Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 251

hard-sphere interaction radius R HS corresponds to the average distance between the
crystallites. The values of R HS are of the order of 25 nm, whereas the average size of
crystallites (R ~3 nm) is close to the value determined by X-ray diffraction. The size
polydispersity of the of crystallites is remarkably small, close to 1.1 at N c =9.

8.6 Cryogels from polysaccharides

Cryogels from polysaccharides have been investigated for a number of years, although
most work has been performed on the galactomannans, particularly locust bean gum
(LBG). Galactomannans are essentially linear macromolecules with a backbone of

β

(1 6)-linked

d-galactopyranose (Gal, G) residues. In LBG the mannose/galactose (M/G) ratio is c.4:1
( Chapter 10 ). However, the mechanism is not restricted to these, and cryogels have been
prepared from a number of other polysaccharide systems, including xanthan, curdlan,
certain

4)-linked d-mannopyranose (Man, M) residues to which are attached (1

→ →

polymer solution
is, in thermodynamic terms, a poor solvent system. This implies that polysaccharide

β -glucans and starches. In all relevant cases it seems the water -

-
polysaccharide interactions are relatively favourable. Under equilibrium conditions this
would result in either liquid

-

liquid phase separation or precipitation, but, as we have seen
for the synthetic systems, the process may be frozen, literally, into a non-equilibrium
state. Below we survey some of these systems.
Perhaps the most studied system is LBG in water (McCleary et al., 1981 ; Tanaka
et al., 1998 ; Doyle et al., 2006 ), and here gels form as a result of extended cryogenic
treatments (freezing, frozen storage, thawing), whereas at positive temperatures the
same solutions remain as

fluids during prolonged storage (only after 2 or 3 months do
they transform into very weakly structured systems). The mechanism seems to be quite
straightforward in that the formation of ice crystals raises the concentration of the
polysaccharide and so drives polymer polymer interactions that they become largely

irreversible. When the initial polymer concentration exceeds a certain critical value,
which often depends on the particular type of polysaccharide and of the freezing/
thawing conditions, formation of freeze-induced gels is observed. Gels are thermo-
reversible in nature; they can be melted upon heating and will re-form a cryogel during
the next freeze/thaw cycle. The cryogels resemble soft sponges when the initial
polysaccharide concentration is about 0.05% w/v, and they release free liquid when
lightly compressed; they evolve towards cellular textures at concentrations around
0.75%w/v. The formation of non-spongy and more homogeneous cryogels only occurs
when more concentrated solutions (2.5

-

3% w/v) are used. These materials, in contrast
to the less concentrated sponge-like structures, do not release free liquid under mod-
erate compression after cryogenic treatments.
McCleary and co-workers (McCleary et al., 1981 ) studied a range of galactomannans
produced by enzymatic (

-

α

-galactosidase) treatment of guar (M/G ratio around 2:1) to
convert it to galactose-lean samples. When the M/G ratio lay in the range 80:20 to 85:15,
freeze/thaw gels could be produced, whereas at lower galactose content (90:10) a
precipitate was formed. Lozinsky et al.( 2000 ) provided evidence for the participation