1,720,967 research outputs found
First liquid single crystal elastomer containing lactic acid derivative as chiral co-monomer: Synthesis and properties
No full textFor 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 rang
Smectic A – Smectic C* Transition in a “de Vries” Liquid Crystal by 2H NMR
No full textSolid 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
Orientational and structural properties of ferroelectric liquid crystal with a broad temperature range in the SmC* phase by C-13 NMR, x-ray scattering and dielectric spectroscopy
No full textThermotropic liquid crystalline materials laterally substituted by a methyl group on the aromatic ring of the alkoxybenzoate unit far from the chiral centre exhibit a very broad temperature range in the ferroelectric smectic C* (SmC*) phase on cooling (including supercooling) with a very high spontaneous polarization (similar to 210 nC cm(-2)) and tilt angle (similar to 43 degrees.) at saturation. We are presenting a detailed study of the physical properties of a ferroelectric compound, representative of this category of liquid crystals, by means of solid state (13)C-NMR, small angle x-ray scattering, dielectric spectroscopy and optical methods of the tilted SmC*. Values of the spontaneous tilt angle measured optically are compared to those determined from the x-ray data and discussed. In addition, the viscosity has been determined in the SmC* phase by different experimental methods. (13)C NMR data allowed us to get information about the degree of orientational order of the SmC* phase and revealed the complete unwinding of the helical axis at the magnetic field of 9.4 T. This result is discussed in the framework of recent publications on the effect of the magnetic field on the supra-molecular structure of the SmC* phase
Polar liquid crystalline monomers with two or three lactate groups for the preparation of side chain polysiloxanes
No full textTwo new chiral liquid crystalline monomers with bilactate or trilactate chiral units were synthesized and studied. The monomers show the paraelectric SmA and ferroelectric SmC* phases. The antiferroelectric SmCA* phase was detected only for the monomer possessing the bilactate unit. The temperature dependences of the spontaneous polarization and spontaneous tilt angle were measured. Real and imaginary parts of the complex permittivity were studied as a function of frequency and temperature. The synthesized monomers contain a double bond at the end of the achiral terminal chain, and were used to prepare liquid crystalline polysiloxanes
Supramolecular structure of tilted TGB* phases studied by means of Deuterium NMR line-shape analysis
No full textIn this work the structure of the tilted TGBC* phase is investigated by means of the line-shape analysis of Deuterium NMR spectra. The case of a rod-like mesogen containing a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain, whose acronym is HZL 7/*, is taken. as a representative example for the large temperature range of stability of its tilted TGB* phases. By comparing the experimental and computed Deuterium NMR spectra. several structural features can be extracted. For instance, the analysis of Deuterium NMR line-shapes put in evidence the occurrence of helical deformations due to the effect of the external magnetic field
Orientational Order of a Liquid Crystal with Three Chiral Centers by a Combined 13C NMR and DFT Approach
No full textIn this work, the liquid crystal (S)-2-methylbutyl-[4′-(4′′-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2′)- benzoyl)-propionyl)]-propionate (ZLL 7/*) was investigated by means of 13C NMR spectroscopy. This compound has a very peculiar mesomorphic behavior, showing the following phases: paraelectric SmA, ferroelectric SmC*, antiferroelectric SmC*A, re-entrant ferroelectric SmC*re, and ferroelectric hexatic Sm*HEX. The structural and orientational ordering properties of ZLL 7/* have been determined by exploiting the nuclear chemical shielding properties of 13C. To this aim, solid-state NMR techniques such as CP, SPINAL-64, and SUPER have been used in combination with DFT calculations. The agreement between experimental and in vacuo DFT shielding parameters appears to be satisfactory. The orientational order parameters obtained from the 13C shielding analysis have been discussed, taking into account different data analysis approaches and comparing them to those previously obtained from an independent 2H NMR study
Brief overview on 2H NMR studies of polysiloxane-based side-chain nematic elastomers
No full textThis is a brief overview on recent studies on liquid crystalline elastomers (LCEs) based on polysiloxane chain, in the form of monodomain films, selectively H-2-labeled in different parts of the LCE samples, i.e. on the crosslinker or mesogenic units. H-2 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
1H NMR Relaxometry Study of a Rod-Like Chiral Liquid Crystal in Its Isotropic, Cholesteric, TGBA*, and TGBC* Phases
No full textThe molecular dynamics of a chiral liquid crystal showing a rich variety of frustrated mesophases has been investigated by means of (1)H NMR relaxometry. The interest in this lactate derivative, HZL 7/*, is related to a large range of thermal stabilities of the twist grain boundary (TGB) phases. Dispersions of the (1)H spin-lattice relaxation times, T(1), in the frequency range from 300 MHz to 5 kHz were measured and consistently analyzed in the isotropic, chiral nematic, TGBA*, and two TGBC* phases. In the isotropic and N* phases, a three-exponential magnetization decay was observed and assigned to three specific molecular groups of the HZL 7/* (molecular core, methyl, and methylene groups). In the TGB phases, all T(1) components merge into a single one. The analysis of the T(1) dispersion in the TGBA* phase shows that the translational self-diffusion relaxation mechanism dominates over a broad frequency range and that layer undulations are less relevant than the relaxation contribution associated with the diffusion process across the TGB structure. In the TGBC(1)* phase, the T(1) dispersion presents a strong contribution of in-layer tilt direction fluctuations (T(1)(-1) proportional to v(-1/2)), while, in the TGBC(2)* phase, the linear frequency dependence of T(1) could be associated with a much stronger contribution of layer undulations than for the other TGB phases. This is at present the first molecular dynamics investigation on several TGB phases by means of (1)H NMR relaxometry
1H NMR relaxometry in the TGBA* and TGBC* phases
Full text linkRecent results obtained by 1H NMR relaxometry of liquid crystals having twist grain boundary (TGB) phases are here reviewed. In
particular, three chiral rod-like lactate derivative mesogens were investigated. In the isotropic phase, three-exponential magnetization
decay was observed in all cases and the three distinct spin-lattice relaxation times (T1) were assigned to three specific molecular groups
of these molecules. In the TGBA* and TGBC* phases the magnetization decay is mono-exponential and the main features of the 1H NMR
dispersion curves analyzed through a global target fitting procedure in terms of specific molecular and collective dynamics are discussed
New Liquid Crystalline Elastomeric Films Containing a Smectic Crosslinker: Chemical and Physical Properties
Full text linkSide-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
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