Mesophase Development Research Articles

Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)

Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is explored as a useful characterization technique in the study of carbonization and graphitization of organic precursors. A model series of carbon materials was prepared from highly graphitizable petroleum pitch heat treated in the range 200–2730°C. Initial characterization was performed using the established techniques of X-ray diffraction (XRD), He pycnometry, TEM, electron diffraction and high-resolution lattice imaging (HREM). EELS in the TEM was then examined. Two routes are presented to quantify the change in the proportion of sp 2 type hybridization accompanying the heat treatment as the material transforms to the graphitic state. Both routes suggest an initial relative sp 2 content of ∼70%, rapidly increasing to ∼90% during mesophase development and carbonization, and then slowly increasing to 100% during graphitization. The peak position of the bulk valence plasmon (π + σ) is shown to be an excellent measure of the degree of graphitic character, and its fundamental dependence upon sample density (ρ) is confirmed. The appearance and definition of features within the core loss region representing the density of unoccupied σ* states are demonstrated to be an excellent measure of the extent of order. Finally, a method is established by which to extract the C–C bond length from core loss EELS spectra with an accuracy of ±0.1 pm. This method suggests an average bond length of 1.44 Å in samples with low heat treatment temperatures, decreasing to the theoretical length of 1.42 Å as both the heteroatom content and proportion of non-sp 2-type hybridized carbon atoms decrease.

Interplay of Ionic, Hydrogen-Bonding, and Polar Interactions in Liquid Crystalline Complexes of a Pyridylpyridinium Polyamphiphile with (Azo)phenol-Functionalized Molecules

Most supramolecular side-chain polymers investigated to date are based on a single type of noncovalent link and involve systems that do not test its selectivity in the presence of other potentially interfering interactions. In this paper, we study the selectivity of hydrogen-bond complexation in the presence of both ionic moieties and highly polar groups and evaluate the effect of these potentially interacting functions on the self-assembly and thermotropic behavior of the intended side-chain liquid crystal (LC) polymer complexes. To this end, the weakly mesomorphic polyamphiphile, poly(ω-pyridylpyridinium dodecyl methacrylate) bromide (P12PP), where the side chains are terminated by a hydrogen-bond acceptor in close proximity to an ion pair, is mixed in equimolar proportion with a series of phenol-functionalized mesogenic complexants (X), some of which have highly polar tails, while others have chiral tails and most contain an azo moiety for potential optical responsiveness. It is found that, in all cases, hydrogen-bond complexation is successful and essentially complete, leading to supramolecular homopolymeric complexes P(X) with well-defined thermotropic LC characteristics. These complexes, which are partially crystalline initially, have above-ambient glass transitions and smectic LC mesophases that are monotropic, enantiotropic or effectively enantiotropic, depending on the molecular characteristics of X. The presence of a polar tail hampers recrystallization after melting, with a cyano tail suppressing it completely and also strongly retarding the development of the LC mesophase from the isotropic phase. This might be related to enhanced electrostatic interactions and increased viscosity in the melt. The mesophase periodicity of P12PP is unchanged by complexants with a flexible alkyl tail, suggesting that they can adapt to the packing structure of P12PP (previously reported to be partial bilayer smectic A-like), whereas more rigid albeit shorter complexants lead to a periodicity increase (indicative of an effective single-layer smectic A mesophase). Possible molecular packing models are presented and discussed.

Cyclopalladated Azo- and Azoxybenzene Mononuclear Complexes Containing a Chiral Chelating Ligand

Chiral metallomesogens are materials of great interest for different technological applications. In most of the reported chiral liquid crystals, the chiral centre is located remote from the central core of the molecule. Our goal is to prepare new chiral metallomesogens featuring the stereogenic centre as close to the metal ion as possible. Accordingly, new mononuclear cyclopalladated complexes based on azo or azoxybenzene ligands (4,4′-bis(octyloxybenzoyloxy)azobenzene, HAzoCOO-8, p-azoxyanisole, HAzoxy, 4,4′-bis(hexyloxy)azobenzene, HAzo-6, or 4,4′-bis(hexyloxy)azoxybenzene, HAzoxy-6) containing the chiral chelating ligand (D-(−)-α-Phenylglycinol, H(Phenylgly)) have been synthesized and fully characterized. 1H NMR spectra of the azoxybenzene cyclopalladated complexes show the presence of just one isomer while those of the products obtained from the azobenzene ligands evidence the formation of isomeric mixtures. Nematic mesophases have been observed for complexes [(AzoCOO-8)Pd(Phenylgly)] and [(Azoxy-6)Pd-(Phenylgly)]. Mixtures of HAzoCOO-8 and the corresponding cyclopalladated complex, [(AzoCOO-8)Pd(Phenylgly)], in different molar ratios, have been prepared in order to promote the development of chiral mesophases.

