Molecular engineering of liquid crystal polymers by living polymerization. II. Living cationic polymerization of 11-[(4-cyano-4′-biphenyl) oxy] undecanyl vinyl ether and the mesomorphic behavior of the resulting polymers

The synthesis and living cationic polymerization of 11-[(4-cyano-4′-biphenyl)oxy]-undecanyl vinyl ether ( 6 – 11 ) are described. The mesomorphic phase behavior of poly( 6 – 11 ) with different degrees of polymerization was compared to that of 6 – 11 and of 11-[(4-cyano-4′-biphenyl) oxy] undecanyl ethyl ether ( 8 – 11 ) which is the model compound of the monomeric structural unit of poly( 6 – 11 ). 6 – 11 displays a monotropic S_A and a monotropic nematic mesophase while 8 – 11 an enantiotropic S_A mesophase. Poly( 6 – 11 ) with low degrees of polymerization exhibits an enantiotropic S_A mesophase. Poly( 6 – 8 ) with high degrees of polymerization displays an enantiotropic S_X (i. e., an unidentified smectic phase) and an enantiotropic S_C mesophase. These results demonstrate that the transformation of the nematic mesophase of the monomer into a smectic mesophase after polymerization, occurs at the level of monomeric structural unit.     

Transitions and morphology of a thermotropic liquid-crystalline polyester with a flexible spacer

The mesomorphic transitions, crystallization from the mesophase, and the influence of the specimen preparation method on the solid-state structure of an aromatic polyester containing a triad aromatic ester mesogenic group and a decamethylene flexible spacer in the main chain were studied by DSC, SALS, WAXS, polarizing microscopy, torsional braid analysis, and depolarizing transmittance techniques. The specimens obtained from solution were semicrystalline and exhibited nematic mesophase formation above the melting point T_m, whereas the melt-cast specimens were mesomorphic as cast. A transition from the nematic phase to another mesophase, designated M_x, is proposed to occur below T_m, so this transition is monotropic. It appears that the transition to the M_x mesophase occurs before, and may even be a prerequisite for, crystallization of the melt-cast specimens. The thermal expansion coefficient of the anisotropic melt is close to that of the isotropic melt, and the T_g of the supercooled solid mesophase is close to that of the amorphous phase.     

4′‐[2‐(2‐Ethoxyethoxy)ethoxy]biphenyl‐4‐carboxylic acid: correlation between its crystalline and smectic phases

The crystal structure of the dimeric title compound, C₁₉H₂₂O₅, is dominated by a head‐to‐head hydrogen‐bonding interaction between centrosymmetrically related carboxyl groups in each monomer. The result is a dimeric axis of unusual length (ca 34 Å), but still shorter than what could be expected for a fully extended chain, owing to two turning points in the oligoethoxy ends. This allows for an explanation of the structure of the smectic mesophase exhibited by this compound and at the same time fully validates former geometric estimations based on PM3 calculations.     

New mesomorphic organocyclosiloxanes I. Thermal behaviour and mesophase structure of organocyclotetrasiloxanes

cis-Cyclotetrasiloxanes of the formula cis-[PhSi(O)(OSiMe₂ R)]₄ with R = Me, CH₂Cl, CH CH₂ and cis-[ClC₆H₄Si(O)(OSiMe₃)]₄ were synthesized and investigated in terms of their thermotropic phase transitions. Two ordered phases were observed for the cis-cyclotetrasiloxanes, one at lower temperature exhibiting the properties of a crystal and one at higher temperature exhibiting the properties of a plastically crystalline (3D) mesophase. A detailed examination of the mesophase behaviour and mesophase structure of octaphenylcyclotetrasiloxane was also carried out. It was shown that the thermal properties and structural characteristics of the mesophase are influenced by the structural characteristics of the substituent attached at the silicon atom in the tetracyclosiloxane. The new mesomorphic cis-cyclotetrasiloxanes are by far the largest molecules reported to date as forming plastic crystals, and the temperature region of the mesophase is much broader than in other plastic crystals. All five cyclotetrasiloxanes studied were found to be isomorphous in the 3D-mesophase and the low temperature forms of the two cis-cyclotetrasiloxanes: PhSi(O)(OSiMe₂ R)₄ (R = Me, CH CH₂) were also isomorphous.     

