氢键诱导液晶的DSC和变温红外光谱研究

制备了两个分别基于4,4’-联吡啶(BPy)和丙基反式环己基苯甲酸(PCBA)以及BPy和丙基反式双环己基甲酸(PCCA)的氢键液晶复合物(PCBA-BPy和PCCA-BPy),结合使用偏光显微镜(POM),用差示扫描量热(DSC)和变温傅里叶变换红外光谱对它们的中间相性能和分子间氢键的热稳定性进行了表征。结果表明,这两个氢键复合物都具有由于分子间氢键作用而导致的近晶相,但它们中存在的氢键作用却不相同,在PCBA-BPy中,温度变化时,发生氢键结合的羰基的吸收峰的位置仅在发生晶型转变时有突变,而在PCCA-BPy中,温度变化时,其羰基的吸收峰的位置则基本不发生突变。另外,当温度高于它们的清亮点时,这两个氢键液晶复合物的分子间氢键都发生部分分解。     

Optical modulation in nematic phase of halogen substituted hydrogen bonded liquid crystals

A series of halogen-substituted hydrogen-bonded liquid crystalline complexes have been designed and synthesised. A successful attempt has been made to form complementary hydrogen bonding between the dodecyloxy benzoic acid (12BAO) and halogen-substituted benzoic acids and the physical properties exhibited by the individual complexes are studied. The complexes obtained are analysed by polarising optical microscope (POM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and dielectric studies. The formation of complementary hydrogen bond is confirmed through FTIR spectra. An interesting feature of this series is the observation of a field-induced transition (FiT) in nematic phase. Another interesting phenomenon is the observation of a new smectic X phase (worm-like texture) in all the synthesised complexes. Dielectric relaxation studies in the smectic C phase of these hydrogen bonded complexes along with the Arrhenius and the Cole–Cole plots are discussed. Optical tilt angle in smectic C phase and the corresponding fitted data analysis concur with the Mean field theory prediction.     

Thermal and Optical Properties of Some Hydrogen-Bonded Liquid Crystal Mixtures

Phase transition properties of the mixtures of hydrogen-bonded nematic liquid crystals (HBLC) 4-hexylbenzoic acid (6BA), 4-(octyloxy)benzoic acid (8OBA), and 4-(decyloxy)benzoic acid (10OBA) have been investigated by means of differential scanning calorimetry (DSC) and polarize optic microscope (POM). The DSC and POM results clearly indicate the existence of smectic and nematic phase transitions in binary mixtures. The phase transition temperature values of 6BA/10OBA mixtures have clearly increased with increasing heating rate. The activation energies were calculated for the phase transitions of 6BA/10OBA liquid crystal (LC) mixture. The optical transmittance of these mixed hydrogen-bonded nematic liquid crystals was investigated in terms of temperature variations through electrooptic methods. The electrooptic experiments indicate that, while low in the nematic phase, the optical transmittance is very high at the nematic-isotropic phase transition. The transmitted light intensity values of 6BA/8OBA mixture are somewhat higher than those of other binary mixtures, 6BA/10OBA and 8OBA/10OBA, a result associated with the different alkyloxy chain lengths.     

Design, synthesis and characterization of a linear hydrogen bonded homologous series

A linear hydrogen bonded liquid crystalline homologous series has been synthesized and characterized. Hydrogen bond is formed between p-n-dodecyloxy benzoic acid and various p-n-alkyl benzoic acids whose alkyl chain vary from octyl to ethyl. Synthesized complexes are characterized by FTIR, ¹H NMR and ¹³C NMR studies for inferring the formation of hydrogen bonds. Polarizing Optical Microscopy (POM) and DSC studies reveal various mesophases and their corresponding transition temperatures along with respective enthalpy values. All the seven synthesized complexes exhibit rich liquid crystalline mesomorphism. A new phase namely smectic X has been observed in five of the complexes with a narrow thermal range. This phase has been characterized by optical textural, DSC, tilt angle and helicoidal pitch studies. Smectic X is sandwiched between traditional smectic C and re-entrant smectic C (designated as C_R) phases. Homeotropic transition in nematic phase is observed in all the mesogens and thus these materials can be used as thermally controlled optical shutters. Tilt angle in smectic C, smectic X and smectic C_R phases have been experimentally elucidated for all the mesogens.     

