polymers, and several specific interactions between groups on the polymer
may play a part, implying that precise prediction of phase separation is
generally not possible. Nevertheless, the trends observed generally agree
with theory. When adding ethanol to a polymer solution, wherebyb is
decreased, most polysaccharides will indeed show spontaneous separation
into a highly concentrated viscoelastic phase and a very dilute one.
That even small changes inchemical structurecan have an enormous
effect on solubility is illustrated by the difference between amylose and
cellulose. Both are 1?4 linked linear chains of glucose, but as shown in
Figure 6.18, amylose is a polymer ofa-glucose and cellulose ofb-glucose.
This implies that amylose cannot form a straight chain, but that cellulose
can. The latter molecules can become perfectly aligned, forming stacks that
are almost true crystals, held together by Van der Waals attraction and
hydrogen bonds. Accordingly, cellulose is completely insoluble in water. A
similar structure cannot occur for amylose, which is to some extent soluble
in water. Actually, even amylose can form a kind of microcrystalline
regions, since the linear molecules can readily form regular helices, which
then can become stacked. Dextrans are chemically almost identical to
amylose, but have some branching of the polymer chain, which prevents this
kind of stacking. Consequently, most dextrans are well soluble in water.
Note The term phase separation is often used indiscriminately
when separation into layers is observed. In this section true phase
separation is considered. Although both phases are aqueous
solutions, there is a phase boundary between them, exhibiting an
interfacial tension, albeit small, mostly< 0 :01 mPa?s.6.5.2 Polymer Mixtures
If a solution contains two polymers at high concentration, phase separation
generally occurs, especially if the polymers have a high molar mass. Phase
separation may be of two kinds. It is illustrating to consider Eq. (2.19),
which gives a virial expansion (of the osmotic pressure) for a mixture of two
solutes, 2 and 3, 1 denoting the solvent. The so-called mutual second virial
coefficient B 23 now determines what will happen. If B 23 >0, the two
polymers tend to stay away from each other; they are preferentially
surrounded by identical molecules (or by solvent). This may lead to a
separation in a phase rich in polymer 2 (and poor in 3) and one rich in 3
(and poor in 2). Figure 6.19a gives a hypothetical phase diagram. The
polymers showsegregativephase separation and are said to beincompatible.
If, on the other handB 23 >0, the two polymers attract each other. This
leads toassociativephase separation in a phase rich in both polymers, called