Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

ABSTRACT

New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (T_SmC*-SmA*/T_SmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan’s molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.     

Liquid‐crystalline thermosets from mesogenic dimeric epoxy resins by tertiary amine catalysis

Liquid‐crystalline thermosets (LCTs) were prepared by the curing of difunctional liquid‐crystalline dimeric epoxy monomers with imine moieties in the mesogenic core and central spacers of different lengths. Tertiary amines were used as catalysts in different proportions. The locked mesophases of the LCTs were characterized by polarized optical microscopy and wide‐angle X‐ray scattering and identified as smectic‐C, regardless of their smectic‐A or smectic‐C initial state. The influence of a 7.1‐T magnetic field on the macroscopic orientation of these materials was studied by dynamic mechanical analysis, and the orientation parameter was determined by IR dichroism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3916–3926, 2002     

Synthesis and mesogenic properties of diaromatic cyanates and isocyanates-monomers for liquid crystalline thermosets

Five pairs of mesogens with identical cores having either a cyanate or an isocyanate reactive terminal group have been synthesized. The monofunctional mesogens have a n-butoxy or a methoxy substituent as the second terminal group. The influence of the two isomeric OCN moieties on the formation of a mesophase and on the thermal transitions has been investigated. The mesophases observed were identified as nematic. A tremendous difference in the mesogenic power (stabilizing effect on a mesophase) has been found for the two reactive terminal groups. The isocyanates have lower melting points than the isostructural cyanates and clearing points which are approximately 30°C higher per isocyanate group, as compared to a cyanate moiety.     

Solid-phase synthesis of oxazolidinones by cycloaddition of resin-bound epoxides with isocyanates

The first solid-phase synthesis of oxazolidinones by cycloaddition of resin-bound epoxides with isocyanates is described. Synthesis of the title compounds was achieved by alkylation of resin-bound carbamates with glycidyl tosylate, followed by cycloaddition of the resulting epoxides with isocyanates at elevated temperature in high yields and purity. Because N-aryloxazolidinones have been known to possess various biological activities, this method is useful from the viewpoint of drug discovery.     

Liquid crystal comb polymer with polar mesogenic and aliphatic side groups I. Preparation and structural properties

A liquid crystal comb polymer of potential interest for electro-optical applications has been synthesized and characterized. The polymer has a polyacrylamide main chain and two different types of side group, built up of 4'-undecanoyloxy-4-cyanobiphenyl groups (68 mol %) and of shorter aliphatic groups (32 mol %). The two types of side group occur randomly along the polymer's main chain. The preparation steps and the liquid crystalline structure of the resulting copolymer are compared with those of a homopolymer whose side groups (built up of 4'-undecanoyloxy-4-cyanobiphenyl groups) are all identical. X-ray diffraction shows that the polymers display different mesophases over different temperature ranges. However, the homopolymer exhibits two smectic phases (SmC2 and SmA2), while the new copolymer exhibits a SmC2 phase and a nematic mesophase at higher temperature. The mesophase thermal stability of the copolymer is significantly lower than that of the homopolymer.     

Liquid crystal comb polymer with polar mesogenic and aliphatic side groups I. Preparation and structural properties

A liquid crystal comb polymer of potential interest for electro-optical applications has been synthesized and characterized. The polymer has a polyacrylamide main chain and two different types of side group, built up of 4'-undecanoyloxy-4-cyanobiphenyl groups (68 mol %) and of shorter aliphatic groups (32 mol %). The two types of side group occur randomly along the polymer's main chain. The preparation steps and the liquid crystalline structure of the resulting copolymer are compared with those of a homopolymer whose side groups (built up of 4'-undecanoyloxy-4-cyanobiphenyl groups) are all identical. X-ray diffraction shows that the polymers display different mesophases over different temperature ranges. However, the homopolymer exhibits two smectic phases (SmC₂ and SmA₂), while the new copolymer exhibits a SmC₂ phase and a nematic mesophase at higher temperature. The mesophase thermal stability of the copolymer is significantly lower than that of the homopolymer.     

Chemistry of sulfonyl isocyanates and sulfonyl isothiocyanates. XII. Cyclizations with epoxides

4-Toluenesulfonyl isothiocyanate reacted with 1,2-epoxy-3-phenoxypropane and 2,3-epoxypropyl 4-methoxyphenyl ether to give, respectively, 3-(4-toluenesulfonyl)-5-phenoxymethylene-2-oxazolidmethione ( I ) and 3-(4-toluenesulfonyl)-5-(4-methoxyphenoxymethylene)-2-oxazolidinethione ( II ) in high yields. The sulfonyl isothiocyanate reacted further with styrene oxide to give a mixture of oxazolidinethiones from which a solid III was isolated. The structure of III is either the 4- or 5-phenyl derivative of 3-(4-toluenesulfonyl)-2-oxazolidinethione. Reactions of the isothiocyanate with 3-chloro-1,2-epoxypropane and 1,2-epoxybutane afforded, respectively, 3-(4-toluenesulfonyl)-5-chloromethyl-2-oxazolidinethione ( IV ) and 3-(4-toluenesulfonyl)-4-ethyl-2-oxazolidinethione ( V ). Evidence for structures was by pmr, ir, and elemental analyses.     

