Thermotropic Mesophases Research Articles

Tailoring thermotropic cubic mesophases: amphiphilic polyhydroxy derivatives

Novel amphiphilic polyhydroxy compounds [N-(3,4-dialkoxybenzoyl)-1-amino-1-deoxy-d-glucitols (glucamides), N-(3,4-dialkoxybenzoyl)-1-deoxy-1-methylamino-d-glucitols, N-(3,4,5-trialkoxybenzoyl)-1-deoxy-1-methylamino-d-glucitols (N-methylgucamides), 1-benzoylaminopropane-2,3-diols, 2-benzoylaminopropane-1,3-diols, 2-(3,4,5-tridodecyloxybenzoylamino)-2-(hydroxymethyl)propane-1,3-diol and (3,4,5-tridodecyloxybenzoyl)bis(2,3-dihydroxypropyl)amine] have been synthesized. Their thermotropic liquid crystalline phases were investigated by means of polarizing microscopy, differential scanning calorimetry and X-ray diffraction. Depending on the chain length, and the size of the hydrophilic polyhydroxy units, different mesophases have been found: smectic A phases (SA), inverted bicontinuous cubic phases (CubV2 , Ia3d), hexagonal columnar phases (ColH2) and micellar cubic mesophases (CubI2 , Pn3m or P43n). In strong analogy to lyotropic systems, the type of thermotropic mesophase depends on the ratio between the volume of the lipophilic moiety and the surface area of the hydrophilic moiety at the hydrophilic–lipophilic interface. The crossing from zero interface curvature (SA phase) to the finite negative curvature of the inverted cylindrical aggregates of the columnar mesophase takes place via bicontinuous cubic mesophases. The cylindrical aggregates of the columnar mesophase are stable over a rather broad range of variation of the structural parameter. At a certain degree of the size of the lipophilic moiety in respect to the surface area of the hydrophilic group, however, the transition from the hexagonal columnar to a micellar cubic mesophase takes place. On the basis of proton conductivity measurements and from packing considerations we propose that this cubic lattice is built up by eight closed micelles per unit cell which have a rod-like shape and represent small segments of extended columns. Therefrom we can propose a model for the transformations between these different thermotropic mesophases.

N-Acyl-β-D-glycopyranosylamines containing 1,4-disubstituted cyclohexyl and phenyl rings: mesomorphism and molecularstructure relationships

A variety of N-acyl-β-d-glycopyranosylamines incorporating aliphatic and/or aromatic groups has been prepared regiospecifically and in good yield in a one step reaction of the required acid chloride with commercially available d-glycosylamines. The dependence of mesophase behaviour on the degree and nature of intermolecular hydrogen bonding, as well as the size and nature of the hydrophilic and hydrophobic parts of the core and the linking groups between the two have been studied. Of the compounds investigated only the N-(4-alkoxybenzoyl)-β-d-glucopyranosylamines and N-(trans-4-pentylcyclohexylacetyl)-β-d-glucopyranosylamine exhibited an observable thermotropic liquid crystal phase. The compounds synthesized were essentially insoluble in water and no lyotropic mesomorphism was observed. It is clear that it is the number of groups on the hydrophilic carbohydrate part of the molecule capable of hydrogen bonding, after a crucial length of the hydrophobic part of the molecule has been attained, that primarily determines thermotropic mesophase behaviour. However, it has also been found that an odd number of units in the central linkage between the hydrophilic and lipophilic parts of the molecule leads to significantly higher transition temperatures than those of compounds incorporating linkages with an even number of units. X-Ray diffraction studies imply that there is no intercalation of the hydrophobic parts of the molecule in the crystalline state. However, some intercalation is thought to occur after melting to form the bilayer structure of the smectic A* phase.

Structure formation of functional sheet-shaped mesogens

The combination of anisometric sub-units with an additional intramolecular functionality has frequently resulted in the creation of supramolecular systems not only able to form thermotropic mesophases due to their anisotropic molecular shape but also capable of structure formation resulting from their amphiphilic properties and/or from non-covalent intermolecular interactions with complementary components. This article summarizes different examples of this interplay of structure formation tendencies based on functionalized sheet-like liquid crystals.

