Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 189
solid-liquid coexistence data of benzene in natural rubber and found good agreement to
other data.
Equation [4.4.55] could in principle also be used for the determination of thermodynamic
data from the melting point depression of (semi)crystalline polymers when the subscripts
changed from 1 to 2. This enables a second approach to data for the infinite dilution
range of the solvent in the polymer. Such investigations have been made in the literature.
However, these data are regarded as being less reliable by a number of reasons and no further
discussion will be made here.
4.4.3.2.7 Liquid-liquid equilibrium (LLE)
There are two different situations for the liquid-liquid equilibrium in polymer-solvent systems:
(i) the equilibrium between a dilute polymer solution (sol) and a polymer-rich
solution (gel) and
(ii) the equilibrium between the pure solvent and a swollen polymer network
(gel).
Case (i) is considered now, case (ii) is specially considered below as swelling equilibrium.
LLE-measurements do not provide a direct result with respect to solvent activities.
Equation (4.4.8) says that solvent activities at given temperature and pressure must be equal
in both coexisting phases. Since the solvent activity of such a coexisting phase is a priori not
known, one has to apply thermodynamic models to fit LLE-data as functions of temperature
and concentration. Solvent activities can be obtained from the model in a subsequent step
only.
Figure 4.4.19. Principles of liquid-liquid demixing in polymer solutions, a) - strictly binary polymer solution of a
monodisperse polymer, b) - quasi-binary polymer solution of a polydisperse polymer which is characterized by a
distribution function: C - critical point, dashed lines - tie lines, T(1) - temperature/concentration in the homogeneous
region, T(2) - temperature/concentrations of the cloud point (phase´) and the corresponding shadow point
(phase´´), T(3) - temperature in the heterogeneous LLE region, coexistence concentrations of phase´ and phase´´ at
T(3) are related to the starting concentration = cloud point concentration of (2).