Possibility of isostructural condensed states of matter in the D phase of ANBC and the cubic mesophase of BABH: heat capacity of 4'-n-octadecyloxy-3'-nitrobiphenyl-4-carboxlic acid, ANBC(18)

Heat capacity measurements have been made on ANBC(18) at temperatures from 8 to 490 K by adiabatic calorimetry. All known phases were detected. The temperatures, enthalpies and entropies of transition were determined for the phase transitions observed. On the basis of the entropy of transition to the SmC phase from the D or cubic phases, it is pointed out that the D phase of ANBC and the cubic phase of BABH might be identical in nature. It is shown that the arrangement of 'molecular' cores has a higher degree of order in the isotropic (D and cubic) phases than in the SmC phase, whereas the terminal alkoxy chains are more disordered in the isotropic phases than in the SmC phase. The degrees of disorder in the D and cubic phases relative to the SmC phase are very similar in terms of the entropy of transition per methylene group. The inverted phase sequence in ANBC (SmC D on heating) and BABH (cubic SmC) can be accounted for in terms of the competing roles in the entropy between the molecular core and the chains.     

Electro-optic response of cubic liquid crystal compounds in Kerr cell geometry

We present the results of our investigations on the electro-optic response of the cubic phase liquid crystal compounds 1,2-bis-[4-n-octyloxy-benzoyl]-hydrazine (BABH8) and 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC16) in Kerr cell geometry. The AC electric field response in the BABH8 cubic phase was found to be as small as that of the isotropic phase, even though there was a response in the adjacent smectic C (SmC) phase. The response in the SmC phase means that the BABH8 molecule itself has an electric field coupling ability, but this ability is strongly inactivated in the cubic phase. This inactivity to the AC fields was also found in the cubic phase of ANBC16. This behaviour could be explained by the small structural unit size of the cubic phase.     

Pressure-scanning DTA measurements of the cubic-smectic C phase transition of 1,2-bis-(4-n-octyloxy)- and 1,2-bis-(4-n-dodecyloxy benzoyl)hydrazines

The pressure-scanning differential thermal analyzer (DTA) measurements of the cubic (Cub)-smectic C (SmC) transition of thermotropic cubic mesogens of 1,2-bis-(4-n-octyloxybenzoyl)- and 1,2-bis-(4-n-dodecyloxybenzoyl)hydrazine, BABH(8) and BABH(12), were performed at isothermal condition using a high-pressure differential thermal analyzer. BABH(8) showed the same endothermic peak of the Cub-SmC transition in the pressurizing process as on heating at isobaric condition. On the other hand, BABH(12) showed only the cubic phase between the crystal and the isotropic liquid under pressures up to 16-17 MPa, but a high-pressure smectic C (SmC(hp)) phase was induced instead of the cubic phase under higher pressure. The Cub-SmC(hp) phase transition with a small exothermic peak occurred in the pressurizing process and the transition was observed reversibly. The Cub-SmC(hp) phase transition was in accordance with the morphological and structural observations mentioned before. The strange phenomenon of the inversion of sign of the Cub-SmC transition heat of BABH(n) homologues can be explained by the “Alkyl-chains as entropy reservoir” mechanism proposed by Saito et al.     

Phase behaviour of the thermotropic cubic mesogens 1,2-bis-(4-n-undecyl- and 4-n-dodecyl-oxybenzoyl)hydrazine under pressure

The phase transition behaviour of two optically isotropic, thermotropic cubic mesogens 1,2-bis-(4-n-undecyloxy- and 4-n-dodecyloxy-benzoyl)hydrazine, BABH(11) and BABH(12), was investigated under hydrostatic pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope equipped with a high pressure optical cell. It is found that for BABH(11) and BABH(12), a smectic C (SmC) phase is induced between the isotropic liquid (I) and the cubic (Cub) phases by applying pressures above 10-12 and 16-17 MPa, respectively. A sea-island texture consisting of bright sand-like sea regions (SmC phase) and areas of dark islands (Cub phase) appears in the mesophase under pressures up to 140 MPa, while the sand-like texture of the SmC phase is formed predominantly on cooling under pressure. These observations indicate the destabilization of the cubic phase with increasing pressure. The phase transition sequence of BABH(11) and BABH(12), Cr-Cub-I at atmospheric pressure, changes to Cr-Cub-SmC-I under intermediate pressures and would change to Cr-SmC-I under elevated pressure.     

