Chemistry Reference
In-Depth Information
sealed with a septum by means of a syringe and transferred to a gas chromatograph.
The amount of the solvent is registered either in a flame ionization detector (FID) or
by means of a cell measuring the thermal conductivity of the gas stream. Such
measurements yield the ratio
p
/
p
o
, which in many cases can be taken as the activity
of the solvent. Whether corrections for the nonideality of the gas are required or not
must be checked in each case. The main work required with this method consists of
the optimization of the HS GC, i.e., determination of the best operation procedures
for gas sampling, gas chromatography, and data evaluation. However, once these
parameters have been determined, HS GC offers quick access to thermodynamic
data because the method is automated.
Another possibility for avoiding the measurement of absolute vapor pressures is
provided by sorption methods. In most cases, the polymer is positioned on a quartz
balance and the amount of solvent it takes up via the vapor phase is weighted. The
so-called “flow-through” variant [
32
] works with an open system in contrast to the
previous method.
Isopiestic [
33
] experiments also offer access to chemical potentials. This method
monitors the conditions under which the vapor pressures above different solutions
of nonvolatile solutes (like polymers or salts) in the same solvent become identical,
where one of these solutions is a standard for which the thermodynamic data are
known. These experiments can be considered to be a special form of differential
osmometry (cf. Sect.
3.2
) where the semi-permeable membrane, separating two
solutions of different composition, consists of the gas phase.
3.2 Osmometry and Scattering Methods
Measurements performed to determine the molar masses of polymers yield as a
valuable byproduct information on the pair interaction between the macromole-
cules [
30
]. The composition dependence of the osmotic pressure p
osm
observed
via membrane osmometry is directly related to the chemical potential of the solvent
[cf. (
14
) of Sect.
2
] and provides the second osmotic virial coefficient
A
2
, from
which w
o
, the Flory Huggins interaction parameter in the limit of high dilution
becomes accessible [cf. (
15
)]. Such data are particularly valuable because they can
be measured with higher accuracy than the w values for concentrated polymer
solutions and because they represent a solid starting point for the sometimes very
complex function w(
). In principle, membrane osmometry can also be operated
with polymer solutions of different composition in the two chambers (differential
osmometry) to gain data for higher polymer concentrations; however, little use is
made of this option.
Scattering methods represent another route to
A
2
and w
o
; these experiments do not
monitor the chemical potential itself but its composition dependence. Light scattering
like osmosis can in principle also yield information for polymer solutions beyond
the range of pair interaction, but corresponding reports are seldom. In contrast, small
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