Structural investigations on 'smectic D' and related mesophases

We have performed time resolved diffraction experiments in order to obtain a better insight upon the metastable phases surrounding some thermotropic mesophases of cubic Ia 3 d and Im 3 m symmetries. These metastable phases are columnar hexagonal, smectic or tetragonal, depending on the nature of the mesogenic molecule. Moreover, it appears that the structure of the cubic phase of the 4-alkyloxybiphenyl-4-carboxylic acids previously labelled smectic D varies with the lateral group substituted on the biphenyl core.

Electron and light microscopical investigation of defect structures in mesophases of pharmaceutical substances

Transmission electron microscopy of freeze fractured and replicated samples (TEM) and polarizing light microscopy (PLM) are used to investigate the defect structures of the thermotropic and lyotropic mesophases of the nonsteroidal antiinflammatory drug fenoprofen sodium and of the thermotropic mesophase of the nonionic surfactant sucrose oleate (O1570). All mesophases have a layered, smectic structure. The thermotropic liquid crystal of fenoprofen sodium is an interdigitated smectic A phase (smectic Ad) having the highest viscosity of the investigated samples. The thermotropic mesophase of the sugar ester is also of the type smectic A, likely to be of subtype smectic A2 (bilayered smectic structure). The lyotropic mesophase is of lamellar liquid crystalline nature and has a much lower viscosity than the thermotropic mesophases. In the PLM the lyotropic fenoprofen mesophase has a strong tendency to form a pseudoisotropic texture, indicating a strong tendency to form undisturbed layered structures. Other textures exhibited in the PLM are fan-shaped texture and maltese-cross texture. Confocal domains, cylinders, pits and peaks as well as screw dislocations are found in great number in the TEM. However, no greater regions of undisturbed lamellar arrangement in the lyotropic mesophase could be detected. The only texture of the thermotropic fenoprofen mesophase visible in the PLM is the fan-shaped texture, indicating confocal domains as predominant structural elements. However, no confocal domains (tori or Dupin cyclides) are found in the TEM. In the PLM the sugar-ester mesophase exhibited a fan-shaped texture, maltese crosses and oily streaks as dominant textures. In the TEM only a few +π and −π disclinations and imperfect confocal domains could be detected. The discrepancies in the appearance of defect structures and textures between the mesophases as well as the discrepancies in the findings in the PLM and in the TEM investigations are caused by the different sample preparation and the different viscosities of the mesophases.

N-Acyl-beta-D-glycopyranosylamines and 2-alkylamido-2- deoxy-alpha / beta-D-glucopyranoses: relationship between molecular structure and mesomorphism

A variety of N -acyl- beta -D-glucopyranosylamines, N -acyl- beta -D-mannopyranosylamines and 2-alkylamido-2-deoxy- alpha / beta -D-glucoses has been prepared regiospecifically and in good yield in a one step reaction of the appropriate acid chloride with D-mannosylamine, 2-amino-2-deoxyD-glucopyranose (prepared in one step from glucose and mannose according to literature methods) and D-glucosamine liberated in situ from the commercially available hydrochloride. The dependence of the liquid crystal transition temperatures and mesophase formation on the degree and nature of intermolecular hydrogen bonding has been studied by choosing two different carbohydrate cores and by attaching aliphatic terminal chains via an amide linkage in two different positions. Comparison is made with related compounds reported in the literature. Whereas the N -acyl- beta -D-mannopyranosylamines and N -acyl- beta-D-glucopyranosylamines possess wide-range smectic A* phases, the corresponding 2-alkylamido-2-deoxy- beta-Dglucopyranoses do not exhibit observable mesomorphism. Although some homologues of the compounds synthesized have already been reported in the literature, their liquid crystalline behaviour was not reported. These investigations confirm the general view that it is the number and type of hydrogen bonding groups on the carbohydrate moiety and the aliphatic chain length that primarily determine thermotropic mesophase behaviour, as well as the absolute values of the transition temperatures.

Studies of the thermotropic mesophase behaviour exhibited by a highly asymmetric tetrabenzotriazaporphyrin derivative

The thermotropic mesophase behaviour of an asymmetrically shaped and highly substituted tetrabenzotriazaporphyrin (TBTAP) derivative has been studied by optical microscopy, DSC and X-ray diffraction. The TBTAP macrocycle differs from the more common phthalocyanine ring system by the substitution of a methine for a nitrogen at one meso-position. The TBTAP core was substituted with a single heptadecyl chain at this meso-position and four neopentyl groups were statistically distributed around the peripheral benzene rings. In contrast to an earlier study of this system which described the structure of the mesophase as discotic lamellar, detailed X-ray diffraction study indicates that the TBTAP derivative forms a disordered hexagonal columnar mesophase (Colhd), with a weak tendency towards antiparallel orientation of neighbouring molecules observed in the form of a weak pseudo-centred rectangular packing.

