Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 163
Figure 4.4.9. Schematic diagram of an isopiestic vapor sorption apparatus using
an electronic microbalance: PC - personal computer, MB - microbalance,
WB1-3 - water bath thermostats with T 3>T 2>T 1, V1-3 - valves, WM - W-tube
mercury manometer, S - polymer sample/solution, SV - solvent reservoir, MS -
magnetic stirrer, CT - cold trap, VP - vacuum pump. [Reprinted with permission
from Ref. 92, Copyright 1998, American Chemical Society].
Comparable apparatuses
were constructed
and used, for example,
by Bae et al. 94 or by
Ashworth, 95 Ashworth
and Price 96 applied a
magnetic suspension
balance instead of an
electronic microbalance.
The magnetic suspension
technique has the
advantage that all sensitive
parts of the balance
are located outside the
measuring cell because
the balance and the polymer
solution measuring
cell are in separate chambers
and connected by
magnetic coupling only.
This allows its application
even at very high
temperatures of some
hundred degrees as well
as pressures up to hundreds of MPa.
The most sensitive solvent vapor sorption method is the piezoelectric sorption detector.
The amount of solvent vapor absorbed by a polymer is detected by a corresponding
change in frequency of a piezoelectric quartz crystal coated with a thin film of the polymer
because a frequency change is the response of a mass change at the surface of such a crystal.
The frequency of the crystal decreases as mass increases when the crystal is placed in a gas
or vapor medium. The frequency decrease is fairly linear. The polymer must be coated onto
the crystal from a solution with some care to obtain a fairly uniform film. Measurements can
be made at dynamic (vapor flow) or static conditions. With reasonable assumptions for the
stability of the crystal’s base frequency and the precision of the frequency counter employed,
the piezoelectric method allows the detection of as little as 10 nanograms of solvent
using a 10 MHz crystal. This greatly reduces both the time necessary to attain equilibrium
(3-4 hours) and the amount of polymer required. Saeki et al. 97-99 extensively applied this
method to various polymer solutions in a concentration range between 60 and 100 wt%
polymer. Recently, Wong et al. 100 and Mikkilineni et al. 101 presented some new investigations
with this method. Figure 4.4.10 shows a schematic diagram of the general equipment.
A resolution of nanograms could be realized by Mikkilineni et al. 101 Measurements
were also made as a function of time to obtain diffusion coefficients. Comparison with
gravimetric sorption measurements demonstrated the accuracy of the experiment. Ref. 100
presents some details about the electronic circuit, the mounting arrangements for the quartz
crystals and the sorption cell. Because very thin films are applied, equilibrium solvent absorption
also can be obtained at polymer mass fractions approaching 1 (i.e., for small sol-