Biomedical Engineering Reference
In-Depth Information
Work by Giannouli and Morris (
2003
) has shown that xanthan can also form freeze/
thaw gels. They prepared some of these by heating aqueous xanthan solutions (0.2
-
2%
w/w) to 80°C , freezing rapidly to
20°C, holding for 24 h and then allowing the frozen
mass to warm to 5°C before testing. The resultant gels were relatively stable, but could
be melted out by heating in the range 20
−
-
40°C. According to the authors, the cryogels
have a
'
soft, spreadable texture, similar to the consistency of normal jams and marma-
lades
. However, recovery of structure appeared to occur almost immediately after
spreading.
From this it may be concluded that a number of (perhaps all) thermodynamically poor
aqueous polysaccharide solutions can be converted into cryogels, either by shock cooling
or by employing freeze/thaw cycling. What is perhaps more interesting is to investigate
how stable these gels are. For example, if they are left with in an excess of water at room
temperature, do they then redissolve? In other words, are they merely trapped as a high-
concentration solution that can then redisperse with time, as appears to be the case for
LBG, or do they show equilibrium swelling behaviour? This hypothesis appears not to
have been widely tested.
'
8.7
Conclusions
The review of the mechanisms of gelation of well-known, widely used polymers
presented in this chapter illustrates the very complex behaviour that physical gels can
exhibit. The nature of the solvent and the chain tacticity both play a crucial role in the
ability to create a network and to give solutions with interesting mechanical properties.
However, the various mechanisms of gelation are very dif
cult to classify within a clear
thermodynamic framework since they include crystallization, phase separation, glass
transition or more subtle effects of local conformation, stacking of chains and polymer
solvent complexes. Results from the abundant literature suggest that none of these gels is
in equilibrium. The contradictory results stress the major in
uence of the whole history of
the sample from dissolution to gel preparation and further thermal treatments. This non-
equilibrium,
cult to
characterize from either a theoretical or an experimental point of view. For none of the
gels examined in this chapter do rheological measurements link to structural parameters.
The experimental observations show the in
'
frustrated
'
structure in these and other physical gels is very dif
uence on the viscoelastic moduli of polymer
concentration, temperature, molecular mass, solvent, thermal history and whole process-
ing. At this stage of investigations, no-one is able to relate rheological parameters directly
to topology, morphology or microstructure of the physical gels. However, morphology
obviously has a major effect; for example, by creating different morphologies via
processing, initially poor mechanical properties of a PVA cryogel can be greatly
improved. This ability to modify material properties by various treatments applied to
physical gels reminds us of the traditional metallurgy approach. We believe that this trend
will develop very much in the future.
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