Handbook of Solvents - George Wypych - ChemTech - Ventech!

158 Christian Wohlfarth
ume, degassing, viscosity), long-time
thermostating of the complete apparatus
because of long equilibrium times (increasing
with polymer molar mass and concentration/viscosity
of the solution), correction
of unoccupied vapor space, impurities of
the solvent, connection to the Hg-surface in
older equipment and there is again the problem
of obtaining real thermodynamic equilibrium
(or not) as explained above.
Modern equipment uses electronic
pressure sensors instead of Hg-manometers
and digital technique to measure the vapor
pressure. Also thermostating is more precise
in recent apparatuses. The apparatus
developed by Haynes et al. 66 is shown in
Figure 4.4.5 as example.
Problems caused by the determination
of the unoccupied vapor space were
Figure 4.4.5. Differential vapor-pressure apparatus. 100 avoided by Haynes et al., since they mea-
ml Pyrex flasks connected (a) to a differential pressure
transducer (c) with digital readout (d) and (b) to vacuum sure the pressure difference as well as the
pump (e) and absolute pressure vacuum thermocouple absolute vapor pressure. Also, the concen-
gauge (f). The constant temperature in the water bath is tration is determined independently by us-
maintained by a temperature controller (g). The transing a differential refractometer and a
ducer and connecting glassware are housed in an insulated
box (i) and kept at constant temperature slightly normalized relation between concentration
above the measuring temperature by controller (j). Poly- and refractive index. Degassing of the liqmer
solution and pure solvent (here water) are stirred by uids remains a necessity. Time for estab-
underwater magnetic stirrers (h). [Reprinted with perlishing thermodynamic equilibrium could
mission from Ref. 66, Copyright 1989, American Chemical
Society].
be somewhat shortened by intensive stirring
(slight problems with increasing polymer
concentration and solution viscosity
were reported).
In comparison to absolute vapor-pressure measurements, differential vapor-pressure
measurements with a high resolution for the pressure difference can be applied
even for dilute polymer solutions where the solvent activity is very near to 1. They need
more time than VPO-measurements, however.
(iii) Isopiestic sorption/desorption methods
Isopiestic measurements allow a direct determination of solvent activity or vapor pressure
in polymer solutions by using a reference system (a manometer has not necessarily to be applied).
There are two general principles for lowering the solvent activity in the reference
system: concentration lowering or temperature lowering. Isopiestic measurements have to
obey the condition that no polymer can vaporize (as it might be the case for lower-molecular
oligomers at higher temperatures).
Concentration lowering under isothermal conditions is the classical isopiestic technique,
sometimes also called isothermal distillation. A number of solutions (two as the minimum)
are in contact with each other via their common solvent vapor phase and solvent