New carbohydrate amphiphiles I. Synthesis

The synthesis and characterization of two groups of new amphiphilic carbohydrates, 3,4,5-tris(alkoxy)- N -(1-deoxysorbitol-1-yl)benzamide and 3,4,5-tris(alkoxy)- N -(2-deoxyglucos2-yl)benzamide are described. The phase behaviour was investigated by differential scanning calorimetry and thermo-optical analysis. All the investigated compounds exhibit thermotropic mesophases. Depending on the length of the alkyl chains, hexagonal columnar and cubic mesophases are obtained.

Cholesteric Carbohydrate Liquid Crystals Incorporating an Intact Glucopyranose Moiety

Recently, the first monosaccharide derivatives containing a fully intact monosaccharide and two vicinal OH-groups which display thermotropic chiral mesophases were synthesized. These liquid crystals have a rigid core, with a trans-decalin-like skeleton incorporating the D-glucopyranose ring, substituted with an alkoxylated polarizable aromatic group, e.g. a phenyl 4-octyloxybenzoate, on one side and an alkoxyphenyl group at the anomeric centre. On the basis of the focal-conic fan-like texture displayed and the existence of a blue phase in the UV, we expected the cholesteric helix to have a very short pitch. The compounds possessed an exceptionally high helical twisting power. Extrapolation of these data to dopant concentrations of 100 % also indicates a very short pitch (30–90 nm).

Glasses of new 2,3,9,10,16,17,23,24-octasubstituted phthalocyanines forming thermotropic and lyotropic discotic mesophases

The synthesis and mesogenic properties of a new class of liquid crystalline metal-free and metalated phthalocyanines with different numbers of alkylthio side chains are presented. The effect of the substitution pattern in a series of differently octasubstituted phthalocyanines on the width and order of the thermotropic mesophase is investigated. The exchange of the alkylthio groups from chlorine atoms alters the mesophase width and the melting behaviour. Glassy phthalocyanines, without any tendency to crystallize, can be synthesized in this way. Furthermore, mixtures of differently 4,5-disubstituted benzenedicarbonitriles are tetramerized statistically to the corresponding mixtures of substituted phthalocyanines. The effect of decreasing symmetry at the molecular level on the properties of the liquid crystalline materials has been investigated. Highly oriented columnar systems can be obtained with these mixtures. For all compounds containing less than eight alkylthio groups, lyotropic mesomorphism in hexadecane solution is also found.

Amphiphilic N-benzoyl-1-amino-1-deoxy-D-glucitol derivatives forming thermotropic lamellar, columnar and different types of cubic mesophases

Novel amphiphilic glucamine derivatives have been synthesized. These are N-benzoyl-1-deoxy1-methylamino-D-glucitols and N-benzoyl-1-amino-1-deoxy-D-glucitols carrying one, two or three aliphatic chains (CnH2n 1O- with n 3, 6 and 12) grafted to the benzamido group. The thermotropic mesophases of these compounds were studied by thermal polarizing optical microscopy and differential scanning calorimetry, and some also by X-ray scattering. Depending on the number and the length of the alkyl chains lamellar, bicontinuous cubic, hexagonal columnar or inverted micellar cubic mesophases were detected by analogy with lyotropic systems. In the contact region between lamellar phases of the single chain amphiphiles and micellar cubic phases of the mesomorphic triple chain compounds, hexagonal columnar phases can be induced. A hexagonal columnar phase was also induced in the contact region between a bicontinuous and a micellar cubic mesophase. The lyotropic liquid crystalline behaviour of the dodecyloxy substituted N-benzoyl-1-deoxy-1-methylamino-D-glucitols was investigated by the solvent penetration method using ethylene glycol as protic solvent. On increasing the solvent content, the double chain compound forms a cubic and a lamellar mesophase and the triple chain compound forms a hexagonal columnar lyomesophase. The dodecyloxy substituted compounds were also investigated with respect to their behaviour as thin films at the air-water interface using a Langmuir film-balance. Different types of pi/Aisotherms were observed whereby the molecular areas at collapse were determined either by the size of the carbohydrate head group (single chain compounds) or by the number of alkyl chains (double and triple chain compound).

