Monotropic Mesophase Research Articles

Rod-like phases formed by Ni(II) and VO(II) complexes of tetradentate enaminoketone ligands

Most of the nickel(II) complexes of tetradentate enaminoketone ligands obtained, although not strictly calamitic and with a rather low length to width ratio, form enantiotropic rod-like nematic and smectic A phases. Corresponding vanadyl(II) complexes exhibit only monotropic mesophases. The vanadyl complexes, due to their non-planar structure, are chiral with an asymmetry centre placed at the metal ion.

Synthesis and mesomorphic properties of 2,6-disubstituted derivatives of quinoline: amides and esters

Thesyntheses of N-4-n-alkoxyphenyl-6-n-decyloxyquinoline-2-carboxamide(alkoxy=CnH2n+1O, n=4-10) and 4-n-alkoxyphenyl-6-n-decyloxyquinoline-2-carboxylate (n=4,6,8) are described along with the measurement of their physical properties using DSC and optical methods. The amides all show an enantiotropic smectic A phase while the esters show monotropic mesophases. A study of the structure/mesomorphic activity relationship is also described.

Liquid crystalline thermosets (LCT) from diaromatic mesogenic diepoxides and aromatic diamines: synthesis and phase behaviour of model compounds and intermediate structures

Two mesogenic monoepoxides, nematogenic 4-(2,3-epoxypropoxy)-4′-ethoxy-biphenyl ( 1) and smectic-A 4-butoxyphenyl-4-(2,3-epoxypropoxy)-benzoate ( 2) were synthesized and reacted with 4,4′-diaminobiphenyl ( 3), or 4-aminophenyl-4-aminobenzoate ( 4) to obtain model compounds for the chain extended and branched intermediates (cross-link sites) formed in the amine curing of epoxides. All epoxy/amine-adducts have higher clearing points than the original epoxide/amine mixtures, though with one exception only monotropic mesophases were observed. The order of the clearing points for a given epoxide/amine combination is symm. 2:1>4:1>1:1≈3:1> non symm. 2:1. The type of mesophase of the adducts is that of the monoepoxide used except for the 1:1 adduct of smectic 2 and 4, where a nematic phase was obtained.

Trinuclear Gold(I) Pyrazolate Complexes Exhibiting Hexagonal Columnar Mesophases with Only Three Side Chains1

Reaction of long-chain alkyl-substituted pyrazoles with AuCl(SMe2) yields trinuclear gold(I) pyrazolate complexes, two of which exhibit monotropic mesophases. Optical microscopy and/or X-ray diffraction studies show that these compounds form hexagonal columnar mesophases. They are only the third examples of discotic mesogens possessing only three aliphatic side chains. The X-ray crystal structure of one compound reveals that two molecules form a “dimer” mainly through intermolecular Au−Au interactions. The “dimers” are stacked in a tilted fashion to form columns along the b axis in the crystal phase, which are presumably rearranged to the hexagonal columnar structure in the mesophase.

Mesomorphic 4'-functionalized 6'-phenyl-2,2'-bipyridines: tridentate ligands for organopalladium mesogens

A novel series of 6'-phenyl-2,2'-bipyridines (HL-n) substituted in the 4 position with promesogenic functional groups has been prepared. The materials are strongly luminescent and display nematic mesomorphism. The HL-n behave as C,N,N-tridentate, cyclometallating ligands leading to [Pd(L-n)Cl] derivatives. Metal co-ordination promotes a considerable stabilization of the crystal phase, forcing mesophases to become metastable. All [Pd(L-n)Cl] species indeed exhibit a monotropic mesophase either nematic or smectic A in character depending on the chain length of the substituents on the central pyridine unit.

New metallomesogens core. New copper complexes with calamitic mesophases using tropolone based ligands. Crystal structure of bis(5-octyloxytropolonato)copper

A new class of metallomesogens based on 5-substituted tropolones with both enantiotropic and monotropic mesophase behaviour are prepared, the mesogenic properties are in line with the rod-like, calamitic nature of bis(5-octyloxytropolonato)copper shown in its crystal structure.

A kinetics study of crystallization from discotic mesophases

The polymorphism of hexa- n -octanoate of rufigallol (RHO) was used to investigate the crystallization kinetics when starting from two different discotic mesophases, namely, a hightemperature enantiotropic mesophase and a low-temperature monotropic mesophase. In all cases, an Avrami exponent of n 3 was obtained, suggesting two-dimensional sporadic nucleation and growth. Although the same overall mechanism was predicted to operate for all crystallization temperatures investigated, the Avrami constant b has a temperaturedependence which is different for different starting columnar phases, indicating a strong influence on the rates of nucleation and of growth of the different molecular arrangements of the starting mesophases. Differential scanning calorimetry was used for the studies, supplemented by direct observations through optical polarizing microscopy.

