Mesophase control

Y. C. Liang, M. Hanzlik, and R. Anwander, Periodic mesoporous organosilicas Mesophase control via binary surfactant mixtures. J. Mater. Chem., 16, 1238-1253, 2006. 

Mochida, I., Yoon, S. H. and Korai, Y., Control of transversal texture in circular mesophase pitch-based carbon fibre using non-circular spinning nozzles, J Mat Sci, 1993,28, 2331 2336. 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

Gibaud, A. Grosso, D. Smarsly, B. Baptiste, A. Bardeau, J. F. Babonneau, F. Doshi, D. A. Chen, Z. Brinker, C. I Sanchez, C. 2003. Evaporation-controlled self-assembly of silica surfactant mesophases. J. Phys. Chem. B 107 6114-6118. 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is controlled by structural flaws [58], Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch precursor also contributes to their lower strengths. 

This study demonstrated that the micro-mesoporous composite materials could be synthesized with two-step treatment by microwave using two different templates system with TPABr and MTAB. This formation was controlled by the self-assembly formation of supramolecular templates between MTA micelles and SiO /TPA gels. As varying microwave irradiation time of micro-mesoporous materials, gradually transition from the mesophase to micro-mesophase was occurred. These materials have higher dm spacing of mesoporous materials and lead to transition from mesophase to micro-microphase by an increment of synthetic time, while the calcined products is formed with bimodal and trimodal pore size distribution under microwave irradiation within 3 h. From TG-DTA and PL analysis, the self-assembly formation of supramolecular templates between MTA+ micelles and SiO /TPA+ gels were monitored. 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

The fragments of macromolecules with ordered cholesterol group sequences, that are formed in bad solvents, may serve as nuclei of supermolecular order in films, obtained from these solvents. Structural and optical studies have shown that PChMA-11 films produced by solvent evaporation display different properties those obtained from chloroform and toluene solutions (small relaxation times, see Table 17) are optically isotropic, and those obtained from heptane solutions (large relaxation times, see Table 17) are optically anisotropic, what reflects the differences in conformational state of polymeric chains in these films. Contrary to the optically isotropic films, a high degree of side branch ordering characterizes optically anisotropic films, which is confirmed by X-ray studies. The observed difference of LC polymer structure in the bulk is thus the consequence of their different conformational state in solution this reveals some possibilities for the control of LC polymer structure at the initial steps of mesophase nucleation in solutions. 

The unequivocal characterization of the mesophases is quite often very tricky and problematic. Each phase shows typical textures under a polarizing microscope a lot of them are documented in precise photos and pictures, but the formation of a texture depends strongly on the sample preparation, surface treatment, temperature control and other parameters. DSC curves and the phase-transition enthalpies yield important information, but the only unequivocal and suitable tool for the determination of the mesophases is given in the different x-ray methods, and nowadays modern resonance and atomic probe techniques attract more and more notice and acceptance9-11. A general description is given in the next section. 

Irradiation procedures. Mesophase solutions and neat solid samples of BN were prepared and sealed under N2 or vacuum in Kimax capillary tubes. Isotropic samples were either degassed (freeze-pump-thaw techniques) and sealed in pyrex tubes or saturated with N2 in pyrex tubes. Nitrogen was bubbled through the latter solutions during irradiation periods. When ther-mostatted, samples were placed in a temperature controlled ( 1°) water bath. All samples were irradiated with a 450 W Hanovia medium pressure Hg arc and were stored at -30°C until their futher use. Usually, a "dark sample was prepared and treated in an identical fashion to the irradiated samples except that it was shielded from the light. JSN from each tube was recovered by either column chromatography (silica or alumina and pentane eluant) at 4°C followed by solvent removal at 0°C and reduced pressure or by hplc (tr-hexane) at room temperature followed by solvent removal at 0°C and reduced pressure. Neat solid samples were dissolved in one of either benzene, tetrahydrofuran or toluene and were frozen until analyzed. 

Addition of a ca. 10% concentration of CA to a mesophase solution (mixture E) of BN quenches completely its photoresolution. CA quenches singlet states of BN in n-hexane at a nearly diffusion-controlled rate (kq = 101- -tf -s- assuming tbn = 3 ns (34-36)). Thus, even In a very viscous medium like cholesteric mixture E, static and dynamic quenching should preclude formation of BN triplets. 

With the previously demonstrated ability of trinuclear gold(I) pyrazo-late complexes that possess long-alkyl chains to form columnar mesophases [119-121], Aida and co-workers synthesized a series of luminescent Au3 complexes bearing dendritic poly(benzyl ether) functionalized pyrazole ligands [122]. By controlling the dendron generation, superhelical fibres with extensive inter molecular Au3 stacking could be obtained, which resulted in an... 

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