirmed the isotropic state of the sample in the cubic phase. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR) is a spectroscopic technique widely used to investigate materials and soft matter in particular. In this brief review, the main uses of NMR techniques to investigate different aspects of liquid crystals, such as the orientational and dynamic properties, the supramolecular structure and average molecular conformations, are described. In the second part of the paper, the case study of a ‘de Vries’ liquid crystal is reported and the main results obtained by combining different NMR techniques are discussed. [ABSTRACT FROM AUTHOR]

Liquid crystals that exhibit de Vries smectic A phases are promising materials for new generations of ferroelectric liquid crystal displays and other electro-optical devices. We investigated the molecular dynamic properties of a rod-like de Vries liquid crystal compound, namely the 9HL, a (S)-hexyl lactate derivative, in the whole mesophasic range. This is the first molecular dynamics' investigation on a de Vries phase, and the interest of this system is related to the understanding of the structural and supramolecular organization of de Vries SmA phases, which has been a subject of a broad scientific debate in the last years. Proton NMR spin-lattice relaxation times, T1, were measured in the range 100 MHz to 5 kHz, thus covering a very wide range of motional regimes. 1H NMR dispersion curves and temperature-dependent relaxation rates were analyzed with a global minimum target fitting approach and the main molecular motions, namely reorientational diffusion, translational self-diffusion, layer undulation, and tilting director fluctuations were fully characterized. The molecular dynamics' behavior observed across the SmA-SmC* phase transitions of 9HL was consistent with the proposed cluster diffuse cone model for the de Vries SmA phase. [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]

In this work, we present our recent results obtained in the field of translational self-diffusion studies by means of 1H NMR diffusometry in smectic phases formed by calamitic chiral liquid crystals. In particular, the diffusional behavior of chiral smectic phases with different clinicity, such as the ferroelectric and antiferroelectric phases, will be discussed on the basis of the results obtained on three different chiral smectogens. This research demonstrates that, despite previous assertions, out-of-plane translational diffusion in smectic phases is poorly affected by the clinicity of the phases, showing no discontinuity at the transition from synclinic to anticlinic structures, typical of the smectic C* phases. We can conclude that diffusion basically reflects the layered smectic structure where in-plane diffusion is much faster and less hindered than the out-of-plane one. [ABSTRACT FROM PUBLISHER]

In this paper, some of the most relevant 2H NMR experiments with the aim of determining the biaxial nature of the nematic phase in several thermotropic liquid crystals are overviewed. Advantages and disadvantages of the technique are discussed. This contribution is an extract of the lecture held in Erice (Sicily) in occasion of the “18th Course - Liquid crystal nanostructures and self-assembling: from organic electronics to metamaterials”, the 2nd School of the Italian Liquid Crystal Society. [ABSTRACT FROM AUTHOR]

Solid State 2H Nuclear Magnetic Resonance (NMR) spectroscopy was applied to the structural and orientational studies of a selectively labelled ‘de Vries’ liquid crystal compound. Measurements were performed at five values of the magnetic field, ranging from 4.70 to 18.80 T. The trend of the orientational order S and tilt angle θ of the 2H-labeled moiety in the SmA and SmC* phases at different magnetic fields confirms previous results and allows us to better characterize the SmA-SmC* transition. [ABSTRACT FROM PUBLISHER]

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]

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]

Recently several liquid crystals having a chiral smectic C phase (SmC*) have been studied by means of NMR spectroscopy combined with ab initio calculations. The advantage of exploiting different experimental techiques, such as 2H and 13C NMR, and the possibility to calculate relevant spectroscopic quantities, such as 13C chemical shift tensors, by means of ab initio methods, is here discussed based on recent results obtained on several ferroelectric liquid crystal compounds. Orientational order parameters, conformational and structural features can be determined in a self consistent way by applying this combined approach. [ABSTRACT FROM AUTHOR]

Abstract: Recent studies based on NMR spectroscopy put in evidence the occurrence of a slow motion regime in bent-shaped liquid crystals having the aromatic core formed by five phenyl rings linked by ester groups both in the isotropic and in the nematic phases. In this paper a brief overview of these NMR studies is presented and additional new relaxation data recorded on a banana-shaped mesogen are reported and discussed. In particular, proton spin-lattice relaxation times, T 1, were acquired at different Larmor frequencies from 8 MHz to 5 kHz by means of NMR relaxometry in a wide range of temperatures from the isotropic to the crystalline phases. These data confirm that NMR relaxation in bent-core liquid crystals is affected by much slower dynamics than that observed in common rod-like liquid crystals and that overall molecular reorientations are responsible of this slow motion regime. This finding is discussed in the frame of the results obtained by means of several NMR methods, such as 2H NMR T 2 analysis and 1H self-diffusion NMR measurements, on the same bent-core molecule. [Copyright &y& Elsevier]

We synthesised a series of rod-like mesogens with a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free-standing films, by Grandjean-Cano texture and by NMR studies on a deuterium-labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20-30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in 2H-NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases. [ABSTRACT FROM AUTHOR]

Several 2H NMR studies have been performed on two deuterium labelled banana-shaped liquid crystals, the 1,3-phenylene bis{4-4'-(11-undecenyloxy) benzoyloxy} benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), in the isotropic, nematic and B2 phases.In this article, the state of the art of these studies is drawn and some new 2H NMR measurements on deuterated banana sub-units, namely the 3-(benzoyloxy)phenyl benzoate-d10 (BOB-d10) and its 4-chloro homologue (ClBOB-d10), used as small probes dissolved in two banana-shaped liquid crystals in the nematic phase are reported.As a general result, the orientational ordering properties of these sub-units are discussed in the frame of recent results obtained on the same banana-shaped liquid crystals dissolved in calamitic nematic solvents as well as in the bulk. [ABSTRACT FROM AUTHOR]

