Handbook of Solvents - George Wypych - ChemTech - Ventech!

162 Christian Wohlfarth
Figure 4.4.8. Isopiestic vapor-sorption apparatus with built-in manometer
using a quartz spring: 1 - connection to the vacuum, 2-9 -
stop corks, 10, 11, 12 - connections to nitrogen, 13 - degassing flask
for the pure solvent, 14, 18 - buffers, 15 - cold trap, 16, 19 - Hg-manometers,
17, 20 - mercury float valves, 21 - pure solvent reservoir at
temperature T 1 provided by 22 - thermostat, 23 - temperature controlled
air box, 24 - measuring cell, 25 - quartz spring (four quartz
springs can be inserted into the equilibrium cell, only one is shown),
26 - pan with the polymer solution, 27 - closing plug sealed with epoxy
resin, 28 - heating to avoid solvent condensation.
contact of solvent vapor with the
mercury surface and to get a better
resolution of the measuring pressure.
Isopiestic vapor sorption can
be made using an electronic
microbalance instead of the quartz
spring balance. Again, this was
not originally developed for polymer
solutions but for gas-solid adsorption
measurements where this
is a widespread application. Electronic
microbalances are commercially
available from a number of
producers. Their main advantages
are their high resolution and their
possibility to allow kinetic measurements.
Additionally, experiments
using electronic
microbalances can easily be automated
and provide computing facilities.
The major disadvantage
with some of these microbalances
is that they cannot be used at high
solvent vapor pressures and so are limited to a relatively small concentration range. However,
since thin polymer films can be applied, this reduces both the time necessary to attain
equilibrium (some hours) and the amount of polymer required and equilibrium solvent absorption
can be obtained also at polymer mass fractions approaching 1 (i.e., for small solvent
concentrations). Depending on their construction, the balance head is situated inside or
outside the measuring apparatus. Problems may arise when it is inside where the solvent vapor
may come into contact with some electronic parts. Furthermore, all parts of the balance
that are inside the apparatus have to be thermostated to the measuring temperature to enable
the correct equilibration of the polymer solution or even slightly above to avoid condensation
of solvent vapor in parts of the balance. The allowed temperature range of the balance
and its sensitivity to solvent corrosion determine then the accessible measuring range of the
complete apparatus. Yoo and coworkers 92,93 have recently measured various polymer solutions
with such equipment and Figure 4.4.9 shows some details of their apparatus.
Two thermostats maintain the pure solvent temperature T 1 and the measuring temperature
T 2 as described above for the spring balance technique, thermostat three protects the essential
part of the balance for solvent vapor condensation and damage. A calibrated weight
was loaded on the left side of the balance. A granular type of quartz was used as reference
weight in order to prevent possible solvent vapor condensation. A dish-type quartz sorption
cell was used to load the polymer sample. Platinum wire was used to link both arms to the
balance to prevent possible oxidative corrosion of the arm by the solvent. The vapor pressure
is measured directly by applying a W-tube Hg-Manometer. The manome- ter reading
was made using a cathetometer.