Chemistry of sulfonyl isocyanates and sulfonyl isothiocyanates. XI. Cyclizations with epoxides

4-Toluenesulfonyl isocyanate cyclized with 1,2-epoxy-3-phenoxypropane and 2,3-epoxypropyl 4-methoxyphenyl ether, respectively, to give 3-(4-toluenesulfonyl)-5-phenoxymethylene-2-oxazolidone ( I ) and 3-(4-toluenesulfonyl)-5-(4-methoxyphenoxymethylene)-2-oxazolidone ( II ). Compounds I and II were hydrolyzed in 2 M sodium hydroxide solution to the corresponding uncyclized hydroxy amides, VII and VIII. Compound I was remarkably stable toward 6 M hydrochloric acid and amines. Styrene oxide, 1,2-epoxybutane, 3-chloro-1,2-epoxypropane, and 1-methoxy-2-methylpropylene oxide reacted with the isocyanate to afford 3-(4-toluene-sulfonyl)-4-phenyl-2-oxazolidone (III), 3-(4-toluenesulfonyl)-4-ethyl-2-oxazolidone ( IV ), 3-(4-toluenesulfonyl)-5-chloromethyl-2-oxazolidone ( V ), and 3-(4-toluenesulfonyl)-4,4-dimethyl-5-methoxy-2-oxazolidone ( VI ), respectively. The yield of VI was constant over a temperature range of 25–90°.     

Liquid crystal comb polymer with polar mesogenic and aliphatic side groups II. Noise of the scattered light

Measurements of static light scattering and of the stationary noise of the scattered light have been performed as functions of temperature on the comb polymer whose synthesis and static structural properties are discussed in the preceding paper, Part I. The intrinsic optical noise strongly increases when the nematic phase is entered. An additional maximum in the noise, observed at lower temperatures, is attributed to a rearrangement of optical domains, introducing boundary regions where the fluctuations of loosely packed side groups are enhanced. The spectral density of the observed noise is definitely non-Lorentzian and can be fitted by the superposition of two pure Lorentzian components with different cut-off frequencies, attributed to the statistically independent random motion of mesogenic and aliphatic side chains. The noise level is reduced at all temperatures by an applied electrical field; in particular, the field is effective in selectively damping the fluctuations of polar side groups.     

Liquid crystalline dimers with bent-core mesogenic units

Two series of dimers, in which two identical aromatic bent core mesogenic units are connected via an organosiloxane or an alkylene spacer, have been investigated. The dimers with a spacer consisting of a trisiloxane central group and relatively long alkylene groups show an intercalated tilted smectic structure. The layer spacings appear to be very weakly dependent on the terminal chain lengths. In these dimers the smectic phase is stabilized for the compounds with a short terminal chain. Dimers with an aliphatic alkylene spacer are liquid crystalline only when the spacer is relatively short and the terminal chains are long. For these dimers a monolayer tilted smectic phase is observed.     

Liquid crystal comb polymer with polar mesogenic and aliphatic side groups II. Noise of the scattered light

Measurements of static light scattering and of the stationary noise of the scattered light have been performed as functions of temperature on the comb polymer whose synthesis and static structural properties are discussed in the preceding paper, Part I. The intrinsic optical noise strongly increases when the nematic phase is entered. An additional maximum in the noise, observed at lower temperatures, is attributed to a rearrangement of optical domains, introducing boundary regions where the fluctuations of loosely packed side groups are enhanced. The spectral density of the observed noise is definitely non-Lorentzian and can be fitted by the superposition of two pure Lorentzian components with different cut-off frequencies, attributed to the statistically independent random motion of mesogenic and aliphatic side chains. The noise level is reduced at all temperatures by an applied electrical field; in particular, the field is effective in selectively damping the fluctuations of polar side groups.     

Liquid crystal codendrimers with a statistical distribution of phenolic and mesogenic groups: behavior as Langmuir and Langmuir-Blodgett films

The first series of carbosilane liquid crystal codendrimers with groups of different polarity has been synthesized. The chemical structure of the newly synthesized materials and the composition of the codendrimers were studied by NMR spectroscopy and MALDI-TOF MS. It was found that the codendrimers tend to form stable Langmuir films at the air-water surface. The influence of composition and generation number on surface pressure-surface area isotherms and film stability was studied. Brewster angle microscopy confirmed the different phase behavior for monolayers of different codendrimer composition and generation number. It was found that side groups of fifth-generation codendrimers do not segregate, unlike those of lower generations. Langmuir-Blodgett films on solid substrates were obtained by the vertical dipping method. X-ray diffraction showed that the codendrimers with 75% of hydrophobic mesogenic terminal groups formed ordered layers parallel to the substrate.     

