between amylose and amylopectine in fairly dilute (gelatinized) starch
solutions. The solvent quality also affects phase separation; for instance,
incompatibility is generally less at large sugar concentrations.
Figure 6.19a gives a hypothetical example. The tie lines indicate how
the separation will be. A mixture of composition A will separate into phases
of composition B and C; the ratio of the volumes of the solutions of
composition A to B is as the ratio of the distances AC/AB. The longer the tie
line, the stronger the incompatibility. The dot gives the critical point, i.e., the
composition at which the tie line vanishes. The heavy line giving the
solubility is a binodal. It is mostly not possible to calculate the phase
diagram from the properties of the two polymers.
In the concentration range between the binodal and the spinodal (not
shown), separation may be very slow. Even spinodal decomposition may
take long to become visible, because the system often is concentrated and
very viscous, implying slow diffusion of the polymers; a concentrated phase
may even tend to gel. Mostly, one of the phases forms droplets, and the
system may be called awater-in-water emulsion. Which phase becomes the
continuous one depends on the concentration ratio of both polymers. If
c 3 =c 2 is larger than the ratio at the critical point, thecontinuous phasetends
to become the one rich inc 2 , and vice versa. The interfacial tension between
FIGURE6.19 Idealized cases of phase separation in aqueous mixtures of two
polymers, concentrationsc 2 andc 3. (a) Segregative phase separation or incompat-
ibility. (b) Associative phase separation or complex coacervation. The heavy lines
denote the binodal (solubility limit), the thin ones are tie lines. The dots indicate
critical points.