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO 2:CTAOH:H 2O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N 2 adsorption–desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.

06/01610 Tar yields from low-temperature carbonization of coal facies from the Powder river basin, Wyoming, USA: Stanton, R. W. et al. International Journal of Coal Geology, 2005, 63, (1–2), 13–26

The influence of the addition of first stage FeCl3-graphite intercalated compounds (GIC) or pristine graphite to coal tar pitch (CTP) upon the formation and the development of mesophase has been studied at 400, 430 and 450 °C. Differences in the final samples were principally observed and studied via polarized light microscopic examinations of partially carbonized portions of the green-cokes. The insolubility was also investigated. The addition of pristine graphite to CTP inhibits the development of the mesophase: its presence generates a physical barrier which hinders the mobility and the coalescence of mesophase spherules. In the case of the addition of GIC to the CTP, the Lewis acid FeCl3 desorbed out of the graphene layers plays the role of a catalyst during the early stages of the mixture carbonization, by enhancing the rate of formation of the mesophase.

Hydrothermal synthesis of MCM-41 using differently manufactured amorphous dioxosilicon sources

The influence of differently manufactured amorphous dioxosilicon sources such as fumed silica (FS), spray dried precipitated silica (PS1) and flash dried precipitated silica (PS2) on the course of structural development and properties of hydrothermally prepared siliceous MCM-41 phases have been investigated. Powder XRD, N2 adsorption/desorption and scanning electron microscopy techniques were used for sample characterization. On the basis of results obtained from time dependent study on the progressive development of MCM-41 mesophases, the reactivity trend of dioxosilicon sources observed was as: FS>PS1>PS2. MCM-41 synthesized using least expensive PS2 having less surface area has exhibited thicker pore walls but poor orderness, while MCM-41 prepared from expensive FS having high surface area has thinner pore walls and was well ordered. The extent of contraction caused by calcination, magnitude of the wall thickness and structural stability were found to be dependent on manufacturing process of dioxosilicon sources. The noticeable morphological differences were also observed by variations in the type of dioxosilicon sources used in the hydrothermal synthesis MCM-41 materials.

Pyrolysis behaviour of pitches modified with different additives

The wetting properties of a petroleum pitch were improved by adding three additives from the fine chemical industry in different proportions. The effect of the additives on the subsequent pyrolysis behaviour of the pitches was studied by thermogravimetric analysis and optical microscopy. The results show that the additives modify the TG/DTG profiles of the pitches and increase the carbonaceous residue at 1000 °C. This suggests that there is interaction between pitch/additive components during pyrolysis. The examination and quantification of the different mesophase features (single spheres, domains and mosaics) in the pyrolysis products obtained from the pitches at 430 °C corroborates the concurrence of interactions. The additive consisting of alkyl/carbonyl derivatives (additive B) and the additive composed of alkyl/sulphur derivatives (additive C) accelerate the formation of mesophase without altering the mechanisms of mesophase growth. On the other hand, the additive that is made up of a light fraction of esters and a heavy fraction of phosphorous derivatives (additive A) affects both the kinetics and mechanisms of mesophase development. The cokes obtained at 900 °C clearly reflect the changes produced in the pyrolysis products at 430 °C by the additives.

