Handbook of Solvents - George Wypych - ChemTech - Ventech!

166 Christian Wohlfarth
Figure 4.4.12. Temperature dependence of the specific
retention volume V g of p-xylene in polystyrenes of varying
molar masses, experimental data were measured by
Glindemann. 105
to real thermodynamic vapor-liquid equilibrium
data. As a rule of thumb, the experimental
temperature must exceed the glass
transition temperature by about 50K. Form
and width of the S-shaped region depend
somewhat on the solvent used and, as can
be seen from the picture, there is a certain
dependence on molar mass of the polymer.
Data on the right side at temperatures below
the glass transition describe mainly surface
adsorption effects.
Brockmeier et al. 106,107 showed that
GLC can also be used to determine the partial
pressure of a solute in a polymer solution
at concentrations as great as 50 wt%
solute. In this case of finite concentration
IGC, a uniform background concentration
of the solute is established in the carrier gas.
The carrier gas is diverted to a saturator
through a metering valve. In the saturator it
passes through a diffuser in a well-stirred, temperature-controlled liquid bath. It leaves the
separator with the solute equilibrium vapor pressure in the carrier gas. The solute concentration
is varied by changing the saturator temperature. Precise control of the temperature bath
is needed in order to obtain a constant plateau concentration. Upon leaving the saturator the
gas flows to the injector block and then to the column. As in the infinite dilute case a small
pulse of the solvent is then injected. This technique is known as elution on a plateau, Conder
and Purnell. 108,109 Because finite concentration IGC is technically more complicated, only
few workers have applied it. Price and Guillet 110 demonstrated that results for solvent activity,
activity coefficient or χ-function are in good agreement with those obtained by traditional
isopiestic vapor sorption methods. Whereas the vapor sorption results are more
accurate at higher concentrations, the reverse is true for finite concentration IGC since
larger injection volumes have to be used, which strains the theory on which the calculations
are based. Also, at large vapor concentrations the chromatographic peaks become more
spread out, making the measurement of retention times less precise. Additionally, the concentration
range is limited by the requirement that the saturator temperature must be below
that of the column. Clearly, at higher measuring temperatures, higher solvent concentrations
may be used. Finite concentration IGC can be extended to multi-component systems.
Especially ternary polymer solutions were investigated to some extend with this technique,
e.g., Bonner and coworkers 111,112 or Glover and coworkers. 113-115 Data reduction is somewhat
complicated, however.
Danner et al. 116 tested the frontal analysis by characteristic point (FACP) technique to
measure thermodynamic data for polymer-solvent systems at finite concentrations. In the
FACP technique, a complete isotherm can be derived from the shape of one breakthrough
profile. A point on an isotherm is obtained by measuring the retention volume of the characteristic
point at the corresponding concentration. The methods to determine thermodynamic
data by FACP technique were discussed in detail by Conder and Young. 117