Mesophase detected

MESOPHASE PITCH is a PITCH with a complex mixture of numerous, essentially aromatic hydrocarbons containing anisotropic liquid crystalline particles (CARBONACEOUS MESOPHASE), detectable by optical microscopy and capable of coalescence into the BULK MESOPHASE. 

This kind of mesophase, which was also detected by various experimental methods, possesses variable properties, accomodating the two extremes between inclusions and matrices. 

For crystalline polymer systems, transition from the crystalline structure to a mesophase structure occurs, whereas for amorphous polymer systems, the mesophase occurs after the Tg has occurred. Some polymer LC systems form several mesophases. Mesophases can be detected using DSC, x-ray diffraction, and polarizing microscopy. 

The analogy between three- and two-dimensional phase diagrams can be carried much further. Monomolecular amphiphilic films show ordered phases similar to three-dimensional systems [579], The phases of an amphiphilic monolayer can be detected most conveniently in pressure-area (7r-versus-OA) isotherms. These may look different for different substances. The behavior of simple amphiphilic molecules, like long-chain alcohols, amines, or acids, was extensively investigated (reviews Refs. [580,581]). In monolayers so-called mesophases can occur. In a mesophase the tail groups are ordered over relatively large areas, while the order in the hydrophilic head groups is only over a much smaller distances. 

The growth units of mesophase can be detected of size <0,1 ym diameter in the pitch using transmission electron microscopy (TEM)... 

Composition of Parent Pitch. Once the chemical composition of the carbonizing system moves away from the comparative simplicity of polynuclear aromatic hydrocarbons to that of industrial pitches, then the pyrolysis chemistry incorporates effects caused by the presence of heteroatoms (0, N and S) and alkyl and naphthenic groups. In general terms, the system becomes more Reactive creating higher concentrations of radicals detectable by ESR. This in turn, leads to enhanced cross-linkages and polymerization of molecular constituents of any mesophase which is formed, and this causes enhanced viscosity and a reduction in size of optical texture. 

These methods, described above, are to produce material (still loosely called mesophase) which essentially is a feedstock for other process developments. This "mesophase" prepared at temperatures below normal carbonization temperatures can be called low temperature mesophase pitch (LTMP). The term mesophase pitch has crept into the vocabulary of this subject and is thought to refer mainly to mesophase as a feedstock. Its anisotropy can be detectable by polarized light optical microscopy. 

Detection of thermotropic mesophase presence in PMCS-4 has induced different opinion on poly-organocyclosiloxanes), because heretofore poly(dimethylsiloxane) had been considered one of the most flexible-chain polymers. More thorough study of the spatial structure of PMCS-4, performed by 29Si NMR spectroscopy method, has shown that HFC... 

The effect of molecular mass on occurrence of the mesomorphous phase in POCS-6 polymers is de-tected. Results displayed for POCS-6 polymers, in which configuration sequences of units alternate irregularly along the backbone, indicate that the ability to crystallization in them is disturbed. For the mesomorphous phase, strict regularity of units is not the necessary condition of its occurrence. In connection with the difference between studied POCS-6 polymers by the type of tacticity and mole-cular mass, the authors have separated effects induced by these two factors and detected more accu-rate, which type of configuration sequences creates conditions for mesophase stabilization. 

In this chapter we want to discuss the correlation of the mesophase behavior of a cyanobiphenyl-based SCLCP with its backbone structure. As shown before, the backbone structure, the spacer lengths, and the mesogen density per repeat unit have great influence on the LC mesophase evolved. Ligure 8 shows some examples of backbone structures bearing the cyanobiphenyl-moiety that have been reported in literature. The above-mentioned ROMP-derived polymers poly-(II-n) [39],poly-(IV-n) [42,47],poly-(VI-n) [41],andpoly-(VII-n) [53] will be compared with each other and with acrylate-based [56-59], siloxane-based [60] and vinylcyclopropane-based systems [61]. The detected mesophases and their transition temperatures are summarized in Table 6. 

A mixed [60]fullerene-ferrocene dendrimer (Fig. 65) was also found to be mesomorphic, showing a room temperature smectic A phase which cleared at 157 °C (no glass transition temperature was detected) [294,295]. The mesophase was only slightly destabilized upon the addition of Ceo when com-... 

Three types of copoly(Y-n-alkyl L-glutamate)s with the combination of methyl-hexyl (MH), methyl-octyl (MO) and propyl-octyl (PO) were prepared by alcoholysis of PMLG in ethylene dichloride using p-toluenesulfonic acid as the catalyst at 60 °C. The thermotropic mesophase was detected by the iridescent cholesteric color which appeared by annealing at temperatures in the region 110 190 C. Also the circular dichroism due to the selective reflection was measured at room temperature for the quenched films. Only the films with the copolymer composition of about 50/50... 

The even-odd effect was first detected in the series of nematic polyesters prepared by Griffin and Havens, the mesogenic structure of which is listed as 35 in Table 1 They also found that the temperature range for mesophase stability decreased as the spacer length increased, and the copolymer with the longest spacers in their series (10 and 12 methylene groups) had the lowest transition temperatures . 

The polymers with diethylene oxide spacers (PEO dimer) reportedly had, in addition to a nematic mesophase, a monotropic smectic mesophase that was detected by X-ray diffraction. No confirmation was available by microscopy because almost all polymers developed a homogeneous texture that gave no real detail. 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

Several fundamental studies have shown the importance of monomer sequence distribution on mesophase behavior (26). Simply changing the direction of ester linkages in a chain affects the transition temperatures, the range of the mesophase stability and, in some cases, even the mesophase texture (2Z). Polyester chains are susceptible to transesterification, which raises the question of which sequence structure is actually responsible for the properties observed for a given polymer. A recent study of aromatic LC polymers by neutron scattering indicates that transesterification occurs in the mesophase at rates twice that in poly(ethylene terephthalate) (28). Such behavior has also been observed to occur in other aromatic polyesters where rapid sequence redistribution was detected by nmr, see for example, the chapters by Jin and Economy et al. The temperature dependence of this effect has not been fully explored, and it may not be as pronounced in those polymers which exhibit mesophase behavior at lower temperatures, for example, those with aliphatic spacers. 

Admittedly, the existence of the postulated transient mesophase would still require structural confirmation. Even so, the taking of a sharp drop In viscosity as Indicator of mesophase formation has well established precedents In the liquid crystal field. Such Is e.g. the well documented effect In Kevlar referred to above (12) which In many respects has similarities to the presently discussed PE, except that Kevlar Is "mesogenlc" and can exist as stable liquid crystal under ambient conditions, while the mesophase In the flexible PE Is "virtual". The latter "virtual" phase only becomes "real" transiently, which suffices to dramatically affect the entire flow behavior of the material, the effect through which It Is being detected. 

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