Optical shuttering action in nematic phase of SMHBLC: observation of a ribbon-like texture

In this study, complexes belonging to supramolecular hydrogen-bonded liquid crystal homologous series are synthesized and characterized. Hydrogen bond is formed between p-n-alkyloxy benzoic acids (nBAO, where n?=?5–11) and chlorobenzoic acid (ClBAO), respectively. The isolated homologues are characterized by various techniques such as polarizing optical microscopic (POM) studies, differential scanning calorimetry (DSC), and Fourier transform–infrared spectroscopy. Based on the POM and DSC studies, the phase diagram has been constructed and discussed. A new smectic ordering, labeled as smectic R, has been characterized, which exhibits a ribbon-like texture. This phase is observed in the complexes pertaining to the higher homologous series. Tilt angle in this phase has been experimentally deduced and the results are fitted to the power law which concurs with the mean-field theory predicted value. Optical shuttering action in the homologue has been detected in the nematic phase and the results are also discussed.     

Design,synthesis and characterization of a linear hydrogen bonded homologous series exhibiting reentrant smectic C ordering

Design, synthesis and characterization of seven linear hydrogen bonded liquid crystal complexes derived from mesogenic p–n-decyloxy benzoic acid and p–n-alkyl benzoic acids designated as 10OØn (where n varied from ethyl to octyl) are reported. FTIR studies confirm the hydrogen bond formation in all these complexes. The phase transition temperatures and their corresponding enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. POM and DSC data are further utilized for the construction of 10OØn phase diagram. Two Odd–even effects have been evinced, one in enthalpy values and the other in corresponding transition temperatures across the isotropic to nematic phase transition. An interesting result is the observation of re-entrant smectic ordering, designated as smectic C_R in three higher ordered mesogens. A new smectic ordering, smectic X, has been observed which is sandwiched between traditional smectic C and re-entrant smectic C_R. Magnitudes of optical tilt angle in smectic C, smectic X and smectic C_R are experimentally found to attain saturation with decrement of temperature in the corresponding phase. The occurrence of smectic X and smectic C_R are discussed with relation to the molecular chemical structure. The optical filtering action in smectic C and re-entrant smectic C_R phases have been analyzed.     

Undulation textures at the phase transitions of some alkyloxybenzoic acids

We observed undulated smectic textures for some compounds of the 4,n-alkyloxybenzoic (nOBAC) acid series, at transitions between the smectic and the isotropic phase and between the smectic and nematic phase. Studied compounds were 12OBAC, 16OBAC and binary mixtures of 12- and 16OBAC. The undulations are dressing a usual Schlieren texture. An interesting fingerprint pattern is observed at the smectic-nematic in the case of the binary mixture, approximately 1 : 1 in weight.     

Growth of toric domains in mesophases of oxadiazoles

In a previous paper (Phase Transitions, 80(9), 987, 2007), we discussed textures of some mesomorphic oxadiazole compounds with terminal Cl-substituent. Optical microscope investigations showed a very interesting behaviour of smectic and nematic phases: the smectic phase has fan-shaped and toric textures and the nematic phase has spherulitic domains, which disappear as the sample is further heated, the texture changing into a smooth one. Here, the focus is on the growth of toric domains from the nematic phase and on the role of defects in domain structure. We use compounds with the same structure of oxadiazoles discussed in the previous paper but with terminal Br-substituent.     

