generally smaller for a higher concentration, greater stiffness, and higherb
value.
Phase separation in mixturesof polymers especially occurs at high
concentrations and for large molar mass. The separation can be of two
kinds. If the two polymers show mutual affinity, associative phase
separation or complex coacervation occurs, i.e., a separation into a solution
high in both polymers (a complex coacervate) and a very dilute solution. In
most other cases, segregative phase separation or incompatibility is
observed, i.e., separation in a phase rich in polymer A but poor in polymer
B, and vice versa.
Starch. Starch is a polyglucan mixture, containing the linear
amylose (about 25%) and the very large, heavily branched amylopectin.
Native starch granules are virtually insoluble in water, mainly because part
of the amylopectin is crystalline. The crystals consist of stacks of double
helices, which stacks are arranged in a complex supermolecular structure.
Starch can begelatinizedby heating in excess water, which implies
melting of the crystallites and extensive swelling; amylose and amylopectin
become phase separated. Highly viscous solutions or pastes are obtained,
although starch is not a very efficient thickening agent as compared to most
other polysaccharides, mainly because of the strong branching of
amylopectin.
On cooling and keeping such systems,retrogradationoccurs, which
originates from partial crystallization of the starch and can cause physical
changes like precipitation, gel formation, or gel stiffening (e.g., staling of
bread). Amylose shows much more and quicker crystallization than does
amylopectin. The original supermolecular structure is not regained. Retro-
gradation itself is reversible: it can be undone by heating and starts again
upon recooling. The retrogradation rate increases with decreasing tempera-
ture and increasing water content. The crystallization in gelatinized starch is
similar to the partial crystallization in concentrated linear polymer systems.
BIBLIOGRAPHY
The classical treatment of the chemistry and physics of polymers is very thorough,
but not easy to read; it is
P. J. Flory. Principles of Polymer Chemistry. Cornell Univ. Press, Ithaca, 1953.
A somewhat easier book, more up to date and laying stress on concepts and
principles, is
P. G. de Gennes. Scaling Concepts in Polymer Physics. Cornell Univ. Press, Ithaca,
1979.