Solvent blends evaporation behavior

A mathematical model of solvent blend evaporation was developed by Walsham and Edwards (61). The model accounts for the nonideal behavior of solvent blends in terms of component activity coefficients. The model allows accurate prediction of blend evaporation time by computer calculations. The technique provides a means to follow residual solvent composition (solvent balance) as evaporation proceeds. 

Evaporation of solvent blends can be quite complicated because of intermolecular interactions that cause them to deviate from ideal behavior as expressed by Raoult s Law. The dotted line in Figure 8 shows ideal behavior of an equal volume blend of two solvents, and the dotted line in Figure 9 shows the evaporation behavior of a solvent blend that has positive deviation from Raoult s Law. 

Solvents have been classified on various arbitrary bases (1) boiling point, (2) evaporation rate, (3) polarity, (4) industrial applications, (5) chemical composition, (6) proton donor and proton acceptor relationships, and (7) behavior toward a dye, Magdala Red, Thus on the basis of industrial application one can classify solvents as those for (1) acctyl-ccliulosc, (2) pyroxylin, 13) resins and lubber, (4) cellulose ether, (5) chlorinated rubber, (6) synthetic resins, and (7) solvents and blending agents for cellulose ester lacquers. Solvents classified according to chemical composition are noted below. 

In the PANI.TSA/PLA blended electrospun nanofibers no phase segregation of PANI in a PLA matrix was observed, while phase segregation was observed in cast films with the same composition. Due to rapid solvent evaporation in the electrospinning process, no crystalline structures in fiber mats were formed compared to cast films. Highly homogeneous electroactive fibers can be useful in the construction of electronic devices and sensors. Similar behavior was observed in the PVDF-TrFE/PANI-PSSA electrospun nanofibers. 

Methyl methacrylate Single Tg WAXD NMR Blends prepared by precipitation of THF solutions in n-heptane or cast from hot THF solutions phase behavior governed by kinetics of solvent evaporation rate (Woo and Su (1996b)) Asano et al. (1992), Chiow et al. (1987), Kyu and Saldanha (1988, 1989), Landry and Hendrichs (1989), Park and Hong (1998), Saldanha and Kyu (1987), Uyar et al. (2005), Woo and Su (1996a, b)... 

It can be seen that the less solvent that is present in the blends the faster the phase separation is initiated and propagated. This behavior can be explained by the system being quenched more deeply into the miscibility gap by evaporation of the solvent, as can be seen in Figure 15.19 this phenomenon is also implied by the free energy depiction in Figure 15.18. Evaporation of the solvent will increase the total free energy level of the system, which in turn would make the polymer pair more likely to phase separate. 

In contrast to PPSu which is a new polyester, PCL is well-known and extensively studied polymer. It is also biocompatible and has found many applications in pharmaceutical technology and medicine. Thus, it was also an interesting idea to explore miscibility and biodegradation behavior of blends made of these two very important polyesters. PCL/PPSu blends with concentrations 90/10, 80/20, 70/30 and 60/40 w/w were prepared by solutioncasting [47]. Proper amounts of both polymers were dissolved in chloroform as common solvent, at room temperature. Sonication was applied in order to achieve complete dissolution and fine mixing of the components. The blends in the form of thin films (200-250 pm) were set up after solvent evaporation at room temperature, under a gentle air stream. They were characterized by DSC, WAXD, HNMR, SEM, and Tensile testing. Finally, their enzymatic hydrolysis was studied. The PCL/PPSu blend system however proved to be only partially... 

Miscibility in polymer blends is controlled by thermodynamic factors such as the polymer-polymer interaction parameter [8,9], the combinatorial entropy [10,11], polymer-solvent interactions [12,13] and the "free volume effect [14,15] in addition to kinetic factors such as the blending protocol, including the evaporation rate of the solvent and the drying conditions of the samples. If the blends appear to be miscible under the given preparation conditions, as is the ca.se for the blends dcscibcd here, it is important to investigate the reversibility of phase separation since the apparent one-phase state may be only metastable. To obtain reliable information about miscibility in these blends, the miscibility behavior was studied in the presence and absence of solvents under conditions which included a reversibility of pha.se separation. An equilibrium phase boundary was then obtained for the binary blend systems by extrapolating to zero solvent concentration. 

As described previously in Section 8.02.2.4, blending block copolymers with either nonselective or selective solvents alters the polymer phase behavior. Stated another way, the thermodynamically stable state of a solvent-swollen block copolymer is different from that of a neat block copolymer. In the case of solvent annealing, the swollen state may be maintained upon evaporation of the solvent. After solvent evaporation, the block copolymer is no longer in a thermodynamically stable state. However, as long as the Tg of the two blocks is sufficiently higher than the temperature at which the film is kept (typically, room temperature), there is little effective mobility of the polymer chains - the morphology is kinetically trapped. As discussed above, Tg s of polymer thin films can be altered from the bulk state strong interactions (attractive or unattractive) with the substrate (Section 8.02.2.3.4). " However, if the Tg is sufficiently above room temperature, little mobility would be expected even in the case of very thin films. 

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