Physical Chemistry of Foods

Nevertheless, a polymer molecule cannot be treated as a simple,
compact colloidal particle. Because at all monomer–monomer bonds
rotation about the bond angle is possible, the molecule can assume
numerous different conformations. (This leaves the configurationof the
molecule intact, since a change in configuration involves breakage or
formation of covalent bonds.) A polymer molecule in solution, the common
situation in foods, would unwind to a considerable extent, leading to even
larger dimensions, about 100 nm for the present example. This is because of
Brownian motion. Solvent molecules collide with the individual monomer
segments, causing a continuous change in conformation of the polymer.
Assuming three degrees of freedom for each monomer–monomer bond (i.e.,
three essentially different orientations), a conformational entropy of
Rln3^10 ;^000 & 105 J?mol^1 ?K^1 can be calculated from Eq. (2.1). By
comparison, Eq. (4.7) would yield an entropy of (translational) mixing of
the polymer with water of the order of 10^2 J?K^1 per mole of polymer, for a
typical dilute solution. The conformational entropy in solution is thus of
overriding importance.
Altogether, polymer solutions show essential differences with normal
solutions as well as with colloidal dispersions and need a special treatment.
Highly concentrated polymer systems behave differently, again, and tend to
form amorphous solids. Typical concentrated synthetic polymers are
plastics and rubbers.
Until now, identical linearhomopolymerswere considered, but such
molecules are rather exceptional. Polymers can beheterogeneousor more
complicated in various ways:

Most polymers, especially synthetic ones, vary indegree of polymer-

izationand thus in molar mass. Moreover, long polymer molecules

will generally contain a few irregularities, since polymerization

without error is almost impossible.

Heteropolymers are (purposely) built of more than one type of

monomer. Some are copolymers, i.e., constituted of two different

monomers (A and B). The latter can be arranged in various ways:

evenly (e.g., alternating),

22 A 22 B 22 A 22 B 22 A 22 B 22 A 22 B 22 A 22 B 22 A 22 B 22 A 22 B 22 A 22 B 22 A 22

at random,

22 B 22 A 22 A 22 A 22 B 22 A 22 B 22 B 22 A 22 B 22 B 22 B 22 A 22 A 22 A 22 A 22 B 22

or in fairly long blocks built of one monomer.

22 A 22 A 22 A 22 A 22 A 22 A 22 A 22 A 22 A 22 A 22 B 22 B 22 B 22 B 22 B 22 B 22 B 22