Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Enhanced conductivity and dielectric absorption in discotic liquid crystalline columnar phases of a vanadyl complex

A liquid crystalline vanadyl complex has been studied by DSC, polarizing optical microscopy, the reversal current technique, X-ray diffraction and frequency domain dielectric spectroscopy. The compound exhibits three columnar phases: rectangular ordered (Col_ro), rectangular disordered (Col_rd), and hexagonal disordered (Col_hd), all of which show a dielectric relaxation process at low frequencies. In the Col_ro low temperature phase this process seems to be connected with a slow relaxation of polarized polymeric chains inside the columns (mHz frequency range). However, in the Col_hd high temperature disordered phase this relaxation is faster (Hz range). It is interesting that the liquid crystalline phases studied show enhanced conductivity which changes by four orders of magnitude from 10^-9 S m^-1 in the orientationally disordered crystal (an ODIC phase) to 10^-5 S m^-1 in the Col_hd high temperature phase. Such a value of the conductivity is typical for semiconducting materials.     

Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals

Thermotropic ionic liquid crystals based on the flavylium scaffold have been synthesized and studied for their structure-properties relationship for the first time. The mesogens were probed by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Low numbers of alkoxy side chains resulted in smectic (SmA) and lamello-columnar (Lam_Col) phases, whereas higher substituted flavylium salts showed Col_ro as well as ordered and disordered columnar (Col_ho, Col_hd) mesophases. Mesophase width ranged from 13 K to 220 K, giving access to room temperature liquid crystals. The optical properties of the synthesized compounds were probed towards absorption and emission properties. Strong absorption with maxima between 444 and 507 nm was observed, and some chromophores were highly emissive with quantum yields up to 99 %. Ultimately, mesogenic and dye properties were examined by temperature-dependent emissive experiments in the solid state.     

Structural,thermal, dielectric and optical behaviour investigations of water-free ZnO/lyotropic liquid crystal nanocolloids

ABSTRACT

Zinc oxide (ZnO) nanoparticles of spherical symmetry (average size of ≈ 20 nm) have been synthesised via a non-aqueous lyotropic liquid crystalline (LLC) templating process. Lyotropic liquid crystalline nanocolloids are prepared via dispersing 0.05, 0.1 and 0.5 wt.% ZnO nanoparticles in non-aqueous lyotropic phase. No structural phase change has been seen with the doping of nanoparticles as stable lamellar phases are observed in all the cases. Stability of the lamellar structure and orientation of the ZnO nanoparticles in the liquid crystalline matrix may be attributed to the interfacial surface charge interactions. A significant increase and pronounced dispersion in dielectric permittivity of the ZnO/LLC nanocolloids could be the result of parallel coupling among guest/host, higher dipole- moment of the ZnO nanoparticles and Maxwell-Wagner polarisation. The variation of relaxation parameters has also been discussed and correlated with the dielectric and structural parameters. ZnO/lyotropic nanocolloids devices exhibit dc conductivity of the order of 10^?5S/m owing to the increase in the number of ions (of the order of 10¹⁹m^?3) in the doped systems. Nanocolloids exhibits, the refractive index of range 1.40 to 1.45 and the wide bandgap of the range 4.1–4.5 eV.     

Self‐assembled diols: synthesis,structure and dielectric studies

In single‐, double‐, and triple‐chain amphiphilic diols the CONH group was replaced by CON(CH₃) in order to reduce the number of proton donor groups available for intermolecular hydrogen bonding. The resulting three new liquid crystalline diols were studied by DSC, X‐ray and dielectric measurements, and show the mesophases SmA, Col_H2 or Cub_I2, depending on the number of decyloxy groups in the hydrophobic part of the molecule. The process of self‐assembly to different liquid crystalline phases is well seen in the dielectric spectrum and details of this process are discussed together with results from the X‐ray measurements. All the compounds show a high frequency dielectric absorption caused by the dynamics of the network of hydrogen bonds. An additional low frequency process related to the internal dynamics of the columns is seen only in the columnar phase.     