Synthesis and characterization of poly(3-substituted thiophene)s having liquid crystalline moiety

3-Substituted thiophene derivatives having a biphenyl or a cyclohexylphenyl moiety as the mesogenic core were synthesized and polymerized. Thiophene monomers with a cyanobiphenyl or a biphenylketone moiety showed a monotropic mesophase, but these phases are unstable by comparing with those of corresponding models. Cyclic voltammetry of these monomers showed that oxidation potentials of the thiophene moiety were more anodic than those of mesogenic cores. Oxidative polymerization by FeCl 3 or electrochemical manners gave soluble and fusible polythiophenes, but these polymers had no mesophases.

Synthesis and thermotropic properties of new discotic 1,3,5-trisubstituteds-triazinetriones obtained by cyclotrimerization of isocyanates

Abstract The synthesis of a number of s-triazine-2,4,6-triones (isocyanurates) by cyclotrimerization of 4-monoalkoxy-, 3,4-dialkoxy-, or 3,4,5-trialkoxyphenyl-, benzyl-, or styryl-isocyanates is reported. The thermal and phase behaviour of the novel compounds were investigated by optical polarized light microscopy and thermal analysis. No mesophase could be observed for any of the precursors except 4-decyloxybenzoic acid. Mesophases of the isocyanurates were observed only in the series containing 3,4-dialkoxyphenyl moieties. A vinylene group between the inner s-triazine ring and the outer phenylene rings gives low melting points and enantiotropic mesophases for C8 and longer alkyl chains. The phenylisocyanurates may exhibit a monotropic mesophase and normally have the highest melting points of the three types, whereas a methylene spacer suppresses formation of a mesophase. According to the textures observed, the mesophases are hexagonal columnar. This will have to be confirmed by X-ray scattering techniques.

Synthesis and liquid-crystalline properties of thermotropic poly(ester-urethane)s

Seven new mesogenic diol monomers CmCn, consisting of two bis( p-oxybenzoyl) units interconnected by a polymethylene segment containing n methylene groups and flanked by two linear ω-hydroxyalkyl segments containing m methylene groups were reacted with 1,6-hexamethylenediisocyanate ( HDI) to give a new series of liquid-crystalline poly(ester-urethane)s HDI-CmCn. All of the precursors CmCn exhibited a nematic phase, and in two of them a smectic mesophase also occurred. Poly(ester-urethane)s HDI-CmCn were semicrystalline and formed a monotropic mesophase. The mesomorphic properties of the polymers were studied as a function of their chemical structure by changing alternatively m or n. The effect of the molecular weight of the polymers on the stability of the mesophase was also investigated for various samples of poly(ester-urethane) HDI-C6C6 ( M n = 3000-22,000). It was found that a competition existed between the tendencies of the polymer to crystallinity and liquidcrystallinity, and the molecular weight dependence of the crystallization temperature determined the onset of a monotropic mesophase.

Crystallization behavior of polyethers containing odd numbers of methylene spacers from the isotropic and liquid crystalline states

A series of thermotropic polyethers synthesized from 1-(4-hydroxyphenyl) - 2 - (2 - methyl - 4 - hydroxyphenyl) - ethane and α,ω-dibromo-n-alkanes with odd numbers of methylene units (MBPE-n = odd) shows monotropic mesophase behavior. In isothermal differential scanning calorimetry (DSC) experiments, two—sometimes even three—exothermic transition processes can be observed when the crystallization temperature is below the mesophase transition temperature, while only one exothermic process is present above the mesophase transition temperature. The melting behavior of the crystals grown from the mesophase and from the isotropic melt states is different. The crystals grown from the mesophase state exhibit a larger overall heat of transition and a higher transition temperature compared with those grown from the isotropic melt. This may be attributed to the molecular interfacial connections between the crystal and amorphous regions when MBPEs crystallize from the mesophase state. The difference in morphology between the crystals grown from the different states has also been studied with polarized light microscopy (PLM) and transmission electronic microscopy (TEM). The structures of the crystals grown from the different states are, however, the same, as evidenced through wide-angle X-ray diffraction (WAXD) measurements. From the banded morphology of MBPE samples observed from PLM, the defect textures observed through TEM and the results of WAXD experiments, this mesophase can be identified as a nematic liquid crystal state.

