Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.2 Effect of system variables on solubility 129
Figure 4.2.1 Two contributions to the χ 1 parameter.
1-χ H,2-χ S, 3 - the total value of χ 1.
Figure 4.2.2 The χ1 parameter as a function of pressure
at T = 300K, r1 = 3.5. The curves are for the following
* *
values of ε / ε = 0.85; 1.0 and 1.3 (After refs. 9,28 ).
11 22
perature raise can cause improvement of compatibility. Such systems are considered to
have the upper critical solution temperature (UCST). If the system of two liquids becomes
compatible at any ratio at the temperature below the defined critical point, the system is considered
to have the lower critical solution temperature (LCST). Examples of a system with
UCST are mixtures of methyl ethyl ketone-water (150 o C) and phenol-water (65.8 o C). An
example of a system with LCST is the mixture of water-triethylamine (18 o C). There are systems
with both critical points, for example, nicotine-water and glycerol-benzyl-ethylamine.
Presence of UCST in some cases means a rupture of hydrogen bonds on heating; however,
in many cases, UCST is not determined by specific interactions, especially at high
temperatures, and it is close to critical temperature for the liquid-vapor system.
There are suppositions that the UCST is the more common case but one of the critical
points for polymer-solvent system is observed only at high temperatures. For example,
polystyrene (M = 1.1×10 5 ) with methylcyclopentane has LCST 475K and UCST 370K.
More complete experimental data on the phase diagrams of polymer-solvent systems are
published elsewhere. 27
The solubility of crystalline substances increases with temperature increasing. The
higher the melting temperature of the polymer, the worse its solubility. Substances having
higher melting heat are less soluble, with other characteristics being equal. Many crystalline
polymers such as polyethylene or polyvinylchloride are insoluble at the room temperature
(only some swelling occurs); however, at elevated temperature they dissolve in some solvents.
The experimental data on the temperature dependence of χ 1 of polymer-solvent sys-
tems are described by the dependence χ α β
1 = + /T. Often in temperatures below 100o C,
β