Mesophase forming samples

Extracted pitch behavior is also dependent on the pltch/solvent ratio used for the separation. Altering ratios will affect both the weight percent dissolved, and the properties of an extract. Increasing the pltch/solvent ratio of a polar or hydrogen bonded solvent can decrease their effectiveness and shift and reduce the size of the enhanced solubility region on solubility plots. In this way an over-effective solvent can be moderated so it extracts a mesophase forming sample. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

An examination of Table 1 reveals some interesting trends. As the solubility parameter of the extraction solvent increases, the mesophase content of the extracted fraction goes to >99%. Samples 3, 4, 5 and 6 are "neomesophases"0) in that upon melting, they instantly transform to a liquid crystalline state. Their quinoline insolubles contents were less than 1%. The extraction process breaks down the micelle and removes the non-mesomorphic species in the pitch which had inhibited the coalescence of the mesophase forming species. Sample 2, the heptane extracted fraction, did not exhibit neomesophase behavior. Extended heat treatment at 400+ C was required to convert this sample to a mesomorphic state. The heptane did not remove a sufficient amount of the non-mesophase formers. 

The properties of manufactured graphites are determined by the microstructure of the carbonaceous mesophase which is formed during pyrolysis, usually between the temperatures of 370 C and 500°C. The characteristics of the final product can be measured in a qualitative way by examination of the mesophase micro-structure. Several coal liquid asphaltene and petroleum pitch samples have been screened in this way to determine their suitability as precursors for graphite materials. The physical and chemical properties of the mesophase formed from the samples and their pyrolyses residues were studied(1). It was found that the phenolic oxygen present either in the precursor or by addition during heat treatment suppresses mesophase formation by crosslinking and preventing the development of fluidity(2J>... 

Typical examples of the temperature dependence of the relaxation rates of the a- and 6-relaxation (log and log 1 5) are plotted in Figures 8 and 9 for the polymers PM4 and PMg, respectively. The high temperature 6-relaxation process shows a behavior which is also found for liquid-crystalline polymethacrylates carrying phenylbenzoate as mesophase-forming unit (16,23). This means a step-like change of the temperature dependence of log 5 at the transition from the isotropic to the liquid-crystalline state. For the samples PMg and PM5 which have a smectic Wgh temperature phase no further step-like change at the phase transition fi om the smectic high temperature phase to the nematic low temperature phase is observed. This implies that... 

Generally mesophases form rapidly which leads to a diffusion-limited growth. Times of order seconds or less have been reported in T-jump experiments where a homogeneous sample is subjected to a temperature change and the time for the mesophase to form is measured 13), However, in some penetration scan experiments times much longer than a second have been observed. 

A helical arrangement within columns was also found for other metal 3-diketonate complexes provided with chiral side chains (32) by Serrano and co-workers.35,36 These compounds form rectangular columnar mesophases with helical order within the columns. A spin-coated sample of 32 showed a positive exciton-splitted signal in the CD spectra, which was interpreted as a left-handed (M) helix. Annealing of the film resulted in much higher optical activities and a shift of the absorption maxima. The observed optical changes clearly point to a chiral organization of the columns in the mesophase. 

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

An interesting variable in all this is the position of the melting point in the sequence (here, the term melting point indicates the temperature at which the solid melts to form a disordered phase, while clearing point indicates the temperature at which the isotropic hquid forms from a mesophase), for there is no real reason for the sohd to be less stable then the mesophase(s). Thus, Fig. 31 shows three possible examples of situations that could occur. In the flrst (A), the sample melts to the mesophase (LC), which then clears to give the isotropic liquid, and on cooling, the whole things reverses. This mesophase is termed enantiotropic. In the second example (B), the sohd melts directly to the isotropic liquid and then supercools into the mesophase (LC), which then crystallizes, here, the mesophase is termed monotropic. In the... 

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