Vapor liquid equilibria, solvent-polymer

Figure 3. Polymer/solvent vapor-liquid equilibrium...

The current state-of-the-art is such that there are no reliable methods of predicting liquid-liquid equilibria of polymer-solvent systems. Thus, the recommended procedures and computer programs included in this Handbook treat only vapor-liquid equilibrium. A discussion of the correlation of LLE data is included in Chapter 2. 

Figure 3.24 Comparison of the schematic P-T diagrams for small molecule systems with those for polymer-solvent systems (A), type-lll system for a small molecule system where the vapor-liquid equilibrium curves for two pure components end in their respective critical points,...

Wen, H., Elhro, H. S., and Alessi, R, Polymer Solution Data Collection. I. Vapor-liquid equilibrium II. Solvent activity coefficients at infinite dilution III. Liquid-liquid equilibrium. Chemistry Data Series, Vol. 15, DECHEMA, Frankfurt am Main, 1992. 

SUR Surana, R.K., Danner, R.P., DeHaan, A.B., and Beckers, N., New technique to measure high-pressure and high-temperature polymer-solvent vapor-liquid equilibrium. Fluid Phase Equil., 139, 361, 1997. 

Solvent activities of polymer solutions have been measured for about 60 years now. However, the database for polymer solutions is still modest, in comparison with the enormous amount of data available for mixtures of low-molecular substances. Three explicit databases have been published in the literature up to now. The database prepared by Wen Hao et al. is summarized in two parts of the DECHEMA Chemistry Data Series. Danner and Higtf provided a database and some calculation methods on a floppy disk with their book. WoUfarth prepared the most complete data collection regarding vapor-liquid equilibrium data of polymer solutions. His annually updated electronic database is not commercially available however, personal requests can be made via his e-mail address given above. 

An up-to-date list of all polymer-solvent systems for which solvent activities or vapor pressures from vapor-liquid equilibrium measurements were published in the literature is provided in Appendix 4.4. A of this Subchapter 4.4 (please see below). 

In summary, investigations on vapor-liquid equilibrium of polymer solutions are the most important souree for obtaining solvent aetivities in polymer solutions. Therefore, emphasis is laid in this subehapter on the experimental methods, whieh use this equilibrium. 

The data reduction of vapor-pressure osmometry (VPO) follows to some extent the same relations as outlined above. However, from its basic principles, it is not an equilibrium method, since one measures the (very) small difference between the boiling point temperatures of the pure solvent drop and the polymer solution drop in a dynamic regime. This temperature difference is the starting point for determining solvent activities. There is an analogy to the boiling point elevation in thermodynamic equilibrium. Therefore, in the steady state period of the experiment, the following relation can be applied if one assumes that the steady state is sufficiently near the vapor-liquid equilibrium and linear non-equilibrium thermodynamics is valid ... 

Equation-of-state approaches are preferred concepts for a quantitative representation of polymer solution properties. They are able to correlate experimental VLE data over wide ranges of pressure and temperature and allow for physically meaningful extrapolation of experimental data into unmeasured regions of interest for application. Based on the experience of the author about the application of the COR equation-of-state model to many polymer-solvent systems, it is possible, for example, to measure some vapor pressures at temperatures between 50 and 100 C and concentrations between 50 and 80 wt% polymer by isopiestic sorption together with some infinite dilution data (limiting activity coefficients, Henry s constants) at temperatures between 100 and 200 C by IGC and then to calculate the complete vapor-liquid equilibrium region between room temperature and about 350 C, pressures between 0.1 mbar and 10 bar, and solvent concentration between the common polymer solution of about 75-95 wt% solvent and the ppm-region where the final solvent and/or monomer devolatilization process takes place. Equivalent results can be obtained with any other comparable equation of state model like PHC, SAFT, PHSC, etc. 

Wen Hao, H. S. Elbro, P. Alessi, Polymer Solution Data Collection, Ptl Vapor-Liquid Equilibrium, Pt.2 Solvent Activity Coefficients at Infinite Dilution, Pt. 3 Liquid-Liquid Equilibrium, DECHEMA Chemistry Data Series, Vol. XIV, Pts. 1,2+3, DECHEMA, FrankfUrt/M., 1992. 

SER Se, R.A.G. and Aznar, M., Vapor-liquid equilibrium of polymer -t- solvent systems Experimental data and thermodynamic modeling, Polymer, 48, 5646, 2007. 

Vapor-liquid equilibrium experiments for ternary polymer solutions have in their result solvent activities, U2 or fl3, and when neglecting real gas behavior, partial vapor pressures of the solvent, p2 or pj,. It is common practice to use the virial equation of state for the purpose of reducing primary VLE-data of polymer solu-... 

As an example for modeling of a vapor-liquid equilibrium in a polymer/solvent system, Fig. 6 shows the results for the polyethylene/toluene binary mixture. The experimental data shown were determined for two different (relatively low)... 

Polymers are often polydisperse with respect to molecular weight. Whereas this is of minor importance for the solvent sorption in polymers (vapor-hquid equilibrium), this fact usually remarkably influences the polymer solubility (liquid-hquid equilibrium). Therefore, polydispersity needs to be accounted for in interpretation and modeling of experimental data. This can be done by applying continuous thermodynamics as well as by choosing a representative set of pseudocomponents. It was shown that a meaningful estimation of the phase boundary is possible when using only two or three pseudocomponents as soon as they reflect the important moments (Mn, Mw, Mz) of the molecular weight distribution. 

Vapor-liquid equilibrium experiments on mixtures of complex molecules, including polynuclear aromatics, polymers and highly polar solvents such as glycols, phenollcs and other "nasty" liquids. The systems water-ethanol and benzene-cyclohexane have each been studied about 50 times. Enough of that. Let s measure equilibria in systems where we cannot now estimate the results within even an order of magnitude. 

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