04/00643 Mesophase development in petroleum and coaltar pitches and their blends: Mora, E. et al. Journal of Analytical and Applied Pyrolysis, 2003, 68–69, 409–424

04/00643 Mesophase development in petroleum and coaltar pitches and their blends: Mora, E. et al. Journal of Analytical and Applied Pyrolysis, 2003, 68–69, 409–424

Emissive Anisotropic Polymeric Materials derived from Liquid Crystalline Fluorenes

The synthesis and characterization of photoluminescent crosslinking agents containing a fluorene core are described. They exhibit an efficient fluorescence and have a promesogenic molecular shape that depending on the substitution results in the development of thermotropic mesophases. A difunctionalized fluorene derivative has been incorporated into a liquid crystal host in order to produce polarized light emitting films. These films have been prepared by the in-situ photopolymerization technique.

Influence of Granular Carbons on the Thermal Reactivity of Pitches

Mixtures of various pitches and granular carbons were pyrolyzed at 430 °C in order to study the influence of different granular carbons on pitch pyrolysis behavior. The toluene-insoluble content was used to monitor the degree of polymerization of the pitches during thermal treatment. The influence of the presence of granular carbons on mesophase development was studied by optical microscopy. Important differences in the pyrolysis behavior of the pitches were observed with the different granular carbons used. The differences were more pronounced for green petroleum coke and graphite. Green petroleum coke accelerated condensation and polymerization reactions. Graphite, however, slowed polymerization of the pitch components, causing mesophase to form and grow on the surface of the graphite particles.

Mesophase development in petroleum and coal-tar pitches and their blends

The behaviour of mixtures of an impregnating coal-tar pitch and a petroleum pitch during the early stages of carbonisation was studied, focusing on the formation and development of mesophase and the role played by primary quinoline insolubles (QIs). The blends were prepared in different proportions ranging from 20 to 80% of the impregnating pitch. The parent pitches and their blends were thermally treated at 430 °C for 3 h and the resultant products were characterized by thermal analysis and optical microscopy. The results showed that primary QIs not only control the coalescence of mesophase spheres but also the nucleation of new spheres in both parent pitches and their blends. Moreover, only the petroleum pitch was able to generate a massive amount of small spheres throughout the isotropic phase.

The effect of the reinforcing carbon on the microstructure of pitch-based granular composites.

Carbon composites were prepared with four pitches (a commercial impregnating coal-tar pitch, two thermally treated pitches and an air-blown pitch) and four granular carbons (anthracite, graphite, green petroleum coke and foundry coke). Granular carbon/pitch proportions were optimized for each composite and differed in the characteristics of the single components. Interactions of the pitch with the granular carbons during pyrolysis and their subsequent effects on the microstructure of the final composite were monitored by light microscopy. The results show that the light texture of the matrix and the porosity of the composite depend not only on the chemical composition of the pitch but also on the specific granular carbon used as reinforcing material. The same pitch may generate different light textures depending on the characteristics of the carbon. Composites from thermally treated pitches and graphite show highly ordered matrices orientated in the direction of graphite planes. Graphite particles seem to exert a huge influence on mesophase development during the pyrolysis of the treated pitches, affecting not only the orientation of the mesophase, but also reducing the rate of mesophase formation. On the other hand, when green petroleum coke is used with the thermally treated pitches, matrices show a small size light texture, due to the high reactivity of the pitch in the presence of this granular carbon. The porosity of the composites is controlled by both the pitch and the granular carbon.

Development of Mesophase from a Low-Temperature Coal Tar Pitch

In this study, a low-temperature coal tar pitch (CTP1) was subjected to carbonization in order to study the formation, growth, and development of mesophase achieved under different conditions. The results were compared to those obtained with a petroleum pitch (PP) and a conventional high-temperature coal tar pitch (CTP2). CTP1 was converted to mesophase at a considerably lower temperature, and also mesophase develops and coalesces at a markedly higher rate for this pitch. Volatile matter removal is a key factor in mesophase formation, but chemical composition also plays an important role, with substituted cata-condensed compounds constituting very reactive components in terms of anisotropic material development. More specifically, the oxygenated functional groups and aliphatic substituents facilitate the growth of mesophase. In particular, the high concentration of phenolic compounds present in CTP1 seems to contribute to the fast coalescence of mesophase spheres. The effect of several additives was also ...

