Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 147
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. 1-3 The database prepared by Wen
Hao et al. 1 is summarized in two parts of the DECHEMA Chemistry Data Series. Danner
and High 2 provided a database and some calculation methods on a floppy disk with their
book. Wohlfarth 3 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.
Some implicit databases are provided within the Polymer Handbook 4 by Schuld and
Wolf 5 or by Orwoll 6 and in two papers prepared earlier by Orwoll. 7,8 These four sources list
tables of Flory’s χ-function and tables where enthalpy, entropy or volume changes, respectively,
are given in the literature for a large number of polymer solutions. The tables of second
virial coefficients of polymers in solution, which were prepared by Lechner and
coworkers 9 (also provided in the Polymer Handbook), are a valuable source for estimating
the solvent activity in the dilute polymer solution. Bonner reviewed vapor-liquid equilibria
in concentrated polymer solutions and listed tables containing temperature and concentration
ranges of a certain number of polymer solutions. 10 Two CRC-handbooks prepared by
Barton list a larger number of thermodynamic data of polymer solutions in form of polymer-solvent
interaction or solubility parameters. 11,12
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).
Solvent activities in polymer solutions can be determined by rather different techniques.
However, no one is really a universal method but covers a certain concentration
range of the polymer solution. Figure 4.4.1 explains in short the situation.
Corresponding to the different regions in the diagram, different experimental techniques
were used for the measurement of the solvent activity in a homogeneous polymer solution:
(i) Solvent activities of highly diluted polymer solutions can be obtained from scattering
methods such as light scattering, small angle X-ray scattering and small angle neutron
scattering via the second osmotic virial coefficient, which is often related to investigations
for polymer characterization. These methods are able to resolve the very small difference
between the thermodynamic limit of 1.0 for the activity of the pure solvent and the actual
value of perhaps 0.9999x at the given (very low) polymer concentration.
(ii) Solvent activities of polymer solutions with polymer concentrations up to about
30 wt% can be measured by osmometry (membrane as well as vapor-pressure osmometry),
light scattering, ultracentrifuge (of course, all these methods can also be applied for polymer
characterization and can be extrapolated to zero polymer concentration to obtain the second
virial coefficient), and differential vapor pressure techniques. Cryoscopy and ebulliometry
can also be used to measure solvent activities in dilute and semidilute polymer solutions, but
with limited success only.
(iii) The concentrated polymer solution between 30 and 85 wt% is covered by vapor
pressure measurements which were usually performed by various isopiestic sorption meth-