Thermal and optical characterization of a novel series of supramolecular liquid crystals

Supramolecular hydrogen bonded liquid crystal (SMHBLC) homologous series is synthesized and characterized. Hydrogen bond is formed between p-n-alkyloxy benzoic acids (nOBA, where n=7 to 12) and Iodo substituted benzoic acid (I_mBA) respectively. The isolated homologs are characterized by various techniques like Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), ¹H Nuclear Magnetic Resonance Spectroscopy (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). From the POM and DSC studies phase diagram has been constructed and discussed. A new smectic ordering, labeled as smectic R, has been characterized which exhibits a ribbon like texture. This phase is observed in a complex pertaining to the undecyloxy carbon number. Tilt angle in smectic R phase and in traditional smectic C of all the complexes have been experimentally deduced and the results are fitted and so obtained β value concurrences with the Mean field theory predicted value.     

Study of phase behavior in a system of linear hydrogen-bonded carboxylic acid homologues

The system of hydrogen-bonded liquid crystals formed from binary mixtures p-n-heptyloxybenzoic (I) acid and p-n-undecloxybenzoic (II) acid has been investigated by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, and scattering and dielectric measurements. The T–X phase diagram was obtained for this system. All mixtures show enantiotropic smectic and nematic phases. The crystalline phase represents a (α, β) solid solution. At the 2:1 ratio of I to II, an intermediate phase – co-crystal γ – is formed. The co-crystal γ possesses a much wider mesophase range than the corresponding initial components. In contrast to mixtures based on the solid solutions having a positive anisotropy, the co-crystal shows a negative dielectric anisotropy in the mesophase.     

Synthesis,characterization and mesomorphic properties of new liquid-crystalline compounds involving ester–chalcone linkages

We report the synthesis and evaluation of thermal behaviour of two novel series of chalcone-based liquid-crystalline compounds. The flexibility in these systems is provided by attaching straight alkoxy chain at one end. All the mesogens have been characterized by spectroscopic methods such as ¹H-NMR, FTIR, mass spectra, UV/Vis, and also elemental analysis. Their thermal behaviour was evaluated mainly by polarizing optical microscopic observation and DSC studies. It has been observed that the majority of the compounds display nematic and/or smectic mesophases. In both the series, compounds having C₃ to C₇ tail display only enantiotropic nematic phase, whereas C₈ and C₁₀ homologues exhibit enantiotropic smectic C and nematic phases. The higher homologues with C₁₂, C₁₄ and C₁₆ tails show the smectic C phase only.     

A study on polymorphism of hydrogen-bonded thermotropic liquid crystals

A novel mesogenic homologous series comprising of eight hydrogen-bonded liquid crystalline complexes are isolated and analyzed by forming a hydrogen bond between p-n alkyloxy benzoic acids (where n represent alkyloxy carbon number which varies from 5 to 12) and mesaconic acid, respectively. Eight synthesized complexes are subjected to Fourier transform infra-red spectroscopy, polarizing optical microscopy and differential scanning calorimetry studies to meet the basic characterization. The variation of optical tilt angle with respect to temperature in various conventional and smectic X phases are investigated and analyzed. A phase diagram is constructed to elucidate the mesogenic behavior of novel liquid crystalline series. The Cox ratio that reveals the order of various mesophases and the stability factor, to invoke the thermal stability of mesophases, is studied under the results of DSC thermogram.     

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids (n indicates the homologue number). The homologues with 7?≤?n?≤?13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originating from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic sub-phases.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

Synthesis and characterization of wide range mesogenic esters based on asymmetrical 2,5-disubstituted 1,3,4-thiadiazole

A homologous series of new 13 esters, 4-(5-(p-tolyl)-1,3,4-thiadiazol-2-yl)-phenyl-4-alkoxybenzoate, (IV_n), based on 1,3,4-thiadiazole core has been synthesized. The structures of these esters were confirmed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and mass techniques. Their mesophases behavior was investigated with hot-stage polarizing optical microscope and differential scanning calorimetry. The thermal stability for most of these derivatives was measured by thermal gravimetric analysis. All the target esters showed enantiotropic mesomorphic behaviors with nematic and nematic/smectic C phases. The phase transition temperatures and liquid crystalline properties were affected by the nature of heterocyclic ring and the length of the alkoxy chain. Only the nematic phase was observed in the first 10 derivatives, (n = 1–10), while the last 3, (n = 12, 16 and 18) showed nematic and smectic C phases. These compounds demonstrated high liquid crystalline ranges, both in heating and cooling cycles. The mesomorphic results obtained were compared with the reported analogs of similar constituents.     

