Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 331

ɸ m there is a dramatic increase in modulus, and an approximate

fit found to the classical
Hertz elastic equation, while at lower volume fractions steady shear measurements could
be made. Results from these suggest an apparent yield stress, and were related to classical
hard-sphere repulsion models. The ' 'fluid gel ' method has been reviewed by Frith ( 2010 ).

Since gelling under shear flows produces quite complex microstructures, such properties

as the interfacial tension(s) become increasingly important. By controlling these, more
precisely ellipsoidal microgel particles can be produced. The phase diagram (strictly, the
state diagram) of a mixture which is both phase separating and gelling while being
deformed, for example in a shear or extension field, may be perturbed. This route can help

to produce novel ' textures ' of use in particular applications.

11.1.4 Multi-membrane hydrogels

Ladet et al.( 2008 ) have described the formation of so-called multi-membrane structured
materials based on purely physical gels. They applied this approach to both chitosan and
alginate ( Chapter 5 ), but they claim the method can be generalized to other natural
polyelectrolytes such as hyaluronic acid. In the multi-stage process for chitosan, an
'

first formed by evaporating water at 55°C from a 50:50 water:alcohol
solution of the polymer, and with the concentration above that for chain entanglements.
When the water is almost fully removed, a gel is formed. The system is then neutralized
with NaOH, and subsequent washings in water yield a material that contains only water
(over 95 wt%) and chitosan in the free amine form. When gelation is achieved, almost
40% NH 3 + groups are still present and have to be neutralized to favour inter-chain
interactions allowing the formation of a stable material ( Figure 11.4 ). The neutralization
of NH 3 + sites removes ionic repulsion between polymer chains and, they suggest,
produces physical cross-links from hydrogen bonding, hydrophobic interactions and
crystallite formation. There appear to be parallels between these structures and the
freeze/thaw gels described in Chapter 8 .
Modi

alcohol gel ' is

cation of the balance between hydrophobic and hydrophilic interactions de-swells
the neutralized gel, and helps create a separate gel membrane structure with what the
authors term an interphase of lower polymer concentration between the neutralized
and alcohol gels. The concentration of NaOH determines the kinetics of neutralization.
At high NaOH, chains are rapidly and completely neutralized, and their aggregation is

Non-
neutralized

[ c NaOH ]

1 M 2 M 4 M7 M

2.2 cm 1.3 cm

Variation of hydrogel shrinkage during neutralization as a function of concentration of NaOH.
The initial chitosan concentration in the non-neutralized alcohol gel was constant and close to
4.5% w/w. Adapted from Ladet et al.( 2008 ) with permission from Nature Publishing Group.

Figure 11.4