Calorimetric study of the cubic mesogen,ACBC(16)

The heat capacity of the cubic mesogen ACBC(16) was measured between 16 and 500?K by adiabatic calorimetry. As well as the known condensed phases, a new crystalline phase was found to undergo a glass transition at around 165?K. Phase transitions between crystal, SmC, cubic, and isotropic liquid phases took place at 399.16, 431.15, and 474.30?K, respectively. As in the case of ANBC, a broad hump was observed in the heat capacity of the isotropic liquid phase. The first order nature of the SmC–cubic phase transition was confirmed for the first time by the observation of supercooling of the cubic phase. The broad hump in the isotropic liquid phase was shown to extend to a low temperature side if the isotropic liquid was supercooled, suggesting that the event occurring at the hump is not directly related to the cubic–isotropic liquid phase transition.     

Thermodynamic investigation of the thermotropic cubic mesogen, 1,2-bis(4- n -octyloxybenzoyl)hydrazine

Themolar heat capacity of the thermotropic cubic mesogen 1,2-bis(4- n -alkoxybenzoyl)hydrazine, BABH(8) for short, with a purity of 99.43 mol% has been precisely measured with an adiabatic calorimeter at temperatures between 14 and 480 K. The enthalpy and entropy gained at each phase transition across the phase sequence of \[crystal(2) crystal(1) cubic mesophase SmC isotropic liquid] have been determined. The existence of a solid-to-solid phase transition with a fairly large entropy change seems to be necessary for the alkyl moieties attached to both sides of the molecule to play the role of 'solvent' in the cubic mesophase. On the basis of curvature elasticity considerations, the small energy difference between the cubic and SmC phases is favourably accounted for in terms of the jointed-rod micelles model. The reason for the immiscibility of BABH(8) with the cubic D mesogen, 4- n -hexadecyloxy-3- nitrobiphenyl-4-carboxylic acid is discussed in terms of the large difference in their molecular size and of 'structure breaking' arising from the admixture of heterogeneously hydrogen-bonded materials.     

Calorimetric study of the cubic mesogen, ACBC(6)

The heat capacity of the cubic mesogen ACBC(16) was measured between 16 and 500 K by adiabatic calorimetry. As well as the known condensed phases, a new crystalline phase was found to undergo a glass transition at around 165 K. Phase transitions between crystal, SmC, cubic, and isotropic liquid phases took place at 399.16, 431.15, and 474.30 K, respectively. As in the case of ANBC, a broad hump was observed in the heat capacity of the isotropic liquid phase. The first order nature of the SmC-cubic phase transition was confirmed for the first time by the observation of supercooling of the cubic phase. The broad hump in the isotropic liquid phase was shown to extend to a low temperature side if the isotropic liquid was supercooled, suggesting that the event occurring at the hump is not directly related to the cubic-isotropic liquid phase transition.     

Do alkoxy chains behave like a solvent in the D phase? DSC study of binary systems, ANBC (nC) (nC = 8, 16 and 18)-n-tetradecane

The phase behaviour of the binary systems ANBC(nC)-n-tetradecane, for nC = 8, 16 and 18, was examined using DSC, paying special attention to the role of the alkoxy chain of the ANBC molecule in the D phase . The dependence of the SmC-D and D-isotropic liquid transition temperatures upon the apparent average number of paraffinic carbon atoms closely resembles the nC dependence in the series of neat ANBCs, demonstrating that the alkoxy chain behaves, at least in part, like the solvent in lyotropic liquid crystals. The D phase was not detected in ANBC(8)-n-tetradecane.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr 2 )-high temperature crystal (Cr 1 ) - cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30-40 MPa, in conjunction with the disappearance of the Cr 1 phase. The transition sequence changes to Cr 2 -SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (d T /d P ) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0 ±1.0°C for the SmC, Cub and Cr 1 phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

Phase behaviour of a thermotropic cubic mesogen of 1,2-bis(4′-n-hexyloxybenzoyl)hydrazine under pressure

The phase behaviour of the thermotropic cubic mesogen 1,2-bis(4′-n-hexyloxybenzoyl)hydrazine [BABH(6)] was investigated under pressure up to about 55 MPa using a polarising optical microscope equipped with a high-pressure optical cell. BABH(6) shows the crystal (Cr)–cubic (Cub)–isotropic liquid (I) phase transition at ambient pressure on heating. The smectic C (SmC) phase was induced above 32 MPa, showing the unusual phase sequence of Cr–Cub–SmC–I, similar to those in BABH(n) (n = 8–10). The boundary between the Cub and SmC phases exhibited a negative slope dT/dP of about –1.0 ºC MPa^?1.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4-n-octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr₂)-high temperature crystal (Cr ₁)- cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30–40 MPa, in conjunction with the disappearance of the Cr₁ phase. The transition sequence changes to Cr₂-SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (dT/dP) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0±1.0°C for the SmC, Cub and Cr₁ phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