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Abstract

Proton spin-lattice relaxation studies were carried out in the S_A and S*_C phases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques. T ₁ dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 10² to 3 × 10⁸ Hz. In both mesophases the T ₁ data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D ₁ was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S_A and the S*_C phases.     

Lateral protrusion and mesophase behaviour in pure and mixed states of model compounds of the type 4-(4′-substituted phenylazo)-2-(or 3-)methyl phenyl-4'-alkoxy benzoates

Two groups of the title compounds were prepared and investigated for their mesophase formation and stability. Each group constitutes five homologous series that differ from each other by the polar substituent X (CH₃O, CH₃, H, Cl, and NO₂). Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 8, 10, 12, 14 and 16. The difference between the two groups of compounds lies in the orientation protrusion of the lateral methyl group attached to the central benzene ring. In the first group (Group I) the methyl substituent, introduced into the o-position with respect to the ester group, makes an angle of 60° with the long axis of the molecule. In the other series of compounds (Group II), the orientation angle is 120° as it is introduced into the position-3. All possible binary phase diagrams could be constructed in which the two components are corresponding positional isomers from either group. The study aimed to investigate the effect of inclusion of the lateral methyl group, as well as its spatial orientation, on the mesomorphic properties of the produced derivatives in their pure and mixed states. The compounds prepared in both groups were characterised for their mesophase behaviour by differential scanning calorimetry and polarised light microscopy. The nematic phase is the mesophase observed in most of the compounds prepared and their binary mixtures.     

Synthesis and characterization of novel thermotropic liquid crystalline copoly(ester imide)s

Novel liquid crystalline copoly(ester imide)s were synthesized via polyesterification of triethyleneglycol bis(4-carboxyphenyl) ether ( 1e ), diacetoxybiphenyl, and diacids with imide moieties. The effects of composition on the changes of T_g, T_m, and T_i were examined by global TSC and DSC. Thermal gravimetric analyses (TGA) found that 4a–d and 5a–g possess higher thermal stability. Strong stir opalescence phenomenon and observations from polarized optical microscopy identified that 2b–e and 3a–d possess the typical schlieren texture of an enantiotropic nematic mesophase. The birefrigent melts of 4a–d and 5a–g, however, displayed particular liquid crystalline behavior. Copolymers with higher aromatic imide ring content ( 4a–d, 5a–g ) form a layered structure and an enantiotropic smectic mesophase in the melting state. The melt viscosity of the semetic mesophase was higher than the nematic mesophase which was observed by capillary rheometer. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1791–1803, 1998     

Effect of lateral bromo substituent on the phase behavior of four-ring azo/ester/azo liquid crystalline materials

ABSTRACT

In order to study the influence of lateral Br substitution on mesophase behaviour, five homologous series of 4-substituted phenylazo phenyl 4?-(3?-bromo-4?-alkoxyphenylazo) benzoates (In_a–e) have been synthesised. Within each homologous series, the alkoxy group varies from 6 to 16 carbons, while other terminal group substituents, X, are CH₃O, CH₃, H, Br and NO₂ groups; the mesophase behaviour of these series is compared with previously prepared laterally neat analogues, 4-substituted phenylazo phenyl 4?-(4?-alkoxyphenylazo) benzoates (IIn_a–e) and laterally methyl analogues, 4-substituted phenylazo phenyl 4?-(3?-methyl-4?-alkoxyphenylazo) benzoates (IIIn_a–e). Similar to lateral methyl analogues, the present series, lateral Br substitution showed that, independent of the polarity of the substituent X or the alkoxy-chain length, the nematic phase is predominant with relatively high stability and broad temperature ranges. The mesophase stability varies between 204.0°C and 335.0°C for the nematic phase and 169.6°C and 281.0°C for the SmA phase. Their total mesophase temperature ranges vary between 87.2°C and 201.4°C. All compounds were found to be thermally stable within the mesophase temperature range, except the lower homologue of the nitro and Br substituted derivatives. The obtained results are discussed in terms of molecular polarisability.     

