Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 167
Figure 4.4.13. Schematic of applying head-space
gas-chromatography (HSGC) to VLE-measurements in
polymer solutions (drawing provided by B. A. Wolf,
Univ. Mainz, Germany).
Wolf and coworkers 118,119 applied another
GLC technique to VLE-measurements
for polymer-solvent systems, the
so-called head-space gas-chromatography
(HSGC). This is practically a combination
of static vapor pressure measurement with
gas-chromatographic detection. HSGC experiments
were carried out with an apparatus
consisting of a head-space-sampler and
a normal gas chromatograph. Figure 4.4.13
shows a schematic diagram of the equipment.
The pneumatically driven thermostated
headspace-sampler samples a con-
stant amount of gas phase (that must be in equilibrium with the liquid polymer solution, of
course) and injects this mixture into the gas chromatograph. Helium is used as carrier gas.
After separation of the components of the gaseous mixture in a capillary column they are
detected individually by a thermal conductivity detector. The signals are sent to an integrator
which calculates the peak areas, which are proportional to the amount of gas in the sample
volume and consequently to the vapor pressure. Calibration can be made by measuring
the pure solvent in dependence on temperature and to compare the data with the corresponding
vapor pressure vs. temperature data. Measurements can be done between about 25 and
85 wt% polymer in the solution (again depending on temperature, solvent and polymer investigated).
In order to guarantee thermodynamic equilibrium, solutions have to be conditioned
for at least 24 h at constant temperature in the head-space-sampler before
measurement. Degassing is not necessary and solvents have to be purified only to the extent
necessary to prevent unfavorable interactions in the solution. The experimental error in the
vapor pressures is typically of the order of 1-3%. Details about theory and practice of HSGC
were discussed by Kolb and Ettre. 120 One great advantage of HSGC is its capability to measure
VLE-data, not only for binary polymer solutions but also for polymer solutions in
mixed solvents, since it provides a complete analysis of the vapor phase in equilibrium. This
is usually not the case with the classical isopiestic sorption balances where PVT-data and a
material-balance calculation must be included into the data reduction to calculate vapor
phase concentrations, e.g., Refs. 121-123
(v) Ebulliometry (boiling point elevation of the solvent)
As pointed out above, dynamic vapor-liquid equilibrium measurement methods are not
very suitable for concentrated polymer solutions, especially due to their heavy foaming behavior.
For dilute polymer solutions, however, there is continuing application of
ebulliometry as an absolute method for the direct determination of the number-average molecular
mass M n. Dedicated differential ebulliometers allow the determination of values up
to an order of 100,000 g/mol. Ebulliometry as a method for molar mass determination was
recently reviewed by Cooper, 33 Glover, 34 and Mays and Hadjichristidis. 40
The major requirements for a successful ebulliometry experiment are thermal stability,
equilibration of both concentration and temperature, temperature measurement and
control and pressure measurement and control. It is an advantage of ebulliometry to know
very exactly the constant pressure applied since pressure constancy is a prerequisite of any