Low-frequency dielectric relaxation in columnar hexagonal and micellar inverse cubic mesophases

Dielectric measurements on four diols showing columnar hexagonal and micellar cubic phases were carried out. Besides the known fast local reorientation in theGHz-range, a new absorption in theMHz region was detected. This mechanism was interpreted as a collective motion of the molecules inside the micelles or cylindrical aggregates. The absorption was also found in the isotropic phase. Based upon the interpretation given, the molecular aggregates should also exist in the isotropic state. The electrical conductivity strongly supports the classification as inverse phases.     

Freeze-fracture electron microscopy of thermotropic cubic and columnar mesophases

Freeze-fracture electron microscopy has been applied to study thermotropic columnar and cubic phases of a polycatenar liquid crystal of silver(I). The columnar phases displayed hexagonal type II characteristic fracture planes. The cubic phases showed a fracture behaviour highly characteristic of the Ia 3 d type II (Q230II) cubic space group, namely the high frequency of well ordered domains perpendicular to the \[2 1 1] crystallographic axis and, much less frequently, the fragmented aspect of domains perpendicular to the \[1 1 0] crystallographic axis. For both types of domain, the observed 2-D lattice parameters were in excellent agreement with the ideal values expected for the Q230 cubic space group.     

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).     

Induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system

The induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system is achieved by mixing hexakispentyloxytriphenylene 1 with a long chain derivative of trinitrofluorenone 3. The difference in chain length has a strong influence on the packing behaviour due to steric effects. The long hydrocarbon chains of the acceptor introduce a strong asymmetry into the electron donor acceptor complex. It could be shown by differential scanning calorimetry, optical microscopy and X-ray measurements that a nematic columnar phase is formed. In this mesophase the triphenylenes form columns but no hexagonal or orthorhombic lattice is built up. Each column behaves like a rod-like nematic mesogen. To prove that the long hexadecane alkyl chains of the acceptor are responsible for this induction, the acceptor 3 was mixed with the non-liquid-crystalline triphenylene derivative 2 containing six hexadecyloxy side groups. The long alkyl chains of the acceptor dissolve perfectly in the side chain region of the discs. No asymmetry is induced and the columns formed can be arranged on a hexagonal lattice resulting in a D_ho phase.     

A positron annihilation study on phase transitions in trans-stilbene single crystal

Positron lifetime spectroscopic and Dopplerbroadening measurements have been carried out on trans-stilbene single crystals. The results (including the complementary DSC measurement) give new evidence for the existence of phase transitions in trans-stilbene at temperatures between 20 and 300 K.     

Thermotropic liquid crystals of 1H-imidazole amphiphiles showing hexagonal columnar and micellar cubic phases

The linear and polycatenar type 1H-imidazole amphiphiles showing a strong self-assembly tendency to build various supramolecular structures in bulk were synthesized by the esterification reaction of 4′-alkyloxy phenols (for 1-4) and hydroxyphenyl trialkyloxybenzoates (for 5-7) with 4-chlorocarbonyl imidazole. The linear 1H-imidazole amphiphiles formed thermotropic smectic phases, but compound 4 with an ethyl group instead of hydrogen on 1N did not show a mesophase. The polycatenar type 1H-imidazole amphiphiles (5-7) formed thermotropic hexagonal columnar and cubic phases. A phase transition was observed from a columnar phase to a cubic phase as the temperature increased on heating, and vice versa on cooling. In the POM study for compounds 5-7, optically isotropic phases first appeared on cooling from the isotropic melts, and then birefringent mesophases with a nonspecific texture appeared on further cooling. The X-ray analysis shows that the optically isotropic phases were very likely micellar cubic phases with Pm3n symmetry. The birefringent phases were confirmed as hexagonal columnar phases.     

On structural transitions in a discontinuous micellar cubic phase loaded with sodium diclofenac

An intermediate mesophase of lyotropic liquid crystalline structure from the ternary mixtures of glycerol monooleate, water, and ethanol was recently characterized in our lab. This mesophase, termed Q(L), consists of discrete discontinuous micelles arranged in a cubic array. The Q(L) phase can solubilize very significant loads of water-insoluble anti-inflamatory drug sodium diclofenac (Na-DFC). Close examination of the internal structures of the lyotropic liquid structure upon increasing the solubilization loads reveals the existence of three structural transitions controlled by the Na-DFC levels. Up to 0.4 wt% Na-DFC, the Q(L) structure remains intact with some influence on the hydration of the headgroups and on the intermicellar forces. However, at 0.8 to 1.2 wt% Na-DFC, the discontinuous micellar cubic phase is transformed into a more condensed mesophase of a bicontinuous cubic phase. At > or =1.2 wt% Na-DFC, the cubic phase is converted into a lamellar phase (L(alpha)). Within 5.5 to 7.3 wt% Na-DFC the mesophase is progressively transformed into a less ordered lamellar structure. At 12 wt% Na-DFC crystals tend to precipitate out. At low Na-DFC concentrations the drug behaves like a lyotropic or kosmotropic salt and can salt-out the surfactant from its water layer, but at higher levels it behaves like a hydrotropic, chaotropic salt and can salt-in the surfactant. The Na-DFC location and position within the interface as well as its polarization and partial ionization are strongly affected by its solubilization contents and the structure that it is inducing. In the cubic phase the drug is located less close to the hydration layer while once transition occurs it is exposed more to the water layer and the surfactant headgroups.     

