Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 155
sion effects (in or out of a non-equilibrium-state polymer solution). Furthermore, only the
solvent coexists in both phases because polymers do not evaporate. The experimental techniques
used for the measurement of vapor pressures of polymer solutions have to take into
account all these effects.
Vapor pressures of polymer solutions are usually measured in the isothermal mode by
static methods. Dynamic methods are seldom applied, see under ebulliometry below. At
least, one can consider measurements by VPO to be dynamic methods, where a dynamic
(steady-state) balance is obtained. However, limits for the applicable ranges of polymer
concentration and polymer molar mass, limits for the solvent vapor pressure and the measuring
temperature and some technical restrictions prevent its broader application, see below.
Static techniques usually work at constant temperature. The three different methods
(i)-(iii) were used to determine most of the vapor pressures of polymer solutions in the literature.
All three methods have to solve the problems of establishing real thermodynamic
equilibrium between liquid polymer solution and solvent vapor phase, long-time temperature
constancy during the experiment, determination of the final polymer concentration and
determination of pressure and/or activity. Methods (i) and (ii) were mostly used by early
workers. The majority of recent measurements was done with the isopiestic sorption methods.
Gas-liquid chromatography as IGC closes the gap at high polymer concentrations
where vapor pressures cannot be measured with sufficient accuracy. HSGC can be considered
as some combination of absolute vapor pressure measurement with GLC. The following
text will now explain some details of experimental equipment and measuring
procedures as well as of data reduction procedures to obtain solvent activities. A recent review
by Williamson 55 provides corresponding information related to low-molecular mixtures.
4.4.3.1.1 Experimental equipment and procedures for VLE-measurements
(i) Absolute vapor pressure measurement
Absolute vapor pressure measurement may be considered to be the classical technique for
our purposes, because one measures directly the vapor pressure above a solution of known
polymer concentration. Refs. 56-65 provide a view of the variety of absolute vapor pressure
apparatuses developed and used by different authors. The common principle of an absolute
vapor pressure apparatus is shown in Figure 4.4.2.
Vapor pressure measurement and solution equilibration were made separately: A
polymer sample is prepared by weighing, the sample flask is evacuated, degassed solvent is
introduced into the flask and the flask is sealed thereafter. All samples are equilibrated at elevated
temperature in a thermostat for some weeks (!). The flask with the equilibrated polymer
solution is connected to the pressure measuring device (in Figure 4.4.2 a
Hg-manometer) at the measuring temperature. The vapor pressure is measured after reaching
equilibrium and the final polymer concentration is obtained after correcting for the
amount of evaporated solvent. Modern equipment applies electronic pressure sensors and
digital techniques to measure the vapor pressure, e.g. Schotsch and Wolf 57 or Killmann et
al. 58 Data processing can be made online using computers. Figure 4.4.3 shows a schematic
diagram of the equipment used by Killmann and coworkers. 58
A number of problems have to be solved during the experiment. The solution is usually
in an amount of some cm 3 and may contain about 1gofpolymer or even more. Degassing
is absolutely necessary. For example, Killmann et al. 58 included special degassing
units for each component of the entire equipment. All impurities in the pure solvent have to