Synthesis of unsymmetric triphenylene discotic liquid crystals with a semi-fluorinated chain and the influence of fluorophobic effect on mesogenic behavior

The fluorophobic effect of the semi-fluorinated chain of discotic molecules efficiently improves the stability of columnar mesophase.Low symmetric discogens exhibit decreased melting points and wide mesophase ranges.A series of triphenylene derivatives 1,C18H6(OCnH2n+1)4(OMe)(O2CC2H4C6F13),and their hydrocarbon analogues 2,C18H6(OCnH2n+1)4(OMe)(O2CC8H17),n = 4―8,were synthesized for the investigation of the influence of semi-fluorinated peripheral chain and molecular symmetry on mesomorphism.The structures ...     

Thermotropic copolyamides from triethyleneglycol bis(4-carboxyphenyl)ether,aromatic diamines,and p-aminobenzoic acid modified by the monomer sequences in the copolyamides

Copolycondensations of triethyleneglycol bis(4-carboxyphenyl)ether (PEG₃), p-amino-benzoic acid (PABA), and p-phenylenediamine (PPD) were studied in order to examine the effect of monomer sequences in the copolyamides upon the transition temperatures of the resulting copolymers. Random copolymerization of these monomers resulted in the copolymers exhibiting a nematic mesophase in a wide range of PABA content. On the other hand, the sequential copolymers composed of PABA/PPD = 1/1 and 2/1, which were prepared from 4,4′-diaminobenzanilide (DABA) and N,N′-bis(4-aminobenzoyl)-p-phenylenediamine (BAB-PPD), respectively, did not show a mesophase. The copolymers of a diamine combination of DABA and PPD exhibited a nematic mesophase, despite the lack of a mesophase for each homopolymer. The combinations of DABA and other diamines were also examined. © 1994 John Wiley & Sons, Inc.     

Improved mesophase stability of benzoxazole derivatives via dipole moment modification

By modifying the molecular dipole moments with lateral monofluorine substituent, improved mesophase stabilities were obtained for novel benzoxazole derivatives, 2-(4?-alkoxy-3-fluorobiphenyl-4-yl)-benzoxazole liquid crystals (coded as nPPF(3)Bx). The series of nPPF(3)Bx with lateral monofluorine substituent ortho to benzoxazole group have larger calculated dipole moments by about 2 D than the corresponding fluorine-substituted analogs (compounds I) with lateral monofluorine ortho to alkoxy group; it is interesting to note that they show lower melting and clearing points but better mesophase stability with wider mesophase ranges for the molecules with both polar (NO₂, Cl) and nonpolar (CH₃, H) terminal groups. Meanwhile, compounds nPPF(3)Bx show greater red-shifted photoluminescence emissions than compounds I, which suggest that π–π interaction between molecules is reinforced by the enhanced dipole–dipole interaction caused by increased dipole moments. These results suggest that modification of the molecular dipole moment is an effective method to improve the mesophase stability of the classical mesogenic compounds.     

Gas transport in poly [bis(trifluoroethoxy) phosphazene] above the T(1) transition

Gas permeation measurements were performed by the time-lag method for 13 gases in poly [bis(trifluoroethoxy) phosphazene] above the mesophase transition, T (1) transition, of the crystals. Hysteresis was found in the temperature dependence of the permeability coefficients for CO₂, O₂, and N₂ in the transition region as observed by dilatometry and differential scanning calorimetry. Independently the solubility of CO₂ was determined gravimetrically above and below T(1). Noticeable change and hysteresis were also observed in the temperature dependence of the solubility coefficients. Diffusion of CO₂ in the measophase is suggested from the permeation and sorption measurements. Gas transport parameters of CO₂ in the mesophase are estimated, and the contributions of the mesophase and the crystalline phase to gas transport are considered.     