Synthesis and characterization of random semi‐flexible thermotropic liquid crystalline poly(ester‐imide)s

AbstractA series of novel poly(ester‐imide)s were prepared by the reaction of meta‐ and para‐substituted trimellitimide dicarboxylic diacid chlorides with various diols containing four, five, six, seven, eight, nine, 10, and 12 methylene groups by a solution polymerization technique utilizing refluxing 1,2,4‐trichlorobenzene as a solvent. The poly(ester‐imide)s were characterized by dilute solution viscosity, infrared spectroscopy, differential scanning calorimetry, and polarized light microscopy. The inherent viscosities of the meta‐substituted poly(ester‐imide)s ranged from 0.06 to 0.25 dL/g while those of the para‐substituted poly(ester‐imide)s ranged from 0.10 to 0.65 dL/g and were obviously of higher molecular weight. The meta series were amorphous and showed no mesophase formation. All para‐substituted poly(ester‐imide)s exhibited monotropic mesophase identified as smectic A order. © 1994 John Wiley & Sons, Inc.

Synthesis and Properties of Comb-Like Liquid Crystalline Polymers with Electrooptically Active Mesogenic Side Groups

Abstract A system of non-centrosymmetric mesogens (4, nitrobenzylidene-2′-methylphenol(M2) and 4, cyanobenzylidene-2′-methylphenol(M5)) and their corresponding alkanol phenoxy ethers (M2C8 and M5C8) was investigated. The mesomorphic properties of mixtures of these low molecular mass compounds were established. The properties of homo and random copolymers of M2 and M5 attached as side-groups via a C8 spacer to a poly(methyl methacrylate) backbone were studied. A phase diagram was obtained. The thermo-mechanical relaxation properties of this system were also investigated by means of thermo-stimulated currents. While monotropic namatic mesophases dominate the low molecular mass M2C8-M5C8 system, the copolymers display an enantiotropic nematic behaviour. Two relaxation processes dominate the thermomechanical response of this copolymer system. One is interpreted as related to the segmental motion of the backbone (α-peak), the second, to the motion of the mesogenic groups (δ-peak). While the parameters related ...

Thermotropic liquid crystal polyesters derived from 4,4′‐biphenyldicarboxylic acid and oxyalkylene spacers

AbstractSeveral polyesters derived from bibenzoic acid and oxyalkylene glycols have been prepared and the ability of formation of mesophases was studied. The phase behaviour was investigated by means of differential scanning calorimetry, wide‐angle X‐ray diffraction and real‐time variable‐temperature small‐angle X‐ray diffraction. A common feature of all the studied polyesters, with ether groups in the spacer, is the inhibited ability for developing three‐dimensional structures, in contrast to the polymers with all‐methylene spacers. Moreover, the latter polymers show a monotropic mesophase behaviour but the former exhibit an enantiotropic transformation. Smectic structures of different types have been detected: The polyester derived from 3,3'‐oxybis(1‐propanol) produces a smectic SA structure, while the polyester from triethylene glycol leads to a smectic SC mesophase. The influence of the spacers on the dynamic mechanical relaxations of these polybibenzoates has been studied and discussed.

Synthesis and thermotropic behaviour of liquid crystals containing tolane-based mesogenic units

Abstract Tolane-based liquid crystal materials with three different terminal groups: trifluoromethyltolanes, cyanotolanes, and nitrotolanes, and with different chain lengths of alkenyloxy groups, have been synthesized. The phase behaviour of these liquid crystal materials has been characterized by differential scanning calorimetry, optical polarized light microscopy and X-ray diffraction. The mesophase behaviour of the materials was shown to be significantly influenced by the polarity of the terminal groups. The trifluoromethyltolanes exhibit an enantiotropic smectic E mesophase and the cyanotolanes exhibit a monotropic nematic mesophase. The most polar among the three types of liquid crystal material; the nitrotolanes do not display any liquid crystalline phases, except for the homologue with an eleven methylene unit chain.

Crystal structure and mesophase morphology of MBPE polyethers containing odd‐number methylene spacers

AbstractA series of polyethers was synthesized from 4‐[2‐(4‐hydroxyphenyl)ethyl]‐3‐methylphenol and α, ω‐dibromoalkenes, MBPE‐n. The polymers with odd‐numbered methylene spacers show a monotropic mesophase behavior during cooling from the isotropic melt before crystallization. The crystal structures of MBPE (n = odd) polyethers have been investigated through wide angle X‐ray diffraction fiber patterns. Two crystallographic periods along the c‐axis are found, one of which corresponds to the length of methylene spacers, and the other is close to an integral number times the length of the repeating unit. Polarized light microscopy observations of the uniaxially oriented samples obtained through mechanical shear show a banded texture, which is a typical morphology found in liquid crystal polymers. Transmission electron microscopy experiments provide morphological evidence to identify that this mesophase is a nematic liquid crystal state based on the disclination defect patterns defined by Frank. A relationship between splay elastic constant k11 and bend elastic constant k33 is discussed through lamellar decoration of disclination defects.

