Handbook of Solvents - George Wypych - ChemTech - Ventech!

176 Christian Wohlfarth
Vapor pressure/vapor sorption measurements cover nearly the complete concentration
range if different apparatuses are used. Measurements can be made with good accuracy.
Principal limits with respect to temperature or pressure do not exist, but most apparatuses in
the literature are constructed only for temperatures between 20 and 100 o C, sometimes up to
150 o C, and pressures between 1 and 100 - 200 kPa. Vapor pressure/vapor sorption measurements
are very time-consuming experiments. To obtain a complete isotherm one needs usually
about a month with conventional techniques or, at least, some days with microbalances
or piezoelectric sensors. This demands long-time stability for thermostating and precise
temperature control. Furthermore, the equilibrium cell has to be sealed in such a way that air
leakage is avoided for the complete duration of the measurement. Experimentators need
quite a lot of experience until they observe really good data. Experiments can only partially
be automated depending on the method and equipment applied. The accuracy of the final
data depends on the method applied, the temperature or pressure investigated, and also the
given concentration. Measurements above about 85 wt% polymer showed sometimes sorption-desorption
hysteresis. Solvent degassing is absolutely necessary with the exception of
the apparatus proposed by Sadowski where degassing takes place automatically during the
experiment (see above). The solvent must be purified from all other impurities. This is true
of course also for the polymer investigated. According to their capabilities, different apparatuses
should be used: differential pressure measurements for 5-30 wt% polymer in the solution,
isopiestic sorption techniques for 30-85 wt% polymer, piezoelectric or microbalance
detection for 60-99 wt% polymer. These limits can change somewhat with molar mass of
the polymer. Oligomer solutions are easier to handle and can be measured even with conventional
VLE-technique as developed for low-molecular liquid mixtures. There may be
limits in temperature and pressure that depend on the nature of the solvent/polymer pair.
Usually, the solutions investigated should not show liquid-liquid demixing and solutions
should not become solid. Thermodynamic equilibrium data can only be obtained if the polymer
is investigated well above its glass transition temperature. There is a depression of the
glass transition temperature with increasing solvent concentration, but there are polymers
that can be investigated only at temperatures above 100 o C and more, even in concentrated
solutions.
VPO is more limited with respect to the measurement of solvent activities. It is designed
only for dilute polymer solutions (in the maximum up to 40 wt% polymer), optimum
temperature and pressure (well below normal pressure) ranges and molar masses up to
about 20,000 g/mol for the polymer. Not all solvents can be applied. On the other hand,
VPO is a well-established method, commercially available, possessing a high resolution for
very small differences of solvent activities with respect to the pure solvent and does not
need much time. Steady-state conditions are obtained within minutes and quite a lot of measurements
can be made during a working day. There are no problems with external
thermostating or long-time stability. Experimental results from VPO are in good agreement
with measurements from scattering techniques. VPO measurements close the gap between
0 and 30 wt% polymer in the solution with respect to conventional vapor pressure/vapor
sorption measurements (of course, only within its limits explained). Experimentators easily
acquire the necessary experience with the measuring equipment.
The piezoelectric sorption technique is a method that is especially suitable for the low
solvent concentration range. It is the most sensitive solvent vapor sorption method. A resolution
of nanograms can be realized. Measurements can also be made as a function of time