Mesomorphic phase transitions of a series of D-phase compounds

Mesomorphic phase transitions of 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids (ANBC) with numbers of carbons (n) in the alkoxy group ranging from 11 to 22 have been studied by differential scanning calorimetry (DSC) and polarizing optical microscopy. The D phase, a mesophase of particular interest through its being optically isotropic, was observed for the n = 17, 19, 20, 21, and 22 members of the ANBCs, as well as for the n = 16 and 18 members, as reported previously. The S_c-D phase transition temperature decreased with increasing n, so that the temperature range of the D phase extended over 64° at n = 22. In the n = 15 member, the D phase was certainly observed on first heating, but was not seen on subsequent cooling and second heating processes.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis(4-n-decyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis(4-n-decyloxybenzoyl)hydrazine, BABH(10), was investigated under pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope (POM) equipped with a high pressure optical cell. The reversible change in structure and optical texture between the cubic (Cub) and smectic C (SmC) phases was associated with a change from a spot-like X-ray pattern and dark field for the Cub phase to the Debye-Sherrer ring pattern and sand-like texture for the SmC phase under both isobaric and isothermal conditions. The Cub phase was found to disappear at pressures above about 11 MPa. The phase transition sequence, low temperature crystal (Cr₃)-intermediate temperature crystal (Cr₂)-high temperature crystal (Cr₁)-Cub-SmC-isotropic liquid (I) observed at atmospheric pressure, is maintained in the low pressure region below 10 MPa. The transition sequence changes to Cr₃-Cr₂-(Cr₁)-SmC-I in the high pressure region. Since the Cub-SmC transition line determined by POM has a negative slope (dT/dP) in the T-P phase diagram, a triple point is estimated approximately at 10-11 MPa, and 143-145°C for the SmC, Cub and Cr₁ phases, giving the upper limit of pressure for the observation of the cubic phase.     

14N NMR studies on an optically isotropic liquid crystalline D phase of 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids (ANBC)

14N nuclear magnetic resonance (NMR) measurements have been carried out for three members of 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids (ANBC-n, where the number of carbon atoms in the alkoxy group, n, is 14, 16, and 22) in the temperature range 400-500 K. ANBC-16 and-22 show an optically isotropic D phase. The 14N NMR spectrum of the D phase showed a single peak, which may result from isotropically averaged quadrupole interactions around the 14N nucleus. Relaxation time measurements indicate the existence of two relaxational processes, faster anisotropic and slower isotropic motions, and suggest that in both cases ANBC molecules act as a dimer. The present 14N NMR results may be interpreted in the framework of the IPJR model, indicating that the structure of the D phase is a three-dimensional network continuous over the unit lattice.     

Mesomorphic phase transitions of a series of D-phase compounds

Abstract

Mesomorphic phase transitions of 4′-n-alkoxy-3′-nitrobiphenyl-4-carboxylic acids (ANBC) with numbers of carbons (n) in the alkoxy group ranging from 11 to 22 have been studied by differential scanning calorimetry (DSC) and polarizing optical microscopy. The D phase, a mesophase of particular interest through its being optically isotropic, was observed for the n = 17, 19, 20, 21, and 22 members of the ANBCs, as well as for the n = 16 and 18 members, as reported previously. The S_c-D phase transition temperature decreased with increasing n, so that the temperature range of the D phase extended over 64° at n = 22. In the n = 15 member, the D phase was certainly observed on first heating, but was not seen on subsequent cooling and second heating processes.     

Isotropic cubic phase of 4-n-pentadecyloxy-3-nitrobiphenyl4-carboxylic acid (ANBC-15): DSC, microscopic and dynamic viscoelastic studies

The phase behaviour of 4-n-pentadecyloxy-3-nitrobiphenyl-4-carboxylic acid (ANBC-15) was investigated by differential scanning calorimetry, polarizing optical microscopy, and dynamic viscoelastic measurements. The phase sequence for the virgin sample of ANBC-15 is crystalsmectic C (SmC)-cubic (D)-smectic A (SmA)-'structured liquid' (I1)-isotropic liquid (I2) on first heating. It was found that the appearance of the D phase on the second heating depends on the top temperature of the first heating. It was also shown that the D phase is formed on heating when the preceding SmC phase is 'solid-like' from the viscoelastic point of view, i.e. storage modulus (G) loss modulus (G) at 62.8 rad sec -1, while the 'liquid-like' SmC phase is transformed directly into the SmA phase without showing the D phase on heating. The isothermal frequency scans at a temperature in the D phase showed the existence of a cross-over point, G G , with G G in the lower frequency side, suggesting a structural fluctuation of the D phase. These results are ascribed to the molecular structure of ANBC-15, which has a critical alkoxy chain length in ANBC series.     