Mesogenic behaviour of two series of orthopalladated polar inline derivatives and their organic ligands (II)

Abstract

One series of 4-n-octyl-N-(4-X-benzylidene)anilines and two series of polar orthopalladated complexes derived from these of type Pd₂(μ-Y)₂ p-X-C₆H₃-CH = N-C₆H₄-C₈H_{17 2}; X: -H, -F, -Cl, -Br, -NO₂, -CN, -CH₃, -OCH₃, -CF₃, -COOCH₃, -OCOCH₃ and -OCOQH₅; Y: -OAc and -Cl; have been synthesized and their mesogenic properties studied. In the polar Schiff bases used as organic ligands, the polar end group determines both the presence of the mesophase and the type of mesophase exhibited. In the complexes, however, it is the central structure of the molecule that practically always determines mesogenic behaviour. No acetato-bridged complex is mesogenic. All the chloro-bridged complexes, however, show mesogenic behaviour. All these compounds show smectic A mesophases with the exception of the CN compound, which only exhibits a nematic mesophase.     

Thermal behaviour of lanthanum(III) alkanoates

The mesophase behaviour of the lanthanum(III) alkanoates [La(C_xH_2x+1COO)₃] (x =3-19) has been investigated by hot-stage polarizing optical microscopy, differential scanning calorimetry and high-temperature X-ray diffraction. Lanthanum(III) butyrate monohydrate shows no mesomorphism, whereas for the remaining short chain homologues (x = 4-9) a highly viscous mesophase M and a smectic A phase were observed. The longer chain lanthanum(III) soaps (x = 10-19) exhibit only a smectic A phase. However, the chain length has a pronounced effect on the transition temperatures. The thermal behaviour of lanthanum(III) alkanoates is compared with that of other lanthanide(III) alkanoates.     

Mesomorphic properties of a tetraphenylporphyrin metallomesogen with a weak hydrogen bond interaction between axial ligands: dihydroxo[5,10,15,20-tetrakis(4-n-dodecylphenyl)porphinato]silicon(IV)

A new metallomesogen, the dihydroxo[5,10,15,20-tetrakis(4-n-dodecylphenyl)porphinato]silicon(IV) complex, C₁₂TPPSi(OH)₂, was synthesized and its mesomorphism was investigated in terms of the axial hydrogen bond interaction in the stacked columnar structure. It was found that this compound exhibits a 3D plastic lamellar mesophase with a columnar structure, and the axial hydroxyl groups are connected by a very weak hydrogen bond interaction in the column. This causes a dramatic increase of the clearing point for the mesophase, even though the stacking periodicity is far larger (c. 9?Å) than that typically found for a columnar meosphase (c. 3.5?Å).     

Overlapping spheres Xα calculatons of the electron affinities and ionization potentials of ethylene,parabenzoquinone and their perfluoro derivatives

Microscopy and deuterium NMR spectroscopy are employed to demonstrate the existence of a thermodynamically independent nematic amphiphilic mesophase in mixtures of perfluoro-octanoate and water between 37.0 to 87.0% by weight of ²H₂O and 284.0 to 348.3 K at 1 atmosphere pressure. This phase is in equilibrium with an isotropic micellar solution and a smeetic lamellars mesophase to, respectively, high and low temperatures. The lamellar to nematic transition line changes from first to second order at a tricritical point corresponding to 42.5% of ²H₂O and 328.0 K, whilst the nematic to isotropic micellar solution transition is first order at all compositions. The measurements suggest the nematic phase is a solution of disc shaped micelles which orient with their unique axis parallel to the direction of an applied magnetic field.     

Influence of lateral methyl and terminal substituents on the mesophase behaviour of four rings azo-ester liquid crystal compounds

ABSTRACT

The effect of introducing a lateral methyl substitution into the previously investigated laterally neat four-ring analogues, 4-substituted phenylazo phenyl 4?-(4?-alkoxyphenylazo) benzoates (In_a–e), on their mesophase behaviour was investigated for the newly prepared five homologous series of 4-substituted phenylazo phenyl 4?-(3?-methyl-4?-alkoxyphenylazo) benzoates (IIn_a–e). Within each homologous series, the alkoxy group varies between 6, 8, 10, 12, 14, and 16 carbons, while the substituent, X, is a polar group that alternatively changes between the electron-donating (CH₃O and CH₃) groups, and the electron-withdrawing (Br and NO₂) groups, including the unsubstituted homologues (IIn_c). Their mesophase stabilities were determined by DSC and phases identified by PLM. The results showed that independent of the alkoxy-chain length or the polarity of the substituent X, the nematic phase is predominant with relatively high stability and wide temperature ranges. All compounds show a good thermal stability in the mesophases domain, except the nitro and Br substituted derivatives bearing short alkoxy chain length. Comparison of the mesophase behaviour was also made between the present series and corresponding three-ring laterally CH_3-substituted azo/ester analogues. UV-vis absorption spectra revealed that derivatives with electron donating or an electron withdrawing groups exhibited redshifts of the π→π* transition compared with unsubstituded derivative.     

