Polymer–Polymer. The following alternative situations may occur.- The two polymers form a homogeneous solution. If now the
conditions are changed so that polymer A would form a gel, e.g., by
lowering temperature, it will also do so in the mixture, and the rheological
properties will not be greatly different from those of a gel of polymer A at
the same concentration. The permeability will be decreased, owing to
polymer B forming a viscous solution in the pores of the gel network. Such
systems are quite rare, however. - Phase separation occurs because ofincompatibility; see Section
6.5.2. This is a very common situation, unless the polymer concentrations
are quite low. Consider, for instance, a mixture of gelatin and a nongelling
polysaccharide, e.g., dextran. The mixture is made at a temperature above
the gel point of the gelatin, and phase separation sets in. Eventually two
layers will be formed, but that takes a long time, and meanwhile one of
the polymers is (predominately) in a continuous phase, the other in drops.
If gelatin is in the continuous phase (which depends on the relative
concentrations of the two polymers), and the mixture is cooled, gelation
occurs, and the nonequilibrium situation is ‘‘frozen.’’ The modulus of the
gel will be markedly higher than for a gelatin solution of the same overall
concentration, because its concentration c in the continuous phase is
significantly increased, and the modulus is, e.g., proportional toc^2. The
fracture stress would be higher owing to the higher concentration, but
lower because the presence of the drops decreases the actual fracture
surface; consequently, a general rule cannot be given. If gelatin forms the
disperse phase, cooling leads to a dispersion of gel particles in a dextran
solution. - Weak attractionbetween the polymers may cause gel formation,
even if both are nongelling. Such mixtures are often used to form weak gels,
e.g., to prevent sedimentation of particles. An example is given by xanthan
and some galactomannans, such as locust bean gum. The mixture is first
heated and then cooled. Mixed junctions are formed, presumably involving
a xanthan helix and a stretched part devoid of bulky side groups (i.e., a
straight zigzag) of the locust bean gum chain. In this way 0.1%of each
polymer may jointly form a gel with a yield stress of order 10 Pa. Another
example is given by the addition of, say, 0.03%k-carrageenan to milk. It
then interacts with thek-casein in the casein micelles, forming a weak gel;
the interaction is due to electrostatic attraction. - Stronger mutual attraction of the polymers or higher concentra-
tions may lead to phase separation by formation of acomplex coacervate.
This often results in lumps of coacervate in a dilute solution, a system that is
generally useless.