Yoji Maeda

Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) under hydrostatic pressure

The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 500 MPa using high pressure optical and DTA measurements. The known enantiotropic phase transitions of HHTT, i.e. crystal (Cr)-helical phase (H), H-hexagonal columnar phase (Colh) and Colh-isotropic liquid (I) were observed up to 32 MPa. Application of hydrostatic pressures above 32 MPa results in the H and Colh phases becoming monotropic, depending upon the applied pressure. The H phase was observed as a monotropic phase in the pressure region between 32 and about 180 MPa. Thus, the I →Colh →H →Cr transition sequence appeared only on cooling under these pressures, while the Cr →Colh →I transition occurred on heating. Further increases in pressure above a second limiting value leads to the Colh phase becoming monotropic. Thus the I →Colh →Cr transition sequence appeared on cooling, while the Cr →I transition was observed on heating. The T vs. P phase diagram based on the data obtained in the heating mode contains two triple points; one is estimated as 40 MPa, 77.2°C for the Cr-H-Colh triple point and the other is extrapolated as 285 MPa, 118.3°C for the Cr-Colh-I triple point. These triple points define the upper limits for the appearance of the stable H and Colh phases, respectively.

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.

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.

Thermal behaviour under pressure of the thermotropic cubic mesogen 4′- n -alkoxy-3′-nitrobiphenyl-4-carboxylic acids

The thermal behaviour of members of a homologous series which exhibits the optically isotropic cubic phase, the 4′-n-alkoxy-3′-nitrobiphenyl-4-carboxylic acids having alkoxy chains containing 16, 20 and 22 carbon atoms (referred to as ANBC-16, -20 and -22, respectively) was investigated under pressures up to 200–400 MPa by high pressure differential thermal analysis. In the phase diagram of ANBC-16 obtained on heating, a triple point was estimated at 54 ± 1 MPa and 205 ± 1°C for the SmC, Cub and SmA phases. It was found that the X phase is formed on cooling under all pressures, while appearing on heating at high pressures above about 54 MPa. Thus the X phase appears monotropically between the SmA and Cub phases in the low pressure region and enantiotropically between the SmA and SmC phases under higher pressures. It is strongly suggested that the X phase is a columnar mesophase. For ANBC-20 and -22, the cubic phase tends to be destabilized with increasing pressure. The temperature region of the cubic phase of ANBC-20 becomes narrower with increasing pressure and a triple point for the SmC, Cub and I phases is estimated to be at about 309 MPa. On the other hand, the cubic phase of ANBC-22 is still observed at the highest pressure examined.

In situ observation of the pressure-induced mesophase for 4′-n-hexadecyloxy-3′-nitrobiphenyl-4-carboxylic acid

In situ observation of the optical texture, and X-ray patterns of the pressure-induced mesophase seen for 4′-n-hexadecyloxy-3′-nitrobiphenyl-4-carboxylic acid (ANBC-16) was performed under hydrostatic pressures up to 100MPa using a polarizing optical microscope equipped with a high pressure hot stage and a wide angle X-ray diffractometer equipped with a high pressure vessel respectively. It was found that the pressure-induced mesophase (hereafter refered to as ‘X’) appeared at pressures above 60 MPa, and exhibits a birefringent broken-fan or a sand-like texture that remain unaltered in the SmC phase. The POM-transmitted light intensity curve measured on heating clearly showed the Cr4 → Cr1 → SmC → ‘X’ → SmA → I transition sequence at 80 MPa. The optical texture and the POM-transmitted light intensity measured during a pressure cycle at 185°C showed a reversible change between the cubic and ‘X’ phases. The WAXD pattern of the ‘X’ phase showed a spot-like pattern, suggesting no layered structure for this phase, and also revealed a substantial decrease in the d-spacing of the low angle reflection at 80 and 100 MPa, compared with the d-spacings of the (0 0 1) reflection of the SmC phase and also the (2 1 1) reflection of the cubic phase. It is concluded from these data that the ‘X’ phase is a birefringent hexagonal columnar phase.

