Solvent removal processes

Fiber cross sections are also deterrnined by the coagulation conditions or, in the case of dry spinning, by the solvent evaporation process. The skin that forms early in the solvent removal process may remain intact as the interior of the filament deflates from solvent removal. Wet spun fibers from organic solvents are often bean shaped, while those from inorganic solvent systems are often round. Dry spun fibers, such as Du Font s Odon, are... 

Fu et al. (2002) report the optimization of a fabrication procedure for microspheres based on the poly( anhydride-co-ether) P(SA-EG). The microspheres are fabricated by solvent removal process that produces a porous structure with densities in the range of 0.344 and 0.077 g cm-3 and sizes that are optimized for delivery to the deep lung by inhalation (Fu et al., 2002). An appropriate in vitro cell culture model for characterization of the particle-epithelia system was also developed (Fiegel et al., 2003). 

The solvent removal process in coatings has been addressed as a transport problem (8,9) and from a mechanics viewpoint. Several methods have been developed to measure residual stresses in coatings. For organic coatings many of these methods utilize plate or beam deflection (6, 10-13). Only recently have they been applied to PI coatings. 

To further understand the solvent removal process, strain as a function of time (and temperature as a function of time) was obtained. Results are illustrated in Figure 6. A constant load of... 

These results indicate that the solvent removal process in poly(amic acid) film formation is dependent on the residual stress level. Solvent evaporation creates volume changes which in turn generate shrinkage stresses ( 8 MPa at room temperature). This will continue until there is a balance between the internal stress and the driving forces for solvent removal. Further solvent may be removed if this balance is altered by a change in stress or temperature. 

BTF is not miscible with water and hence, it can be used for extraction purposes. Like dichloromethane, it is heavier than water. We observed, however, that phase separation between water and BTF is sometimes difficult. BTF can be used as solvent for chromatography as well. However, it has a typical intense aromatic odor, which is not convenient when using large quantities. In addition,because of the high boiling point, the solvent removal process is relatively slow (compared to solvents like ether and dichoromethane) and loss of relatively volatile products can occur. Because of these practical disadvantages, we have limited the use of BTF as a chromatography solvent to the purification of fluorous compounds (see 4.0). 

The movement of material from cell wall to cell junctions is believed to occur due to perturbations of closed cell geometry from that of perfect sphere. Since the material inside the closed cells (solvent-rich material) has better wetting properties than that around the cells (polymer-rich material), these perturbations are unstable. The tendency of the system is to form larger spherical domains from the smaller solvent-rich spherical cells (as in the case of gas bubble coalescence in a liquid medium). Clearly, as a first step toward this process, closed cells would coalesce with one another the smaller cells tending to coalesce with the bigger ones. During the removal of the solvent to form the solid product, the wall thinning and rupture processes continue to occur. The extent of this second mechanism depends on how efficient is the solvent removal process. 

Fig. 3.7 Particle formation during solvent removal process by spray drying versus microprecipitation...

Solidification of the dry spinning filament is due to the evaporation of the solvent component from the spin line. There are three mechanisms in the solvent removal process ... 

The sulfonimidoyl chlorides 3a-f can not be isolated since they are unstable to any solvent removal process that we have attempted. They appear to cleanly decompose to oxothiazene oligomers or low polymers (broad NMR signals for S-alkyl group) via elimination of Me3SiCl. This condensation may be catalyzed by the amine hydrochloride salts (chloride ion) present in the reaction mixture. A noteworthy observation is that no cyclic oxothiazene species has been detected (by NMR) in is condensation. 

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