Thermal and dielectric studies of self-assembly systems formed by hydroquinone and alkyloxy benzoic acids

Self-assembly systems formed by hydroquinone and alkyloxy benzoic acids are isolated and characterized. Hydroquinone formed double hydrogen bonds with p-n-alkyloxy benzoic acids. Various hydrogen bonded complexes have been synthesized with hydroquinone and pentyloxy to dodecyloxy benzoic acid, respectively. FTIR studies confirm the hydrogen bond formation in the complex. Polarizing Optical Microscopic (POM) studies revealed the textural information while the transition and enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. Phase diagram has been constructed from the POM and DSC data. The members of the present homologous complexes are characterized by POM, DSC, optical tilt angle and dielectric studies. Odd-even effect has been evinced in enthalpy values and transition temperatures corresponding to the isotropic to nematic phase transition. Optical tilt angle in Smectic C phase data has been fitted to a power law and it has been observed that the temperature variation of the tilt angle follows Mean Field theory prediction. Results of Fourier Transform Infrared Spectra (FTIR), POM, DSC, tilt angle and dielectric studies are discussed.     

High-pressure DTA-investigations on liquid crystals

Abstract

High pressure studies on the phase behaviour of liquid crystals up to a maximum pressure of 7 kbar were performed using a computer-assisted high-pressure differential thermal analysis apparatus that has been developed for the present experiments. T (p) - phase diagrams were determined for selected liquid crystals differing systematically in structure.

Data are presented for some phenylcyclohexanes, some bicyclo-hexanes and cyclohexylcarboxylic acid phenyl ester being substituted with different functional groups e.g. alkyl, alkoxy and/or cyano. Pressure-induced nematic and smectic phases were found, and for one substance even an “interdigitated” smectic B phase could be stabilized under pressure.     

Characterization of phase transition in liquid crystals by voltaic potential difference

Our investigations show that the determination of voltaic potential differences is a useful method to characterize the phase transition N → I and I → N for a liquid-crystal (LC) mixture and for a phase transition from one liquid-crystalline phase into another. The ΔV/T curves show reversible as well as an irreversible behaviour. Moreover, the transition from the smectic into the nematic phase is demonstrated on liquid-crystalline polymers.     

Comparative study of liquid-crystalline ordering in a monomer,linear polymer,and graft copolymer by the photon transmission technique

The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylate monomer, 6-(4-cyanobiphenyl-4′oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer (GLC6) with polytetrahydrofuran grafts. The phase transitions were also confirmed by DSC and polarizing microscopy. We observed the phase transition sequence isotropic–nematic–smectic A–smectic C in the LC6 monomer. In PLC6 and GLC6 polymers, the nematic and smectic A phases appear dominant. The apparent nematic–smectic A transition is of first order in PLC6 and of second order in GLC6, with the transition temperature remaining the same. The effects of quenched random constraints introduced in GLC6 are consistent with the theory of quenched random interactions. The critical exponents were also evaluated.     

Universal scaling of dielectric response of various liquid crystals and glass-forming liquids

Abstract

We present a new generalized scaling relationship accounting both for the real and imaginary parts of the complex permittivity data. The generalized scaling procedure has been successfully used for various relaxation processes in liquid crystals (4-bromobenzylidene-4′-pentyloxyaniline, 4-bromobenzylidene-4′-hexyloxyaniline, 4′-butyl-4-(2-methylbutoxy)-azoxybenzene, 4-ethyl-4′-octylazoxybenzene), and in glass-forming liquids (glycerol, propylene carbonate, salol, cresolphthalein-dimethylether). As it is shown, one obtains common master-curve for liquid-like phases (isotropic liquid, cholesteric, nematic, smectic A), solid-like phases (smectic B, conformationally disorder crystal) and supercooled liquid phase.