Chemistry of the co-pyrolysis of an aromatic petroleum residue with a pyridine–borane complex

Synthesis of boron-doped mesophase by controlled co-pyrolysis of an aromatic petroleum residue and a pyridine–borane complex, PB, as a boron source, was carried out. Pyrolysis was performed at temperatures ranging from 400 to 440 °C, under 1 MPa nitrogen atmosphere. Soaking time was varied between 1.5 and 6 h, and the boron concentration in the pyrolysis mixtures ranged from 0 to 1 wt.%. The effect of the presence of boron upon solid yield, insoluble content, mesophase development and microstructure was studied. PB reacts with molecules of the petroleum residue and creates B–C bonds, B substituting C in the polyaromatic molecules, with sp 2 hybridisation, and probably also creating B crosslinking between polyaromatic units. The enhanced reactivity caused by B addition is reflected in an increment of viscosity; as a result, the development of mesophase is strongly increased, but the size of the mesophase structures is decreased to a mosaic texture; B also causes an increase of the interlayer spacing and a noticeable enhancement of insoluble material.

Characteristics and carbonization behaviors of coal extracts

N-methyl pyrrolidone (NMP) raw coal extract (EXT), hydrogenated coal extract (HEXT) and the blend of EXT and HEXT in NMP (BLD), from two bituminous coals, were studied. The extracts were carbonized in both tube-bomb and a temperature programmable furnace. Elemental analysis, FTIR spectroscopy and optical microscopy techniques were employed to characterize the extracts and the carbonization residues. It was found that the extracts resembled petroleum-derived pitches in the hydrogen content and (C/H) atomic ratio. Higher oxygen and nitrogen contents differentiated the coal extracts from commercial petroleum pitch. More carbon and hydrogen, and lesser oxygen and sulfur differentiated HEXT from EXT. The ratios of integrated IR band intensity for aromatic and aliphatic CH stretching indicate that the relative content of aliphatic hydrogen in EXT is higher than in HEXT. HEXT contains comparatively more aromatic hydrogen, a feature necessary for thermal stability and fluidity during carbonization. BLD materials are at a place somewhere in between. Kinetic modeling of the aliphatic hydrogen change during carbonization reveals that EXT has high carbonization rate and low apparent activation energy. This can be related to the optical texture size of carbonization residues. The residues made from EXTs exhibited fine mosaic optical texture and limited mesophase development. HEXTs were readily converted into highly anisotropic coke. BLDs produced carbonization residues with intermediate properties. Extracts with similar activation energies produced similar residues in the same coal series. The degree or extent of anisotropy displayed by the carbonization residues was found to be dependent on the relative distribution of aromatic and aliphatic hydrogen.

A Comparative Tg-Ms Study of the Carbonization Behavior of Different Pitches

The purpose of this work was to study the formation of mesophase spherules from a low-temperature coal tar pitch under carbonization conditions. For comparison, the carbonization of a high-temperature coal tar pitch and a petroleum pitch were also considered. Different operating conditions during the carbonization process were used in an attempt to cover different degrees of mesophase formation and development for each pitch. The parent pitches and the semicokes thus obtained were analyzed by elemental analysis, optical microscopy, and Fourier transform infrared spectroscopy (FT-IR). More significantly, the samples were subjected to thermal decomposition under well-controlled operating conditions from room temperature to 850 °C in a thermogravimetric analyzer (TG). The use of a mass spectrometer linked to the TG (TG-MS) provides additional data about the devolatilization process, yielding information about the evolution of different volatile products and about possible chemical reactions occurring during thermal decomposition. Thus an insight into the process of mesophase formation is obtained. The results from FT-IR, elemental analysis, and the TG-MS tests were compared with the different extents of mesophase formation, checked by optical microscopy. According to the results, several stages can be distinguished as temperature increases in the carbonization process of the pitches. In the low-temperature coal tar pitch, the devolatilization of light components, especially phenols, accounts for the most significant weight loss. Moreover, cross-linking contributes greatly to the formation and development of mesophase, resulting in the predominance of bulk mesophase in a relatively short time in the case of the low-temperature coal tar pitch.

