Phase separation crystallization techniques

The infrared and mechanical data all indicate that in the case of DBDI based PU, the conformational mobility of the two benzene rings around the ethylene bridge induces a specific stress - strain and stress relaxation behaviour in PU, both in tensile and compression experiments. Experimental DSC, IR diehroism, and X-ray diffraction techniques were employed to study the influence of the compatibility of PU microphases (hard and soft blocks) on the PU physical/ mechanical properties, by the study of the structural modifications which appear during mechanical stresses i.e. phase separation, crystallization and orientation phenomena. The morphological study on such polymers have proved a high tendency of crystallisation and hard segment blocks phase separations [2,4]. ... 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

The phenomenology of physical organogels and jellies is extremely rich, and their comportments are similar in some aspects to those of both surfactants in solution (e.g., lyotropism and crystallization) and polymer solutions (6 (e.g.. swelling/shrinking behaviors and microscopic mass motion). Gels can be considered as being at the interface between complex fluids (i.e.. micellar systems) and phase-separated states of matter. The main properties and concepts appropriate to describe the gels and the basic principles of techniques for their study will be reviewed here. 

As is well known, some compounds have never been crystallized, and phase separation results in a stable oil or an amorphous solid. The search for solvents and conditions, or the introduction of foreign particle seeds (e.g., by scratching a glass test tube) to induce crystal formation for a new compound, becomes a matter of trial and error. Combinatorial techniques continue to be developed that can aid in this evaluation. A critical factor for success may be removal of impurities to achieve a very high level of purity, because the effect of even very low levels of impurities on homogeneous nucleation will not be known at this stage. 

Equations 1 and 3 with the initial and boundary conditions as given above were first non-dimensionalized and then solved by the orthogonal collocation numerical technique (20). The concentration in the mobile phase and crystal pore was divided by (C0 V / , "Ve) to give the dimensionless concentration. The parametric analysis shows that varying any of the parameters of n, b and Dp can result in substantial change in the location (retention time) and the shape (broadness and symmetry) of the LC response peaks. A variation on the axial dispersion coefficient D, however, has very small effect on the response peaks for the present LC system. The details of the numerical solution and parametric analysis will be given in a separate paper. 

SMB allows a drastic reduction of the costs of chiral separations, mainly due to a reduction of chiral stationary phase (50-60% lower than in HPLC) and of eluent consumption (up to 10 times compared with the batch chromatographic process). It allows production scales of 10-100 tons per year, with separation costs as low as US 30 per kg of pure enantiomer [155], The coupling of SMB with racemization and/or enantioselective crystallization techniques is even more promising. 

There are a few publications on the use of other spectroscopic techniques such as Brillouin scattering, photoacoustic, and Raman spectroscopy. The primary application of these has been to study the heterogeneities in polymer blends, viz. crystallization or phase separation. 

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