Investigation of transitions and relaxation processes in polystyrene by using positron annihilation

The glass transition and relaxation processes in polystyrene resins with the number average molecular weight ranging from 7.0·10² to 9.8·10⁴ were studied with the positron annihilation technique. The pick-off annihilation lifetime of ortho-positronium (₃) and its intensity (I ₃) were measured in the temperature range from 20 to 430 K. The glass transition temperature (T _g) was determined as an onset temperature coefficient of ₃.T _g shows the molecular weight dependence in these samples. BelowT _g, local motions were detected by measurements ofI ₃. The local motions could be observed above 100 K in this experiment.I ₃ show the minimum at around 250 K and it does not show molecular weight dependence.     

Synthesis and characterization of nanostructural polymer-silica composite: positron annihilation lifetime spectroscopy study

The swelling of poly(TRIM) spherical particles in TEOS is assessed as a potential way for obtaining polymer-silica nanocomposite materials. Silica deposition was achieved by simply stirring of swollen polymer particles in acidic hydrochloric-water solution. This procedure leads to spherical composite particles with dispersed silica gel within the polymer matrix. The resulting material exhibits the same morphology as the initial polymer. Nanocomposite particles are silica rich (about 17 wt.%). Characterization of the nanocomposites was performed using scanning electron microscopy, FT-IR spectroscopy, (29)Si CP MAS NMR spectroscopy and thermogravimetry. Moreover, the use of positron annihilation lifetime spectroscopy PALS to characterize the structural properties of the nanocomposites is presented. This technique gave more realistic pieces of information about the pore structure of the investigated samples in contrast to nitrogen adsorption studies.     

A positron annihilation lifetime study of isothermal structural relaxation in bisphenol-A polycarbonate

Positron annihilation lifetime spectroscopy has been used to study the isothermal relaxation response of compression molded bisphenol-A polycarbonate at temperatures of 263, 273, and 303 K. The temperature dependence of both the lifetime and intensity of the ortho-Positronium (o-Ps) pickoff component is discussed in terms of ductile-to-brittle transition behavior and free volume theory. An additive exponential model and the Williams–Watt model were used to analyze the relaxation as a function of temperature and provided results consistent with the anticipated molecular mobility of polycarbonate at sub-T_g temperatures.     

Liquid crystalline dendrimer of the fifth generation: From lamellar to columnar structure in thermotropic mesophases

The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied. This dendrimer was synthesized by a hydrosilylation reaction and then the cyanobiphenyl mesogenic groups were chemically linked to the dendritic matrix via a-OOC-(CH2)10-Si(CH3)2OSi(CH3)2-spacer. Structural studies carried out by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods revealed unusual phase behaviour. At room temperature the dendrimer forms a lamellar (smectic A) phase which develops in-plane ordering above 40C.This is due toa tendency to form columns ofmolecules which areprobably perpendicular to the layers. Above 121C the material transforms into another more disordered mesophase which is probably a disordered hexagonal columnar phase. The proposed structures and molecular packing in these different types of mesophase are discussed.     

Infrared study of phase transitions in thermotropic liquid crystal polyesters

Three samples of semiflexible thermotropic liquid-crystalline polyesters based on mesogenic aromatic triads and a decamethylene spacer have been investigated by IR spectroscopy at various temperatures between room temperature and 290°C. The crystal-to-liquid crystal (either nematic or smectic) phase transition was accompanied by fairly strong spectral variations, whereas slight, but significant, changes in the IR profiles were detected at the liquid crystal-to-isotropic transition. By comparing the results obtained with the spectral behavior recorded for corresponding structural analogs of low molar mass, it was possible to attribute the spectral variations observed in the polymer samples to a decrease in intermolecular interactions rather than to conformational changes. The thermal transitions indicated by IR spectra were in good agreement with the analogous data obtained by calorimetric or optical microscopy techniques.     

Smectic, columnar and cubic mesophases in binary systems of hemiphasmidic and calamitic amphiphilic diols

A first example of an amphiphilic hemiphasmid consisting of a biphenyl rigid core connected to a hydrophilic 5,6-dihydroxy-3-oxahexyloxy group at one end and carrying two lipophilic dodecyloxy chains at the other end has been synthesized by a Pd-catalysed cross coupling reaction. The liquid crystalline properties of this compound have been investigated by polarized light microscopy, by differential scanning calorimetry and by X-ray diffraction. It exhibits a thermotropic hexagonal columnar mesophase which is stabilized on addition of formamide. On addition of ethylene glycol, a bicontinuous cubic mesophase is induced. Furthermore, binary mixtures of this compound with structurally related single chain amphiphiles have been investigated. Besides a smectic A phase, an induced columnar mesophase and a cubic phase were found in these mixtures.     