Synthesis, characterization and phase transitions of the inorganic-organic layered perovskite-type hybrids [(C(n)H(2n+1)NH3)2PbI4], n = 7, 8, 9 and 10

Four inorganic-organic hybrid materials that consist of 2-D layers of corner-sharing lead(II) iodide octahedra separated by alkylammonium chains have been crystallized and characterized via single-crystal XRD (SCXRD). The four hybrids, represented by the general formula [(C(n)H(2n+1)NH(3))(2)PbI(4)] and abbreviated C(n)PbI, exhibit multiple reversible phase transitions for a narrow temperature range. The transition temperatures were determined with differential scanning calorimetry experiments. The number of transitions and the transition temperatures are dependant on the chain length; for n = 7 and 10, there are three transitions, and for n = 8 and 9, there are two transitions. Regardless of the number of transitions, all four compounds have identical lowest temperature phases, which have inorganic layers that are eclipsed, non-planar conformations of the alkyl ammonium chains and yellow-coloured crystals. The next highest temperature phase for three of the compounds (C(10)PbI goes through an intermediate phase first), has staggered inorganic layers, all-trans planar conformations of the chains and orange coloured crystals. The highest temperature phase for n = 8 and 10 has red-coloured crystals and shows a disordering of the alkylammonium chains over two positions and staggered inorganic layers. The high temperature phase of C(7)PbI retains its orange colour and has only increased thermal motion of its alkylammonium chain. The structure of the high temperature phase of C(9)PbI was not determined. The SCXRD structures of the various phases give clues to the structural changes that the compounds undergo at the phase transitions, which will now enable future studies of their optical and electronic properties to be better understood.     

Raman frequency shifts of an internal mode near the tricritical and second order phase transitions in NH4Cl

This study gives our analysis for the frequency shifts of the v2 (1708 cm-1) Raman mode in NH4Cl close to its tricritical (P=1.6 kbar) and second order (P=2.8 kbar) phase transitions. From our analysis, we extract the values of the critical exponent which describes the critical behavior of the Raman frequency shifts for this internal mode for the pressure conditions studied in NH4Cl. Our exponent value of alpha approximately 0.2 for the tricritical phase transition is close to the values of 1/16 (TTc) for the specific heat, predicted from a 3D Ising model. Our exponent values for the second order phase transition (P=2.8 kbar) for TTc are comparable with those reported in earlier studies.     

Self-assembly of ternary cubic, hexagonal, and lamellar mesophases using the lattice-Boltzmann kinetic method

We use a kinetic lattice-Boltzmann method to simulate the self-assembly of the cubic primitive (P), diamond (D), and gyroid (G) mesophases from an initial quench composed of oil, water, and amphiphilic particles. Here, we also report the self-assembly of the noncubic hexagonal phase and two lamellar phases, one with periodic convolutions. The periodic mesophase structures are emergent from the underlying conservation laws and quasi-molecular interactions of the lattice-Boltzmann model. We locate regions of the model's parameter space where the sequence of appearance of mesophases lamellar --> primitive --> hexagonal is in agreement with pressure jump experiments and the sequence cubic --> lamellar is in agreement with compositional variations reported in the literature. The ability of our lattice-Boltzmann model to simulate self-assembly of cubic and noncubic phases in a unified and consistent manner opens the way for further investigations into the transition pathways and kinetics of the phase transitions between these states as well as of the rheology of these phases.     

Toward rational design of complex nanostructured liquid crystals

An analogy of block copolymer micro‐segregation as a low‐molecular weight nanostructured liquid crystal (LC) was tested with recently found columnar and cubic phase‐forming LC molecules, to clarify the broader applicability of the analogy as a molecular design principle. We found that the copolymer analogy principle also works well for new micellar cubic phase‐forming molecules. For bicontinuous cubic phase‐forming 1,2‐bis(4′‐n‐alkoxybenzoyl)hydrazines (BABH‐n) compounds that cover a much broader core fraction range than that predicted by the copolymer analogy, we propose hierarchical preferential orientation as an additional mechanism for their cubic range broadening. For azo‐dichiral molecules that also do not fit with the above principle, we propose chiral segregation as an alternative origin for their cubic phase formation. DOI 10.1002/tcr.201000025