Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 177
to obtain diffusion coefficients. Comparison with gravimetric sorption measurements demonstrated
the accuracy of the experiment. Because very thin films are applied, equilibrium
solvent absorption also can be obtained at polymer mass fractions approaching 1, as with
the IGC experiment. Comparison to IGC-data gives good agreement. Sorption-desorption
hysteresis has never been observed when using piezoelectric detectors. Measurements are
limited to a concentration range where the swollen polymer film is still stable at the crystal
surface. Equilibrium is rather quickly established, usually after 3-4 hours, i.e., an isotherm
can be measured within some days. With the corresponding equipment, high pressures and
high temperatures can be applied, too.
IGC is the most rapid method and it is the recommended technique for the infinite dilution
range of the solvent in the (liquid, molten) polymer. Measurements can also be made
to obtain diffusion coefficients. Column preparation and finding optimum experimental
conditions are the most time-consuming tasks. These tasks require quite a lot of experience.
The final measurements can be automated and provide quick, reliable and reproducible results.
Temperature and solvent dependencies can easily be investigated. The common accuracy
is 1-3% with respect to data of the χ-function or Henry’s constant. There is no need to
degas the solvents or to purify them except from impurities which may react with the polymer.
Limits are mainly given by the glass transition temperature of the polymer as explained
above. Due to this problem, most IGC measurements are made at temperatures well above
100 o C. On the other hand, temperatures well above 100 o C can cause the problem of thermal
ageing and degradation of the polymer sample if temperatures are too high. In comparison
to IGC, vapor pressure measurements were made in most cases below 100 o C. There were
some special investigations in earlier literature to compare IGC-data at infinite dilution with
those from vapor pressure measurements at concentrated solutions, e.g., Refs. 110,136-138 Differences
between IGC-data and vapor pressure measurements reported in older papers are
mainly caused by errors with the IGC technique. Temperatures were used too near or even
within the glass transition region, unsuitable polymer loading was applied, non-equilibrium
conditions were used. But, there are also errors from/within vapor pressure data, mainly
sorption/desorption hysteresis at too high polymer concentrations because of non-equilibrium
conditions. Today it is accepted that there are no differences between IGC-data and vapor
pressure measurements if all thermodynamic equilibrium conditions are carefully
obeyed. In contrast to vapor pressure measurements, IGC can also be applied with thermodynamically
bad solvents. It is the only method to obtain limiting activity coefficients for
strong non-solvents. Even mass fraction based activity coefficients above 25 or χ-values of
2 or more can be measured.
Finite concentration IGC provides the possibility to connect advantages from IGC and
vapor pressure measurements because it can be applied between 50 and 100 wt% polymer.
However, the experimental technique is more sophisticated, data reduction is more complicated,
and only few workers have applied it. On the other hand, much experimental time can
be saved since finite concentration IGC is a rapid method. One isotherm can be observed
within one day (or two). Price and Guillet 110 or Danner et al. 116 demonstrated that results for
solvent activity coefficients and χ-functions or sorption isotherms are in good agreement
with those obtained by traditional isopiestic vapor sorption methods. The concentration
range of finite concentration IGC is limited by the requirement that the saturator temperature
must be below that of the column. Clearly, at higher measuring temperatures, higher