Polymers in poor solvents experimental observations, existence of demixtion

POLYMERS IN POOR SOLVENTS EXPERIMENTAL OBSERVATIONS. EXISTENCE OF DEMIXTION 

In most cases, the polymer-solvent mixing is endothermic, to dissolve a solute in a solvent at constant temperature and pressure, one has to provide the system with heat, and since the total volume of the system does not vary much, this heat is practically equal to the internal energy. This fact shows the existence of attractive forces between the molecules of the solute in the solution, independently of the excluded volume effects. These forces express the fact that solute-solute and solvent-solvent contacts are preferred to solute-solvent contacts. 

Practically, the hard core effects of quantum origin do not depend on temperature. On the contrary, the effects of the van der Waals attractive forces, which are softer and have a longer range, increase when temperature diminishes. Concurrently, the contribution of the entropy to the free energy of the system decreases. 

These facts have important experimental consequences. Thus, let us consider polystyrene solutions in a poor solvent like cyclohexane at 40 °C. By lowering the temperature gradually, we can observe effects revealing the existence of attractive interactions between the polystyrene chains and also between monomers belonging to the same chain. We shall present here three significant experimental observations. 

Let us first examine how the osmotic pressure varies with concentration p. We have already seen that at 40 °C, the value A2 of the second virial coefficient 

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