Liquid crystalline thermosets from dimeric LC epoxy resins cured with primary and tertiary amines

Liquid crystalline thermosets (LCTs) were prepared by curing difunctional LC dimeric epoxy monomers with imine moieties in the mesogenic core and central spacers of different length. Primary diamines or tertiary amines were used as curing agents obtaining materials with different characteristics. The results obtained were related to the mesogen structure, since dipolar moments in the mesogenic cores affect the ability to form ordered networks.     

Liquid crystal polymers from swallow-tailed mesogens

Swallow-tailed compounds containing an acetylenic unit have been synthesized. Only those compounds with a three ring structure exhibited liquid-crystalline phases. The platinum catalysed addition of these monomers to compounds containing Si-H groups was used to synthesize polymeric materials. Only those polymers with the tetramethyldisiloxane unit showed mesophases. The synthesis, characterization, and phase behaviour are described.     

Synthesis and characterization of novel epoxy monomers and liquid crystal thermosets

Four new epoxy monomers have been synthesized and characterized as part of a program to prepare novel liquid crystal thermoset (LCT) materials. Three of the new epoxy monomers contained a biphenyl mesogen and were not liquid crystalline (LC). The remaining epoxy monomer, which contained a 1,4-dibenzoyloxybenzene mesogen, was synthesized in an overall yield of 30% and displayed a broad (83°C) nematic liquid crystalline phase. The new liquid crystalline epoxy monomer was cured at 120°C and postcured at 175°C with a stoichiometric amount of 1,4-phenylenediamine. The thermal transitions of the resulting LCT were studied by differential scanning calorimetry (DSC), polarized light optical microscopy (POM), thermomechanical analysis (TMA), and wide angle x-ray diffraction (WAXD) as a function of cure time and temperature. A process characterization diagram was constructed which shows that LCTs based on this new LC monomer can be processed in the liquid crystalline phase over a broad range of times and temperatures. Qualitative agreement with previous epoxy LCT results was found, as LCT's with smectic phases and without clearing temperatures were observed at long cure times (high crosslink densities), whereas nematic phases with clearing temperatures predominated in networks at short cure times (low crosslink densities). © 1993 John Wiley & Sons, Inc.     

Isocyanatocyanates and (iso)cyanatocyanurates synthesis and liquid crystal thermosets

Synthesis and liquid crystal properties of a series of trifunctional star-shaped cyanurate rings containing mesogens with trimethylsiloxy-, hydroxy, cyanate- and isocyanate groups and of 4-cyanatophenyl-4-isocyanato benzoate ( 2 ) are described. Urethanes from the isocyanato-cyanurate ( 8 ) and monoalkohols were high-melting monotropic or non-1c-compounds. Cyclotrimerisation of the cyanuratocyanate ( 7 ) in comparison to the parent dicyanate ( 1 ) was studied to prove that the strong increase of isotropisation temperature with conversion was related to the polymer effect and not to the gel point. Cyclotrimerisation of 8 and of the isocyanatocyanate ( 2 ) give networks with identical structure of alternating cyanurate and isocyanurate cross-links, the former with nematic organisation, the latter isotropic, which proves the possibility of kinetic stabilization of a mesophase.     

Liquid crystal properties of N-alkyl(ethylpyridinium) bromides ω-substituted with a mesogenic group

A new series of ethylpyridinium salts N-substituted with a 4'-methoxybiphenyl-4-yloxyalkyl group has been synthesized and the mesomorphic properties examined. The effect of positioning the ethyl group at each of the three available positions (2; 3- or 4-) on the pyridinium ring was examined and compounds with alkyl chains containing odd numbers of carbon atoms from C5 to C11 were investigated. It was found that the longer chain (C9-C11) 2- and 4-ethylpyridinium compounds form smectic phases, whereas the corresponding 3-substituted compounds do not. X-ray diffraction studies indicated that the smectic phases were of types A, B and in one case E, with the molecules lying in a head to tail fashion in single layers with interdigitated alkyl chains. We explain the difference in mesogenic properties of the three different ring-substituted compounds in terms of packing considerations in the ionic regions of the structures. It is suggested that the 3-derivatives pack more efficiently than the 2- and 4-substituted compounds.     