Effect of lateral substitution on supramolecular liquid crystal associates induced by hydrogen-bonding interactions between 4-(4′-pyridylazo-3-methylphenyl)-4′′-alkoxy benzoates and 4-substituted benzoic acids

Five laterally methyl-substituted pyridine-based derivatives of the title compounds (I 8–I 16), with molecular formula 4-C_nH_2n+1O-C₆H₄COOC₆H₃(3-CH₃)-N=N-C₅H₄N were prepared and their molecular formulae elucidated via elemental analyses, infrared, nuclear magnetic resonance and mass spectra. The number of carbon atoms in the alkoxy chain (n) varies between 8, 10, 12, 14, and 16 carbons. The newly prepared pyridine-based derivatives were investigated for their mesophase behaviour by differential scanning calorimetry and polarised optical microscopy; most of them were found to possess monotropic smectic C (SmC) mesophase. Two groups (A and B) of the 1:1 hydrogen-bonded associates, formed between each of the derivatives I 8– I 16 and two types of 4-substituted benzoic acids (II), were prepared and similarly characterised to investigate the effect of lateral methyl substitution on the central phenylene ring, as well as terminal polar substituents and alkoxy-chain length on the stability of the mesophases induced by intermolecular hydrogen bonding. In Group A complexes, mesomorphic 4-alkoxy benzoic acids, that carry the terminal n-alkoxy group of varying chain length, were used. The other series of complexes (Group B) is composed from the same pyridine-based derivatives and each of the non-mesomorphic 4-substituted benzoic acids that carries small compact polar groups, varying between CH₃O, CH₃, H, Cl, Br, and CN. All complexes prepared were investigated for their mesophase behaviour by differential scanning calorimetry and polarised optical microscopy and found to be purely smectogenic, possessing SmC as the only mesophase observed. The formation of the hydrogen-bonded complexes was confirmed by constructing their binary phase diagrams, which cover the whole range of concentration of the two complements.     

Synthesis and mesomorphic properties of the nematic mesophase benzoxazole derivatives with big twist angle of difluoro-biphenyl unit

Modifying the position and numbers of lateral fluorine substituent is a common method to design and adjust the mesophase of liquid crystal compounds. Here, a series of 2-(2,2?-difluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-5-substituted benzoxazole with both non-polar (H, CH₃) and polar (NO₂) groups (coded as nPF(2)PF(2)Bx) is synthesised and characterised. All of the compounds show a conspicuous inter-ring twist angle of 38° compared with corresponding reference compounds I and II which are calculated by density functional theory method, and it is interesting to note that the final compounds nPF(2)PF(2)Bx show only nematic mesophase during heating or cooling. Meanwhile, the UV-vis absorption bands and photoluminescence emission peaks both display remarkable blue-shifted. The aforementioned results reveal that lateral difluoro substituents play a key role to stable the nematic mesophase by increasing the dihedral angle of biphenyl.     

半氟烃链苯并菲盘状液晶的合成及其介晶性

全氟烃链的憎氟效应(fluorophobic effect)可有效地促使棒状分子形成近晶相, 并稳定液晶相. 为进一步探讨氟效应对盘状分子介晶性的影响, 合成了一系列全氟酯链的苯并菲化合物C₁₈H₆(OC_nH_2n＋1)₅(OCOC₂H₄C₆F₁₃) (a), 以及另一系列相对应的不含氟化合物C₁₈H₆(OC_nH_2n＋1)₅(OCOC₈H₁₇) (b), n＝4～9. DSC检测和偏光显微镜观察显示两类化合物都为柱状相热致型液晶. 化合物a与相对应的化合物b比较, 其熔点和清亮点上升, 柱状相的热稳定性增强.