Structural modification of coal-tar pitch fractions during mild oxidation—relevance to carbonization behavior

Modified pitches with softening points of about 175 °C were prepared by air-blowing at 300 °C of coal-tar pitches from different commercial coke-oven tars. The modifications induced by the mild oxidation were monitored using solvent and extrographic fractionation, elemental analysis, 1H NMR and HPLC. Optical microscopy was used to follow the effect of air-blowing on carbonization behaviour. Low molecular weight cata-condensed PAHs and those with basic nitrogen and hydroxylic functionalities present in extrographic fractions F2 and F4, respectively, are preferentially polymerized pitch constituents. In contrast, peri-condensed PAHs in extrographic fractions F2 and F3, are practically unreactive under the oxidation conditions used. The mild oxidation enhances the tendency of quinoline insoluble (QI) particles to form aggregates in an early stage of thermal treatment, modifying the mode of mesophase development and leading to a non-homogeneous optical texture. The enhanced propensity of QI to aggregation is discussed in terms of structural peculiarities of the parent pitch and possible oxidative polymerization reactions.

Solidification behaviour of binary sitosteryl esters mixtures

Phytosterol esters are known for their blood cholesterol lowering effect and have consequently been incorporated into vegetable oil table-spread products. In furtherance to gaining knowledge about these materials in a fat-blend environment, the current work describes the phase-diagram based thermal characteristics of a series of β-sitosterol alkyl-ester homologues prepared as binary mixtures and observed using differential scanning calorimetry. A scanning speed of 10 K min −1 was chosen and, whilst not representing true equilibrium conditions, was more indicative of the likely dynamic melting or cooling conditions found in a fat-blend product during processing. The thermal behaviours presented reflect therefore this concept. Long-chain saturated esters showed ideal solubility behaviour of immiscible solids. In contrast, the shorter-chain homologues produced complex and dynamic phase behaviours, including some mesophase development. In all cases, supercooling was observed on cooling due to retarded crystallisation, which could equally result in two crystal conformations, depending on the actual composition.

Mesophase development in coal-tar pitch modified with various polymers

The effect of 10% addition of various polymers to coal-tar pitch on its conversion into mesophase during isothermal treatment at 450 °C has been studied using polarized light optical microscopy. The additives appear to modify to a different extent the kinetics of the conversion and morphology of the developed mesophase. A common effect is an acceleration of mesophase unit growth in the early stage of transformation, the influence decreasing from polystyrene to cumarone–indene resin, and, at the advanced stage, an increase of proportion of areas of isotropic appearance in the optical texture. The addition of poly(vinyl chloride) leads to non homogeneous texture of mesophase pitch due to early separation of particulate matter and growth of mesophase in a way typical of pitch matrix which is free from quinoline insolubles. Poly(ethylene glycol) and unsaturated polyester resin tend to hinder the ability of mesophase to coalescence giving products of enhanced proportion of spherical mesophase. The cumarone–indene resin is the only additive that distinctly increases residue yield but does not affect its optical texture. Based on the results the behaviour of polymers within pitch matrix under carbonization conditions and possible interactions of polymer degradation products with pitch constituents are discussed.

Co-pyrolysis of an aromatic petroleum residue with triphenylsilane

The effect of silicon on the pyrolysis of a petroleum residue has been studied. In this work, controlled co-pyrolysis of an aromatic petroleum residue and triphenylsilane, TPS, as a silicon source, under 1 MPa nitrogen atmosphere and at 440°C is performed. Soaking time was varied between 1.5 and 6 h, and the silicon concentration added to the petroleum residue ranged from 0 to 2 wt%. Silicon content in the final solid product increases with soaking time. The effect of silicon upon the solid yield, insoluble content and mesophase development is appreciable only when the concentration of silicon in the residue is above 0.4 wt%. Initially TPS acts as a diluting agent and a mild proton donor, inhibiting reactivity of the radicals formed in the system and molecular growth. As reaction progresses TPS reacts, creating tetrahedral bonds within macromolecules and, as a consequence, after a period, molecular growth is enhanced. However, the planar stacking leading to mesophase formation and growth is inhibited by silicon and consequently the development of mesophase structures is limited to small spheres and fine mosaics. Optically isotropic material is obtained when silicon concentration is above 1 wt%.