Characterization of crystalline and amorphous content in pharmaceutical solids by dielectric thermal analysis

Morphological and thermodynamic transitions in drugs as well as their amorphous and crystalline content in the solid state have been distinguished by thermal analytical techniques, which include dielectric analysis (DEA), differential scanning calorimetry (DSC), and macro-photomicrography. These techniques were used successfully to establish a structure versus property relationship with the United States Pharmacopeia standard set of active pharmaceutical ingredient (API) drugs. A distinguishing method is the DSC determination of the amorphous and crystalline content which is based on the fusion properties of the specific drug and its recrystallization. The DSC technique to determine the crystalline and amorphous content is based on a series of heat and cool cycles to evaluate the drugs ability to recrystallize. To enhance the amorphous portion, the API is heated above its melting temperature and cooled with liquid nitrogen to ?120 °C (153 K). Alternatively a sample is program heated and cooled by DSC at a rate of 10 °C min^?1. DEA measures the crystalline solid and amorphous liquid API electrical ionic conductivity. The DEA ionic conductivity is repeatable and differentiates the solid crystalline drug with a low conductivity level (10^?2 pS cm^?1) and a high conductivity level associated with the amorphous liquid (10⁶ pS cm^?1). The DSC sets the analytical transition temperature range from melting to recrystallization. However, analysis of the DEA ionic conductivity cycle establishes the quantitative amorphous and crystalline content in the solid state at frequencies of 0.10–1.00 Hz and to greater than 30 °C below the melting transition as the peak melting temperature. This describes the “activation energy method.” An Arrhenius plot, log ionic conductivity versus reciprocal temperature (K^?1), of the pre-melt DEA transition yields frequency dependent activation energy (E _a, J mol^?1) for the complex charging in the solid state. The amorphous content is inversely proportional to the E _a where the E _a for the crystalline form is higher and lower for the amorphous form with a standard deviation of ±2%. There was a good agreement between the DSC crystalline melting, recrystallization, and the solid state DEA conductivity method with relevant microscopic evaluation. An alternate technique to determine amorphous and crystalline content has been established for the drugs of interest based on an obvious amorphous and crystalline state identified by macro-photomicrography and compared to the conductivity variations. This second “empirical method” correlates well with the “activation energy” method.     

Self-assembly of a coil-rod-coil block molecule complexed with LiCF3SO3

Enantiotropic liquid crystalline phases were induced by the complexation of docosyl 4-(4-oxy-4'-biphenylcarbonyloxy)-4'-biphenyl carboxylate, containing poly(ethylene oxide) of twelve ethylene oxide subunits, with 0.25 mol of LiCF₃SO₃ per ethylene oxide unit. The existence of oblique columnar, rectangular columnar (Col_rec), and smectic A (SmA) mesophases of this complex was demonstrated by a combination of polarized optical microscopy and X-ray diffraction. This complex was observed to transform from the rectangular columnar phase to a lamellar phase on heating. The unusual phase transition behaviour in this complex is believed to originate from the mixing entropy of the added LiCF₃SO₃. Also, the Col_rec-SmA phase transition was accompanied by a change of the packing structure from bilayer to monolayer.     

Highly thermal conductive benzoxazine‐epoxy interpenetrating polymer networks containing liquid crystalline structures

A diamine‐based benzoxazine monomer (Bz) and a liquid crystalline epoxy monomer (LCE) are synthesized, respectively. Subsequently, a benzoxazine‐epoxy interpenetrating polymer network (PBEI) containing liquid crystalline structures is obtained by sequential curing of the LCE and the Bz in the presence of imidazole. The results show that the preferential curing of LCE plays a key role in the formation mechanism of liquid crystalline phase. Due to the introduction of liquid crystalline structures, the thermal conductivity of PBEI increases with increasing content of LCE. When the content of LCE is 80 wt %, the thermal conductivity reaches 0.32 W m^?1 K^?1. Additionally, the heat‐resistance of PBEI is superior to liquid crystalline epoxy resin. Among them, PBEI55 containing equal weight of Bz and LCE has better comprehensive performance. Its thermal conductivity, glass transition temperature, and the 5 % weight loss temperature are 0.28 W m^?1 K^?1, 160 °C, and 339 °C, respectively. By introducing boron nitride (BN) fillers into PBEI55, a composite of PBEI/BN with the highest thermal conductivity of 3.00 W m^?1 K^?1 is obtained. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1813–1821     

Liquid crystals in the series of 2,4,6-tristyryl-1,3,5-triazines

Alkaline condensation reactions of 2,4,6-trimethyl-1,3,5-triazine (1) and substituted benzaldehydes (2a-n) yield 2,4,6-tristyryl-1,3,5-triazines (3a-n). A sufficient number and length of the alkoxy chains at the benzene rings provide liquid crystalline phases Col_hd. A special structure was found for compound 3i with 9 hexyloxy chains; it exists in the solid state in a helical columnar arrangement, which is transformed by heating to a hexagonal columnar mesophase. Irradiation of the mesophases of 3i-3m leads to partial cyclodimerization reactions, which cause different textures and lower the clearing points. The border line between the irradiated and the unirradiated zones is preserved in the solid and the liquid crystalline temperature range but also over a surprisingly long period in the molten state. A detailed study of this imaging technique was performed for the LC phase of 3i.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz-13 MHz) has been used to analyse the frequency, temperature and bias-field dependences of the molecular dynamics of a very high-spontaneous-polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature-dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X-mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*-SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*-SmA phase transition was revealed.     

Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly(ether ether ketone) (PEEK) was studied over a broad frequency range from 10^?3 to 10⁶ Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses ε′′_KK has been determined from the real part ε′_T(ω) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the γ and the β modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main α relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the α_c relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Chasing Self-Assembly of Thioether-Substituted Flavylium Salts in Solution and Bulk State

Two series of flavylium triflates carrying alkoxy side chains in the A-ring (benzo unit of chromylium salt) and thioethers in the B ring (phenyl unit) ( O_n - Fla - S_m ) as well as thioethers at both A and B ring ( S_n - Fla - S_m ) were synthesized in order to understand the effect of thioether functionalization on their self-assembly and electronic properties. Concentration-dependent and diffusion ordered (DOSY) NMR experiments of O₁ - iV - Fla - S₃ indicate the formation of columnar H-aggregates in solution with antiparallel intracolumnar stacking of the AC unit (chromylium) of the flavylium triflate, in agreement with the solid state structure of O₁ - V - Fla - S₁ . Thioether substitution on the B ring changes the linear optical properties in solution, whereas it has no effect on the A ring. According to differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction bulk self-assembly of these ionic liquid crystals (ILCs) depends on the total number of side chains, yielding SmA and Lam_Col phases for ILCs with 2–3 chains and Col_ro, Col_h phases for ILCs with 3–6 chains. Thus, we demonstrated that thioethers are a useful design tool for ILCs with tailored properties.     

Alpha sub-phase in a new ferroelectric fluorinated compound

Dielectric and DSC methods were used to study a new fluorinated liquid crystalline compound exhibiting ferroelectric and paraelectric phases as well as an intermediate alpha sub-phase. Two dielectric relaxation processes were revealed in the SmC* phase: a typical Goldstone mode over the whole temperature range and a soft mode in the pre-transition region on both sides of the SmC*–SmA* transition. From the temperature dependencies of the dielectric increments and critical frequencies for the dielectric relaxation processes observed in all the liquid crystalline phases, as well as from texture observations, it was shown that there is a SmC*_α sub-phase between the ferroelectric SmC* and paraelectric SmA* phases.     

Phase transitions in chlorobenzene-cis-decalin mixtures

The dielectric constant ?' and loss tangent tan δ of chlorobenzene-cis-decalin mixtures have been measured in the temperature range 77 K to 330 K and frequency range 0.1 to 100 kHz. On cooling, ?' increases with decreasing temperature upto about 135 K, after which it drops rapidly with decreasing T followed by a slow decrease. This indicates that the liquid mixture goes to an amorphous phase which transforms to a glass phase of restricted dipole rotation below T_g; however, the peak in ?' is due to relaxation in the amorphous phase (α relaxation) and does not give an exact T_g. On heating, the behaviour of the cooling curve is retraced upto 160 K, after which ?' drops suddenly to a value lower than that at 77 K in the glass phase. This indicates the transition to a crystalline phase in which dipole rotational freedom is completely lost. The crystalline phase changes to a eutectic liquid phase of high ?' at a temperature (200 K) lower than the melting point of chlorobenzene and cis-decalin. Dielectric dispersion is observed only in the glass and amorphous phases. The dielectric relaxation time is independent of the concentration of chlorobenzene.     

Hindered reorientation of chiral molecules around their long axes in the S*C phase

Abstract

In this work, the high frequency dielectric relaxation process has been studied for the N*, S*_A and S*_C phases of the mixture ZLI 3654. To this end, time domain spectroscopy (TDS) was used to cover the frequency range from 10⁷ to 10¹⁰ Hz. The dielectric parameters obtained show that the high frequency spectrum is mostly influenced by the reorientation of molecules around their long axes. In addition, the latter process seems to be strongly hindered in the S*_C phase, because the necessary relaxation time is of the order of 1 ns, whereas for the N* and S*_A phases, the relaxation time is remarkably shorter and equal to 0.3 and 0.4 ns, respectively.     

