Polymer Mixtures - Polymer Science and Engineering - page 230

the blend is not at equilibrium but the user may not be able to distinguish between
a persistent metastable state and true miscibility.
Many investigators have opted to study polymer compatibility in solution in
mutual solvents, because of uncertainty as to whether a bulk mixture is actually
in an equilibrium state. Compatible components form a single, transparent phase
in mutual solution, while incompatible polymers exhibit phase separation if the
solution is not extremely dilute.
Equilibrium is relatively easily achieved in dilute solutions and studies of such
systems form the foundation of modern theories of compatibility. Application of
such theories to practical problems involves the assumption that useful polymer
mixtures require the selection of miscible ingredients and that compatibility can
therefore ultimately be explained in terms of thermodynamic stability of the
mixture.
This assumption is not necessarily useful technologically. A more practical
definition would consider components of a mixture compatible if the blend
exhibits an initially desirable balance of properties that does not deteriorate over
a time equal to the useful life that is expected of articles made from the mixture.
Miscible mixtures are evidently compatible by this criterion. Compatibility is not
restricted to such behavior since a blend of immiscible materials can be very use-
ful so long as no significant desegregation occurs while the mixture is being
mixed.

5.2 Thermodynamic Theories
The terminology in this area is sometimes a little obscure, and Table 5.1 is pro-
vided to summarize the classification of solution types.
Thermodynamic theories assume that a necessary requirement for solution and
compatibility is a negative Gibbs free energy change when the blend components
are mixed. That is to say,

Δ G m 5 Δ H m 2 T Δ S m , 0 (5-1)

where the subscript m refers to the change of state corresponding to formation of
the mixture and the other symbols have their usual significance. There will be no
volume change ( 0) when an ideal solution

is formed from its components. The properties of ideal solutions thus depend
entirely on entropy effects and

Δ V m 5 0) or enthalpy change ( Δ H m 5

Δ G m 52 T Δ S m ð ideal solution Þ (5-2)

5.2.1 Regular Solutions and Solubility Parameter
If H m is not zero, a so-called regular solution is obtained. All deviations from

ideality are ascribed to enthalpic effects. The heat of mixing

Δ

Δ H m can be