Introduction - Physical Gels from Biological and Synthetic Polymers - page 9

Water and electrolyte solutions are often good solvents for biopolymers. However,
after chemical modi

cation via inclusion, grafting or substitution of non-polar (hydro-
phobic) groups, the solubility of such modi

ed polymers may be limited, so that the
aqueous solvent becomes poorer, especially with increasing temperature. With electri-
cally charged polymers (polyelectrolytes), solubility is also reduced when water contains
large amounts of salt (high ionic strength). The overall mechanism of physical gelation
often lies in this subtle interplay between solubility and aggregation. The is a

true solution. Then, with a combination of several factors including temperature, pH,
polymer concentration, polymer molecular mass and ionic strength, there is a reduction
in this ability to solubilize the polymer. This alone would normally produce a precipitate,
a two-phase liquid-droplet morphology or even crystallization. However, in gels there is
still connectivity within polymer-rich domains or phases. The mechanisms reported
below have been identi

' sol state '

ed as being among the features that are characteristic and give
rise to the formation of physical gels.
Some of these structures at different distance scales are shown in Figures 1.1 and 1.2.
Figure 1.1 introduces the differences between chemical, physical and colloidal networks,
drawn to the same scales. Figure 1.2 illustrates the various mechanisms leading to
physical gel structures:

1. Conformational changes of the polymer (e.g. the coil - helix transition) which give rise

to more rigid domains. In this category we triple helix transition in

gelatin ( Figure 1.2a ), the aggregation of helices in certain carrageenans ( Figure 1.2b )
and the double helices in agarose, and the

find the coil -

' egg-box ' structure in alginate gels

( Figure 1.2c ). The first two cases are driven by lowering the temperature; the last

two are induced by temperature changes and/or speci

c ionic content.
2. Denaturation of globular proteins under conditions where the protein remains essen-
tially globular induces aggregation, so producing colloidal type networks formed with
branched structures ( Figure 1.2d ) or linear ones ( Figure 1.2e ). These consist of
particulate networks including the casein networks formed in milk clotting or cheese-
making. Such aggregation is usually irreversible, but shares some characteristics with
the phase separation of model colloidal systems. Since the assembly can involve
mutual hiding of hydrophobic groups exposed during denaturation, it also has features
in common with the systems immediately below.

(a) (b) (c)

Figure 1.1

Types of network structure: (a) point junctions (chemically cross-linked); (b) junction zone
systems; (c) colloidal network strands. These are not drawn to scale, since in (a) and (b) the strands
are typically <0.3 nm in thickness, whereas in (c) this dimension can be >2 nm.