Substitution effects on the liquid crystalline properties of D,L-xylitol amphiphiles

In this article we describe the self-assembling properties of alkyl substituted xylitols in relation to both thermotropic and lyotropic liquid crystalline mesophases. Three series of substituted xylitols were prepared where aliphatic chains of varying length were attached to a xylitol moiety via ether, thioether and ester linking groups. The thermotropic properties were investigated by thermal polarized light microscopy and differential scanning calorimetry, and evaluated as a function of chain length and linking group. The lyotropic phase behaviour was investigated via the addition of water to each material at room temperature. The efficiency for forming thermotropic phases was found to be reversed for the lyotropic phases in respect of the three series, i.e. as a function of the linking unit.

Thermotropic and lyotropic mesophase formation of poly(ethylene oxide) substituted rod-coil oligomer

The synthesis and characterization of the rod-coil oligomer consisting of a molecular rod and a poly(ethylene oxide) with degree of polymerization of 12 (5) are presented. The rod-coil oligomer5 shows a crystalline phase with microphase segregation. However, the complexation of the rod-coil oligomer5 with LiCF3SO3 induces a thermotropic smectic liquid crystalline phase. In aqueous solution, the rodcoil oligomer5 shows various lyotropic mesophases such as lamellar, cubic and cylindrical micellar mesophases, depending on the oligomer concentration. These results characterized by differential scanning calorimetry and optical polarized microscopy are described.

Synthesis and liquid crystal properties of phthalocyanine derivatives containing both alkyl and readily oxidised phenolic substituents

The synthesis, using a mixed phthalonitrile cyclotetramisation, of phthalocyanine derivatives containing both long alkyl side-chains and redox-active, sterically hindered phenolic (3,5-di-tert-butyl-4-hydroxyphenyl) substituents is described. A detailed structural characterisation, including high resolution NMR spectroscopy, revealed that the statistically predicted amounts of regioisomers were prepared for each compound. Some isomers could be isolated using a combination of column chromatography and HPLC. A detailed investigation of their thermotropic mesophase behaviour is reported, including X-ray diffraction structure determination. These compounds are designed to combine interesting oxidative behaviour with liquid crystalline properties.

Preparation and characterization of thermotropic liquid crystal copolyesters containing m-hydroxybenzoic acid units

Wholly aromatic, main-chain copolyesters derived from terephthalic acid and phenylhydroquinone modified with m-hydroxybenzoic acid were prepared and characterized by elemental analyses, 13C nuclear magnetic resonance, thermogravimetric analysis, optical observations, differential scanning calorimetry and wide-angle X-ray diffraction. The phases behaviours depended on copolymer composition. Poly-(p-phenylphenylene terephthalate-co-m-hydroxybenzoate)s with molar percentages of the p-phenylphenylene terephthalate units in excess 70 mol% displayed thermotropic mesophases at relatively low temperatures and exhibited good thermal stability up to about 400°C. Increasing content of m-hydroxybenzoic acid units, which increased the amount of structural irregularity, decreased the isotropization temperature and the temperature at which the melt started to flow, and also caused a loss of both crystallinity and of liquid crystallinity.

Main chain liquid crystalline ionic polymers with thermotropic and lyotropic mesophases

AbstractAn ionogenic main chain liquid crystalline polymer was synthesized. In contrast to previous studies the charged sites were incorporated into the flexible spacer and not into the mesogenic group. This was done through quaternization of 1,2‐bis(4‐pyridylethane) with a biphenyl‐4,4′‐nonanoxytoluenesulphonate. The polymer displayed thermotropic and lyotropic me‐sophases. The thermotropic mesophase was smectic. Organic counterions such as meth‐ylsulfonates and p‐toluenesulfonates were found to be favorable to the development of mesophases, due to the increased solubility and lower isotropisation temperatures, when compared to bromide counterion containing polymers. Easy supercooling, dependence on thermal history, and the development of batonnet textures on cooling from the melt were also observed. © 1995 John Wiley & Sons, Inc.