Evidence of nematic, hexagonal and rectangular columnar phases in thermotropic ionic liquid crystals

Dithiolium salts, with amphipathic character, are compounds of choice for investigations of the influence of an ionic feature upon mesomorphic properties. In this way, salts bearing a branched chain have been studied by SAXS. In spite of their rod-like shape, they exhibit only columnar mesophases, the supramolecular organization of which is close to that of cylindrical inverted micelles. Moreover, the nematic columnar phase, characterized by the loss of lateral positional correlations of the columns themselves, is one of the first examples of such behaviour in the case of thermotropic liquid crystals.     

Radiation induced nano-scale free volume modifications in amorphous polymeric material: a study using positron annihilation lifetime spectroscopy

Radiation exposure modifies the nano scale free volume of the polymers which has direct correlation to certain properties of polymeric materials. CR-39 (DOP) polycarbonate polymeric films were exposed to gamma radiation at different doses for the study of nano scale free volume as well as structural and optical properties. An overall increase in the number of free volume holes at all doses was observed. UV–visible studies showed an improvement in the optical properties due to decrease in direct band gap energy. X-ray diffraction spectra of the polymer samples showed the increased amorphous nature of the polymer.     

A study of inter-crystallite spaces in some polycrystalline inorganic systems using positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) was used for calculation of number density and effective sizes of free volume holes (inter-crystallite spaces) in polycrystal CaSO₄, CaCO₃ (vaterit) and Ca₁₀(PO₄)₆(OH)₂ (apatite). The effect of substitution of two-valence Ca(II) for three-valence Eu(III) on annihilation characteristics of apatite, studied together with the data on thermo-stimulated luminescence (TSL) and low-temperature sorption of gas (N₂), helped to elucidate mechanism of positronium atom (Ps) localization in the free volume holes and perform corresponding calculations. It came out that PALS is more sensitive to inter-crystallite sites (10¹⁶ cm⁻³) in polycrystallites than to the free volume holes in polymer glasses (10¹⁹ cm⁻³). This is due to higher mobility of the precursor of localized Ps in crystallites.     

Study of water in Ca-montmorillonite by thermal analysis and positron annihilation lifetime spectroscopy

Journal of Thermal Analysis and Calorimetry - The aim of this study is to characterize adsorbed liquid in montmorillonite structure for different levels of adsorption by both thermoanalytical and...     

Gas permeation and positron annihilation lifetime spectroscopy of poly(ether imides) containing main chain ethylene oxide segments

This article reports a study of four poly(ether imide)s with varying ethylene oxide (EO) segments lengths using positron annihilation lifetimes spectroscopy, wide angle X‐ray diffraction, and gas transport measurements. The measured properties change with the length of the EO segment. Comparison of the poly(ether imide) containing a single ether linkage with those containing one and three EO units, show progressive changes of the permeability and diffusion coefficient with void size. However, when six EO units are incorporated into the polymer backbone certain of the observed trends are reversed. Incorporation of flexible EO segments in the polymer backbone allows changes in the chain–chain interactions which increases the packing density and changes the void size and influences the solubility coefficients leading to variation of the gas transport characteristics. Differences in the measured solubility parameters reflect the extent to which the gases molecules are able to interact with the polymer matrix. The highest values obtained for the gas separation for carbon dioxide and nitrogen is observed when EO has a value of three. Further increasing of the length of the EO segments in the poly(ether imide) leads to a reduction the gas transport properties and hence the extent to which gas separation would be achieved. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1654–1662     

Formation of columnar mesophases by calamitic molecules: a modulated SmA phase in mixtures of amphiphilic and bolaamphiphilic biphenyl derivatives

Binary mixtures of bolaamphiphilic biphenyl derivatives with each other and with amphiphilic biphenyl derivatives were investigated by means of optical microscopy. The miscibility of the bolaamphiphiles is very sensitive to the molecular length of the components. The SmA phases of bolaamphiphiles with the same length are completely miscible. If the length difference between the two components of the binary system increases, a miscibility gap occurs. Due to their different phase structures (bilayer versus monolayer) no miscibility in the SmA phases was found for amphiphilic and bolaamphiphilic compounds with comparable molecular lengths. However, in some cases a novel mesophase was induced in the contact region. This mesophase was investigated by X-ray diffraction. It represents a two-dimensionally modulated (columnar) phase with a rectangular lattice (Col r), but the local order is similar to that of disordered smectics. Its formation is explained in terms of ribbon structures resulting from the collapse of smectic bilayers, in strong analogy to the antiphases (SmA) of terminally polar calamitic mesogens.