Homogeneous miscibility in polymer blends requires a negative free energy of mixing:
ΔGmix = ΔHmix - T.ΔSmix.
However, if two high molecular weight polymers are blended, the gain in entropy, ΔSmix, is
negligible, and the free energy of mixing can only be negative if the heat of mixing, ΔHmix, is
negative. In other words, the mixing must be exothermic, which requires specific interactions between
the blend components. These interactions may range from strongly ionic to weak and nonbonding
interactions, such as hydrogen bonding, ion-dipole, dipole-dipole and donor-acceptor interactions.
Usually, only Van der Waals interactions occur, which explains why polymer miscibility is the
exception rather than the rule.
The miscibility behaviour of two polymers is strongly dependent on temperature. Each polymer pair is
characterized by an interaction parameter, which usually exhibits such a temperature dependence that
possible miscibility at lower temperatures is lost at higher temperatures, e.g. during processing.