Effect of silver nanoparticles on frequency and temperature-dependent electrical parameters of a discotic liquid crystalline material

The effect of silver nanoparticles (AgNPs) of diameters 6 and 100 nm on a discotic liquid crystalline material, namely 2,3,6,7,10,11-hexabutyloxytriphenylene (in short HAT4), has been observed in thermodynamic, electrical and optical texture studies. Silver nanoparticles (0.6 wt%) of diameter ~6 nm demonstrate a negligible (but ~100 nm shows appreciable) effect on the broad temperature range plastic columnar hexagonal (Col_hex) phase (~65.0°C) of pure HAT4. The dielectric studies have been carried out in the frequency range of 10 Hz–35 MHz under homeotropic anchoring conditions of the molecules. In the low frequency region of pure HAT4 and its AgNP composites, a relaxation mode has been observed. AgNPs of 6 nm elevate the value of dielectric permittivity of the plastic columnar hexagonal phase of pure HAT4. The dc conductivity of pure HAT4 and its AgNP composite (6 and 100 nm) material has been determined. The optical band gap for pure and AgNP composites of HAT4 has been determined by the ultraviolet-visible study. Due to insertion of AgNPs, the optical band gap of HAT4 has reduced.     

Phase behaviour of three homologues of the discotic hexa‐n‐alkoxyanthraquinones under pressure

The phase transition behaviour of three homologous discotic mesogens, the hexa‐n‐alkoxyanthraquinones HOAQ(n), n indicating the number of carbon atoms in the alkoxy group, was investigated under hydrostatic pressures up to 500?MPa using a high pressure differential thermal analyser. The T vs. P phase diagrams of HOAQ(6), HOAQ(8) and HOAQ(9) were constructed for solution‐ (Cr₀) and melt‐crystallized (Cr₁) samples of the compounds. HOAQ(6) shows the reversible Cr₀–rectangular columnar phase (Col_r)–hexagonal columnar phase (Col_h)–isotropic liquid (I) phase sequence at atmospheric pressure. The stable Col_r phase of HOAQ(6) has a decreased temperature range with increasing pressure and then the Col_r phase disappears under pressures above about 350?MPa; instead the Cr₀–Col_h–I phase sequence is exhibited. For HOAQ(8), the solution‐grown sample exhibits the stable Cr₀–Col_h–I phase sequence at atmospheric pressure. Applying pressure to the solution‐grown sample induces the formation of the stable Col_r phase in the pressure region between 10 and 350?MPa, leading to the Cr₀–Col_r–Col_h–I phase sequence. The pressure‐induced Col_r phase disappears under higher pressures. The melt‐cooled sample of HOAQ(8) shows the formation of the metastable crystal (Cr₁), unknown mesophase (X) and Col_r phases at lower temperatures under atmospheric pressure, and exhibits the reversible Cr₁–X–Col_r–Col_h–I phase sequence on subsequent thermal cycles. The metastable phase sequence was observed under pressures up to 100?MPa, but the phase transitions were too small to be detected under higher pressures. In HOAQ(9) the stable Cr₀–Col_h–I phase sequence is observed at all pressures, while the melt‐cooled sample shows the metastable Cr₁–Col_r–Col_h–I phase sequence under pressures up to 300?MPa. The metastable Col_r phase disappears under higher pressures.     

Non-linear dielectric spectra of ferroelectric liquid crystals

Abstract

The non-linear dielectric relaxation spectroscopy has been recently developed and applied to soft materials such as polymers. We have applied this new method to the S*_C phases of some ferroelectric liquid crystals. Under a weak AC electric field, the original and third order harmonic frequency components of electric displacement are proportional to the first and third powers of the applied electric field, respectively. The linear spectrum obtained from the original frequency component shows the relaxation of Debye type and the third order non-linear spectrum shows the relaxation with an extended form of Debye type to the non-linear case. The third order non-linear dielectric increment is found to be negative, which implies that the dielectric non-linearity of the liquid crystal in the S*_C phase is due to the saturation of molecular dipole moments induced by the applied electric field. The temperature dependence of the linear and third order non-linear spectra in the S*_C phase are also studied. Both spectra do not change their forms much through the whole temperature range of the S*_C phase. In the vicinity of the S_A–S*_C transition temperature, the critical behaviour is more remarkable in the third order spectrum than in the linear one.