Experimental evidences of the slow dynamic behaviour of a banana-shaped liquid crystal (LC) detected by means of Deuterium Nuclear Magnetic Resonance (DNMR) in its nematic phase are reported. Deuterium line-shape and line-width trends of DNMR spectra of three selectively deuterium-labelled isotopomers of 4-chloro 1,3-phenylene bis 4-4′-(11-undecenyloxy) benzoyloxy benzoate (ClPbis11BB) are discussed in comparison with those of a deuterated probe dissolved in ClPbis11BB, used as LC solvent. Different dynamic regimes are observed for the central and lateral aromatic rings of the probe. These experimental remarks are discussed in terms of overall and internal molecular motions. [ABSTRACT FROM AUTHOR]

In this work the dynamic behaviour of the ferroelectric liquid crystal (‐)‐( S )‐[4‐(2‐methylbutyloxycarbonyl)phenyl] 4‐ n ‐heptylbiphenylcarboxylate (MBHB) in its smectic A (SmA), unwound chiral smectic C (uSmC*) and chiral smectic C (SmC*) phases has been studied by means of 2 H NMR spectroscopy. Zeeman ( T 1Z ) and quadrupolar ( T 1Q ) spin‐lattice relaxation times have been analysed to extract dynamic parameters (diffusion coefficients and activation energies). The small step rotation diffusion model in the uniaxial approximation has been used to describe overall spinning and tumbling motions, and the strong collision model to describe the internal reorientations of the aromatic fragment. Relaxation data in the SmC* phase have been analysed by using a theoretical approach. The dynamic features obtained in the smectic phases of this mesogen are here presented and discussed in comparison with the results obtained in other ferroelectric liquid crystals, focusing on the fast regime of motions. [ABSTRACT FROM AUTHOR]

The behaviour of the 2 H NMR line-width throughout the whole mesophasic range of the ferroelectric liquid crystal “1-methylheptyl 4'-(4?-n-decyloxybenzoyloxy) biphenyl-4-carboxylate” (10B1M7) is here reported and discussed. Static Fringe Field NMR Diffusometry measurements from the isotropic to the anticlinic phase have been performed leading to a significant result concerning the tumbling diffusional coefficient (D ? ). These results have been compared to X-Ray measurements and 2 H NMR line-widths in order to explain the peculiar behaviour occurring at the transition between the ferroelectric (FLC) and the antiferroelectric (AFLC) liquid crystalline phases. [ABSTRACT FROM AUTHOR]

The dynamics of a chiral polymorphic liquid crystal smectogen (8BEF5), partially deuterated on the aromatic core and on the chiral chain, has been investigated in its smectic A and C* phases. The 2H T1Q and T1Z spin–lattice relaxation times, and, consequently, the spectral densities J1(ω0) and J2(2ω0) have been determined for the aromatic as well as methylene deuterons of the chiral chain closer to the aromatic core. The interpretation of the relaxation behaviour has been carried out by means of the available motional models in the SmA phase. Contrary to what is usually observed for other mesogens, in the case of 8BEF5 the internal rotations are much better described by the strong collision model rather than by the small step rotational diffusion one. [Copyright &y& Elsevier]

In this work, we report a study of two thermotropic liquid crystalline samples showing a not common mesophase behavior. The samples, namely a di-benzyloxy biphenyl derivative labelled 9/2 RS/RS, and a bimesogenic liquid crystal labelled L1, show a direct transition between two isotropic phases followed, at lower temperatures, by the optically isotropic, 3D structured, cubic phase. These systems have been investigated by means of 1H NMR diffusometry and 1H NMR relaxometry in order to characterize their isotropic–isotropic'–cubic mesophase behavior, mainly on the dynamic point of view. In particular, the temperature trend of the self-diffusion coefficients measured for both samples allowed us to significantly distinguish between the two isotropic phases, while the temperature dependence of the 1H spin-lattice relaxation time (T1) did not show significant discontinuities at the isotropic–isotropic' phase transition. A preliminary analysis of the frequency-dependence of 1H T1 at different temperatures gives information about the main motional processes active in the isotropic mesophases. [ABSTRACT FROM AUTHOR]

Abstract Bimesogenic liquid crystal elastomers, composed with controlled concentrations of two mesogenic constituents, specifically a nematogen and a smectogen, exhibit either enhanced nematic or smectic structure. Here we present a thorough examination of thermomechanical and elastic properties of such a system at high crosslinking density, with ensuing disordered smectic structure which prevents the observation of smectic-nematic phase transition by means of thermomechanical response measurements. Yet, the onset of local smectic order is clearly observable in stress-strain measurements via an anomaly in the elastic modulus. Breakdown of the local smectic structure is detected at higher mechanical loads. Bimesogen composition – temperature phase diagram is as well constructed, consistent with the one obtained by complementary experimental techniques. Stress-strain temperature dependence measurements can thus serve as efficient and simple phase diagram characterization method that is able to detect formation of smectic clusters in strongly crosslinked and thus highly disordered smecto-nematic liquid crystal elastomers. Graphical abstract Image 1 Highlights • Strongly crosslinked bimesogenic liquid crystal elastomers with enhanced nematic (N) or smectic A (SmA) structure. • Investigation of thermomechanical and elastic properties. • No SmA-N phase transition seen by thermomechanical measurements due to high crosslinking density. • All phase transitions can be identified with stress-strain studies, as well as the stress-induced SmA-N phase transition. • Simple approach for determination of bimesogen concentration – temperature – external mechanical stress phase diagram. [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]