Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 165
For infinite dilution operation the carrier gas flows directly to the column which is inserted
into a thermostated oil bath (to get a more precise temperature control than in a conventional
GLC oven). The output of the column is measured with a flame ionization
detector or alternately with a thermal conductivity detector. Helium is used today as carrier
gas (nitrogen in earlier work). From the difference between the retention time of the injected
solvent sample and the retention time of a non-interacting gas (marker gas), the thermodynamic
equilibrium behavior can be obtained (equations see below). Most experiments were
made up to now with packed columns, but capillary columns were used, too. The experimental
conditions must be chosen so that real thermodynamic data can be obtained, i.e.,
equilibrium bulk absorption conditions. Errors caused by unsuitable gas flow rates, unsuitable
polymer loading percentages on the solid support material and support surface effects
as well as any interactions between the injected sample and the solid support in packed columns,
unsuitable sample size of the injected probes, carrier gas effects, and imprecise
knowledge of the real amount of polymer in the column, can be sources of problems,
whether data are nominally measured under real thermodynamic equilibrium conditions or
not, and have to be eliminated. The sizeable pressure drop through the column must be measured
and accounted for.
Column preparation is the most difficult task within the IGC-experiment. In the case
of packed columns, the preparation technique developed by Munk and coworkers 103,104 is
preferred, where the solid support is continuously soaked with a predetermined concentration
of a polymer solution. In the case of capillary IGC, columns are made by filling a small
silica capillary with a predetermined concentration of a degassed polymer solution. The one
end is then sealed and vacuum is applied to the other end. As the solvent evaporates, a thin
layer of the polymer is laid down on the walls. With carefully prepared capillary surfaces,
the right solvent in terms of volatility and wetting characteristics, and an acceptable viscosity
in the solution, a very uniform polymer film can be formed, typically 3 to 10 μm thick.
Column preparation is the most time-consuming part of an IGC-experiment. In the case of
packed columns, two, three or even more columns must be prepared to test the
reproducibility of the experimental results and to check any dependence on polymer loading
and sometimes to filter out effects caused by the solid support. Next to that, various tests regarding
solvent sample size and carrier gas flow rate have to be done to find out correct experimental
conditions.
There is an additional condition for obtaining real thermodynamic equilibrium data
that is caused by the nature of the polymer sample. Synthetic polymers are usually amorphous
or semi-crystalline products. VLE-based solvent activity coefficients require the
polymer to be in a molten state, however. This means that IGC-measurements have to be
performed for our purpose well above the glass transition temperature of the amorphous
polymer or even above the melting temperature of the crystalline parts of a polymer sample.
On the other hand IGC can be applied to determine these temperatures. The glass transition
of a polymer does not take place at a fixed temperature but within a certain temperature
range depending on the probing technique applied because it is a non-equilibrium effect.
Figure 4.4.12 demonstrates the appearance of the glass transition region in an IGC-experiment.
The S-shaped part of the curves in Figure 4.4.12 is the glass transition region. Its minimum
describes the glass transition temperature as obtained by IGC. Only data from the
straight line on the left side at temperatures well above the glass transition temperature lead