Liquid crystalline properties of unsegmented and segmented polyurethanes synthesised from high aspect ratio mesogenic diols

A series of novel tetrad high aspect ratio mesogenic diol monomers 4-{[4-(n-hydroxyalkoxy)-phenylimino]-methyl}-benzoic acid 4-{[4-(n-hydroxyalkoxy)-phenylimino]-methyl}-phenyl ester were prepared with varying alkoxy spacer length (n=2,4,6,8,10) by reacting 4-formylbenzoic acid 4-formylphenyl ester and 4-(n-hydroxyalkoxy) anilines. Two series of thermotropic main chain liquid crystalline unsegmented polyurethanes (PUs) were obtained by the polyaddition of the mesogenic diols with hexamethylene diisocyanate (HMDI) and methylene bis(cyclohexylisocyanate) (H₁₂MDI) in dimethylformamide respectively. The effect of the incorporation of a third component namely polyol on the liquid crystalline properties of the polyurethanes was also studied. Linear segmented PUs were synthesised by a two-step block copolymerisation method. The PUs synthesised were based on six spacer mesogenic diol chain extender, soft segments poly(tetramethylene oxide)glycol (PTMG) (M_n= 650,1000,2000) and polycaprolactone diol (PCL) (M_n=530,1250,2000) of varying molecular weights and different diisocyanates including HMDI, H₁₂MDI and methylene bis(phenylene isocyanate) (MDI). Structural elucidation was carried out by elemental analysis, fourier transform infra red (FT-IR), nuclear magnetic resonance (¹H NMR and ¹³C NMR) spectroscopy. Inherent viscosity of the unsegmented polymers measured in methanesulphonic acid at 26°C was in the range of 0.13 - 0.65 dL/g while the molecular weights and molecular weight distribution of the segmented polyruethanes was determined using gel permeation chromatography (GPC). Mesomorphic properties were studied by differential scanning calorimetry (DSC) and hot stage polarising optical microscopy and the thermal stability was determined by thermogravimetric(TG)analysis. The monomeric diols and the polyurethanes exhibited nematic texture and good mesophase stability. It was observed that the partial replacement of the mesogenic diol by the polyol of varying molecular weights influenced the phase transitions and the occurrence of mesophase textures. The phase transition temperatures of the investigated polyurethanes showed dependence on the chain length of the soft segment and on the content of the mesogen moiety. A higher content of mesogenic moiety was needed to obtain liquid crystalline property when the soft segment length was increased as observed in the case of PTMG. Grained and threaded textures were observed depending on the molecular weight of the soft segment, the mesogen content and the diisocyanate. The stress-strain analyses showed that the polymers bused on high molecular weight PTMG soft segment have elastomeric property while the PCL based PUs displayed no elastomeric property.     

Liquid crystalline epoxy thermosets based on dihydroxymethylstilbene: Synthesis and characterization

This article describes the synthesis and characterization of a new series of liquid crystalline thermosets. Nematic epoxy-terminated oligoethers based on dihydroxy-α-methylstilbene were synthesized for this study. These prepolymers were crosslinked within the nematic mesophase using methylenedianiline. Depending upon the molecular weight and polydispersity of the oligoether, the crosslinking reaction resulted in networks with either a smectic or nematic molecular organization in contrast to the simple nematic phase of the oligoether. The formation of a smectic-like structure on curing was found to be related to the breadth of the prepolymer molecular weight. In those networks with a low crosslink density a clearing transition could be observed, whereas in the more highly crosslinked networks the molecular organization was frozen in until decomposition. The glass transition temperature of these LC networks rose as the crosslink density was increased, ranging from 35 to 152°C. In agreement with theory, the clearing transition of the networks was found to be dependent on the phase state during curing © 1992 John Wiley & Sons, Inc.     

Synthesis, characterization and crystal structures of two polyphilic mesogenic compounds

Three homologous series of semi-perfluorinated liquid crystals: 4-(2,2,3,3,4,4,4-heptafluoro-butyloxycarbonyl)phenyl, 4-(2,2,3,3,4,4,5,5-octafluoropentyloxycarbonyl)phenyl and 4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxycarbonyl)phenyl 4-[(n-perfluoroalkyl)alkyloxy]benzoates have been synthesized. Their mesomorphic properties were studied by DSC, microscopic observation and X-ray diffraction. Their layer spacings are smaller than the molecular length (L). The ratio d_SA/L is about 0.7. The crystal structure of two derivatives of the first family have also been investigated. In both structures the molecules pack in smectic C-like sheets where neighbouring molecules are antiparallel, interacting through dipolar and van der Waals forces. The interactions between contiguous sheets, through the ends of perfluorinated chains are very weak. The X-ray diffraction results on the mesophases and on the crystalline structures of two compounds of the first family are compared. From this, we propose a model of the smectic phases with a zig-zag structure.