High pressure differential thermal analysis of dimer liquid crystals: α, ω-Bis [(4,4′-cyanobiphenylyl) oxy] alkanes

Abstract The phase behaviour of dimer liquid crystals (DLC), α, ω-bis[(4,4′-cyanobiphenylyl) oxy]alkanes (CBA-n with n = 9,10) has been studied by differential thermal analysis (DTA) over a pressure range from 0.1 to 150 MPa. Both samples exhibit crystal (Cr)↔nematic (N)↔isotropic (I) transitions under all experimental conditions. The slopes of the phase boundary curve (dp/dt)tr were determined from the P tr vs. T tr phase diagram, where the subscript tr designates CrN or NI. Both transition temperatures T CrN and T NI were found to increase almost linearly as a function of pressure; CBA-9: (dp/dt)CrN = 3.92, (dp/dt)NI = 2.03; CBA-10: (dp/dt)CrN = 3.66, (dp/dt)NI = 2.17, the units being MPaK−1. As a consequence, the nematic region defined by the interval between the CrN and NI transitions becomes broader as the applied pressure increases. While the transition enthalpies δS CrN and the associated entropies δS CrN at the CrN transition decrease substantially with increasing pressure, the corresponding quant...

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 (Colr)–hexagonal columnar phase (Colh)–isotropic liquid (I) phase sequence at atmospheric pressure. The stable Colr phase of HOAQ(6) has a decreased temperature range with increasing pressure and then the Colr phase disappears under pressures above about 350 MPa; instead the Cr0–Colh–I phase sequence is exhibited. For HOAQ(8), the solution‐grown sample exhibits the stable Cr0–Colh–I phase sequence at atmospheric pressure. Applying pressure to the solution‐grown sample induces the formation of the stable Colr phase in the pressure region between 10 and 350 MPa, leading to the Cr0–Colr–Colh–I phase sequence. The pressure‐induced Colr phase disappears under higher pressures. The melt‐cooled sample of HOAQ(8) shows the formation of the metastable crystal (Cr1), unknown mesophase (X) and Colr phases at lower temperatures under atmospheric pressure, and exhibits the reversible Cr1–X–Colr–Colh–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–Colh–I phase sequence is observed at all pressures, while the melt‐cooled sample shows the metastable Cr1–Colr–Colh–I phase sequence under pressures up to 300 MPa. The metastable Colr phase disappears under higher pressures.

Phase behaviour of thermotropic banana-shaped compounds under pressure

The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[N-(2-hydroxy-4-n-alkoxybenzylidene)-4′-aminobenzoate] series was investigated under hydrostatic pressures up to 300 MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr1)–B1 phase–isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70 MPa. At higher pressures a pressure-induced crystalline phase (Cri) appears between the Cr1 and B1 phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9 MPa and 160.3°C for the Cr1, Cri and B1 phases, indicating the lower limit of pressure for the Cri phase. In compound II the reversible transition sequence crystal (Cr1)–B2 phase–I is seen over the whole pressure region, and the temperature range of the B2 phase remains unaltered. It is concluded that both the B1 and B2 banana phases are stable over the whole pressure region studied.

High-pressure DTA study on liquid crystalline polyesters with biphenyl as mesogen

Abstract The phase transition of a homologous series of liquid crystalline polyesters was studied under hydrostatic pressures up to 300 MPa by using a high-pressure DTA apparatus. The thermotropic polyesters, abbreviated as PB- n (where n is an even number of methylene units), were prepared from 4,4′-dihydroxybiphenyl and aliphatic dibasic acids containing an even number of methylene groups from 8 to 22 units. The T vs. P phase diagrams of the PB-10, PB-12, PB-14 and PB-18 polyesters were constructed. PB-10 shows a simple phase transition of melt (I) over the whole pressure region. PB-12 shows a similar region. PB-12 shows a similar KS H -I KS H I transition in the low-pressure region below about 100 MPa. At high pressures above 100–120 MPa, however, PB-12 exhibits a T vs. P relation that is different from that in the low-pressure region. The phase diagram supports the existence of the pressure-induced smectic phase which was assigned to be the smectic-B (S B ) phase. The PB-14, PB-16, and PB-18 samples show an additional first-order transition just below the KS H transition at atmospheric pressure. The new transition peak increases with increasing space n in the PB- n samples, in contrast to the decrease in the KS H transition peak. The transition is a kind of crystal transition between the low- and high-temperature phases, denoted here as K 1 and K 2 . The thermal behavior of these samples can be understood as enantiotropic transitions through the K 1 K 2 S H I route. They show the same type of T vs. P phase diagram, indicating the K 1 K 2 , K 2 S H and S H I transition lines. The phase diagrams indicate that the stable region of the S H phase broadens with increasing pressure. However, the K 2 S H transition peak becomes too small to be detected under pressures of 300–400 MPa.

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.