Fluorinated esters with a very broad temperature range of the antiferroelectric phase
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ABSTRACTAntiferroelectricity is a desirable property of liquid crystalline materials. Therefore, we synthesised and studied two new chiral rod-like mesogens with a molecular core based on two biphenyls connected via ester linkage, with the phenyl substituted by a fluorine atom. The studied mesogens are characterised by nuclear magnetic resonance spectroscopy and mass spectrometry analysis. They have methyl heptyl in the chiral chain and exhibit an antiferroelectric phase in a very broad temperature range. We investigated their mesomorphic properties and confirmed the phase identification by differential scanning calorimetry and broad-frequency dielectric spectroscopy measurements. Additionally, we compared the studied mesogens with previously synthesised analogous materials. Two mixtures were formulated using a base mixture and new mesogens. The helical pitch of the synthesised mesogens and formulated mixtures was estimated using the selective reflection method.KEYWORDS: SynthesisAFLCshelical pitcheutectic mixturesdielectric modes Disclosure statementNo potential conflict of interest was reported by the author(s).Supplementary dataSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2252381.Additional informationFundingThis work was supported by the University Research Grant [UGB 22-801].Keywords:
Antiferroelectricity
Atmospheric temperature range
Fluorine
Abstract The general rule that addition of a non-mesogenic solute causes a sharp decrease in the nematic-isotropic transition temperature (TN→ I) of a nematic solvent is not obeyed when the solute and solvent can enter into a donor-acceptor interaction. Addition of 4-aminobiphenyl to the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (nematic range ∼ 25–35°) leads to an increase in TN→ I and a decrease in the crystal-nematic transition temperature. The maximum nematic range (21–38°) is achieved at ∼ 7 mole % solute.
Mesogen
Atmospheric temperature range
Biaxial nematic
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Antiferroelectricity
Atmospheric temperature range
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Lead zirconate (PbZrO$_3$) is considered the prototypical antiferroelectric material with an antipolar ground state. Yet, several experimental and theoretical works hint at a partially polar behaviour in this compound, indicating that the polarization may not be completely compensated. In this work we propose a simple ferrielectric structure for lead zirconate. First-principles calculations reveal this state to be more stable than the commonly accepted antiferroelectric phase at low temperatures, possibly up to room temperature, suggesting that PbZrO$_3$ may not be antiferroelectric at ambient conditions. We discuss the implications of our discovery, how it can be reconciled with experimental observations and how the ferrielectric phase could be obtained in practice.
Antiferroelectricity
Zirconate
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MHPB(H)PBC and MHPB(F)PBC were studied fairly by means of dielectric measurements. We studied the Fluorine - Hydrogen replacement in the structure of the molecule (in the aliphatic tail) and its influence on dielectric properties of the investigated antiferroelectric liquid crystals. We found that replacement of Hydrogen by Fluorine does not influence on dielectric relaxation in antiferroelectric phase. It is worth to notice that for fluorinated molecules mesophases exist for higher temperature what could confirm that Fluorine in achiral part of molecule stabilizes mesophases. Keywords: Dielectric RelaxationAntiferroelectricity
Antiferroelectricity
Fluorine
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Abstract An approximate solution is derived for the structure of a twist wall between two regions of opposite 180° twist in a nematic liquid crystal film. The result is used to show that a twist wall is unstable unless the elastic constants of the nematic are such that K22≤ ½(K11 + K33). Thus the observation of such walls in nematic liquid crystals is evidence of this elastic anisotropy. Experimental evidence is given for the validity of the approximate solution obtained.
Biaxial nematic
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Abstract Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.
Biaxial nematic
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This chapter contains sections titled: Introduction Elements of Thermodynamics in DSC The Basics of Differential Scanning Calorimetry Purity Determination of Low-Molecular-Mass Compounds by DSC Calibration of Differential Scanning Calorimeters Measurement of Heat Capacity Phase Transitions in Amorphous and Crystalline Polymers Fibers Films Thermosets Differential Photocalorimetry (DPC) Fast-Scan DSC Modulated Temperature Differential Scanning Calorimetry (MTDSC) How to Perform DSC Measurements Instrumentation Appendix Abbreviations References
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Abstract Achiral ‘swallow-tailed’ liquid crystalline materials are known to give alternating-tilt smectic C phases (SCalt) which have structural similarities to the chiral antiferroelectric phases denoted as S∗CA. This paper describes the synthesis and characterization of three achiral branchedalkyl 4-(4′-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates. Optical microscopy and differential scanning calorimetry confirm that these materials show SCalt and overlying SA phases. The compounds were investigated as potential hosts which could be doped with a chiral ferroelectric liquid crystal so as to provide a viable- antiferroelectric mixture. These studies (microscopy and differential scanning calorimetry), to characterize the properties of the mixtures, show that antiferroelectric phases are induced. However, switching studies show that the antiferroelectric phases are extremely stable, a property which is almost certainly a consequence of the length of the lateral branching groups (ethyl, propyl and butyl).
Antiferroelectricity
Branching (polymer chemistry)
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Antiferroelectricity
Atmospheric temperature range
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Abstract The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,th 5U th and 10U th, where U th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.
Homeotropic alignment
Biaxial nematic
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