Novel Ion Containing Liquid Crystals and Liquid Crystalline Main Chain Polymers Based on Trans-1,2-Bis(4-Pyridyl Ethylene) Mesogen

Abstract Ion bearing liquid crystalline compounds based on trans-1,2-bis (4-pyridinium) ethylene were synthesized and studied. These compounds included low molecular mass analogues, twin model compounds and main chain ionomers. Most of these exhibited liquid crystallinity of the smectic type. As a general rule the introduction of charges into the mesogenic moiety increased the stability (transition temperature) of the mesophase. Large supercooling effects, broad mesophase intervals and tendency to polymorphism are common features of these ionic mesogenic compounds. In the case of twin model compounds a variety of smectic phases were observed. The nature of these was dependent on the nature of the counterion and the thermal history of the compound. Tosylate or p.toluene sulfonate counterions promoted a higher degree of order in smectic mesophases as well as higher transition temperatures compared to methylsulfonate counterions. A pronounced odd-even effect of the isotropisation temperature of the twin compounds was observed. Compounds with even numbered methylene spacers display higher transition temperatures than those with an odd spacer. Only polymers with methylsulfonate and toluenesulfonate counterions were found to exhibit both, thermotropic and lyotropic mesophases. Thermotropic mesophases were characterized as smectic mesophases of lower order (S A ). The nature of lyotropic mesophases were not determined. High transition temperatures and concommitant oxydative crosslinking made the study of polymers with halogen counterions difficult. Polymers with halogen and perchlorate counterions were found to be poorly soluble in water precluding the formation of lyotropic mesophases. The introduction of a few siloxane bonds into the flexible spacer joining the charged mesogen moieties has a dramatic effect on lowering the transition temperatures and making possible the study of mesophases up to their isotropisation temperature. Preliminary characterization of polymers with siloxane containing spacer and methylsulfonate/tosylalte counterions suggest a smectic mesophase of lower order.

Nematic, smectic and columnar phases of main‐chain liquid crystal polyethers

AbstractA number of interesting thermotropic mesophases and transitions occur in main‐chain polyethers and copolyethers based on aliphatic spacers and various semi‐flexible mesogens of the general 1‐(4'‐hydroxyphenyl)‐2‐(2‐R‐4‐hydroxyphenyl)ethane (RBPE) or 1‐(4'‐hydroxybiphenyl‐4‐yl)‐2‐(4‐hydroxyphenyl)propane (TPP) type. The unusual effects described here are (i) nematic‐nematic transition, (ii) smectic‐B (crystal‐B) phase and its shear‐induced orientation, (iii) calamitic columnar phase, and (iv) nematic‐columnar phase sequence.

Synthesis of hybrid liquid crystalline block copolymers by combination of cationic or promoted cationic and free-radical polymerizations

The synthesis of two new classes of liquid crystalline block copolymers (1–5) by a two-step combination of cationic and free-radical polymerizations is described. An azomacroinitiator was prepared first by cationic polymerization of tetrahydrofuran or promoted cationic polymerization of cyclohexene oxide, which was then used to initiate the free-radical polymerization of an acrylate monomer containing a variously substituted biphenyl mesogenic group. The semicrystalline and liquid crystalline blocks were essentially microphase-separated and gave rise to different thermotropic mesophases.

Stability of the interphase interaction and changes in the microdestruction mechanisms in low density polyethylene on high energy irradiation

The effect of gamma irradiation on non-isothermally crystallized low density polyethylene (LDPE) has been studied by thermal-mechanical analysis (TMA) and transmission and scanning electron microscopy (TEM and SEM, respectively). Analysis of the TMA curves of the irradiated samples shows that the radiation-induced crosslinking of the macromolecular chains in the dose range 0–10 Mrad causes an increase, decrease and partial freezing of the mechanical microstresses located at the phase surfaces in the bulk of the polymer. These effects are explained in terms of the physical kinetics and macromolecular mobility. At temperatures above 120 °C., gamma-irradiated LDPE with doses above 5 Mrad develops a physical state corresponding to a high elastic state with thermotropic mesophases. Below 5 Mrad, controllable changes of the melting entropy and enthalpy of the crystalline regions and the free energy of the crystalline lamellae can be achieved. The TEM and SEM study shows an increase in the degree of co-operation of the sublevels of the superstructural molecular organization with the irradiation dose, even in areas that undergo plastic microdestruction.