A new series of low molar mass ferroelectric organosiloxanes with unusual electro-optic properties

Abstract A new series of low molar mass chiral organosiloxanes A*B, C*B and a racemic analogue A(±)B have been synthesised and their mesomorphic and ferroelectric properties investigated. A* and C* denote the mesogenic moiety and B denotes the pentamethyldisiloxane tail. The following organosiloxane materials have been prepared: C8A*B, C10A*B, racemic C10A(±)B and C10C*B, where the C8 or C10 prefix indicates the number of methylene spacers between A* and C* and A* contains a 4-[(S)-2methylbutanoyloxy] and C* contains a chloro chiral end group. C8A*B exhibits only an unstable monotropic mesophase, C10A*B and C10C*B both exhibit one tilted enantiotropic ferroelectric smectic-C mesophase over a broad temperature range, with high tilt angles and high spontaneous polarisation (e.g. C10C*B 28 ° and 150nCcm−2 at 20°C). The achiral siloxane derivative C10A(±)B, exhibits one broad enantiotropic smectic-C phase. Electrooptic measurements on C10A*B and C10C*B indicate that the spontaneous polarisation is only weakly...

Ordered structures in a series of liquid-crystalline poly(ester imides)

A series of poly(ester imide)s (PEIMs) has been synthesized from N-[4-(chloroformyl) phenyl]-4-(chloroformyl)phthalimide and different diols with methylene units of 4 to 12 (n). PEIMs with both even and odd methylene units exhibit a monotropic mesophase behavior during cooling due to the supercooling necessary for crystallization. Identification of this mesophase as a monotropic liquid crystal with smectic A order has been carried out via differential scanning calorimetry, wide angle X-ray diffraction (WAXD), small angle X-ray scattering, polarized light microscopy, and transmission electron microscopy. It is found that the methylene units in these chain molecules are also largely responsible for the formation of this liquid crystal phase. After investigating different types of main chain mesogen-nonmesogen liquid crystal polymers, a general concept considering the contributions to enthalpy and entropy changes during the liquid crystal transitions may be associated with the relative rigidity, linearity, and regularity of the mesogenic groups compared to the methylene units. Detailed WAXDR study of fiber and powder patterns indicates that the degree of orientation and the order correlation lengths along and perpendicular to the direction of chain molecules (chain lateral packing and layer structure) not only increase with the number of methylene units but also show an even-odd alternation.more » A possible chain packing model is suggested. The morphology and defects of this smectic A liquid crystal phase are also discussed.« less

Self-regulated phase transitions in poly(4-{2-[4′-(11-vinyloxyundecyloxy)biphenyl-4-yl]ethyl}benzo-15-crown-5) and poly(4-{2-[4′-(11-methacryloylundecyloxy)biphenyl-4-yl]ethyl}benzo-15-crown-5)via molecular recognition

The syntheses of 4-{2-[4′-(11-vinyloxyundecyloxy)biphenyl-4-yl]ethyl}benzo-15-crown-5 (12) and 4-{2-[4′-(11-methacryloylundecyloxy)biphenyl-4-yl]ethyl}benzo-15-crown-5 (14) and their respective cationic and radical polymerizations to yield liquid-crystalline poly(12) and poly(14) are described. Poly(12) with a degree of polymerization of 6 exhibits a monotropic smectic A (SA) phase, while the higher-molecular-weight polymers exhibit an enantiotropic SA phase. Poly(14) displays an enantiotropic SA and an unidentified higher-order smectic phase. Both poly(12) and poly(14) display crystalline phases also. The mesomorphic behaviour of the complexes of both polymers with NaSO3CF3 was investigated. Complexation increases the isotropization temperature of the highest temperature mesophase and decreases the temperature transition of crystalline and higher-order smectic phases. Therefore, this complexation transforms virtual and monotropic mesophases into enantiotropic mesophases and also increases the thermodynamic stability of the highest temperature mesophase. These results have demonstrated the ability of these polymers to self-regulate their phase behaviour via molecular recognition of metal salts by their 15-crown-5 receptor.

Paramagnetic rod-like liquid crystals with lateral branches: bis{N-alkyl[5-(4-decyloxybenzoyloxy)]salicylaldiminate} copper(II)

The synthesis and characterization of a homologous series of copper(II) complexes (CuL2) with a large lateral substituent and of their respective ligands, N-alkyl[5-(4-decyloxybenzoyloxy)]salicylaldimines (LH), are reported. The ligands showed monotropic smectic A mesophases, whereas the copper(II) complexes exhibit monotropic nematic mesophases for n= 3,6,7,8 and 9, enantiotropic smectic C and nematic phases for n= 1, an enantiotropic nematic mesophase when n= 2 and non-liquid-crystal properties for n= 4,5,10,16. The relationship between molecular structure and mesomorphic behaviour is discussed.