Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 299

studying incompatible mixed-gel systems is microscopy, and both electron and optical
microscopy have been used by a number of groups, including early work on BSA and
agarose gels (Clark et al., 1982 ). Since many microscopy stains are taken up preferen-
tially by the protein component, it is relatively simple to study the phase composition
simply by examining images showing these contrasting areas.

10.4.1.1 Gelatin dextran

In recent years several groups have examined the gelatin

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dextran system. Gelatin is
considered in detail in Chapter 7 ; the co-system dextran is a very soluble, and usually
long-chain branched, polyglucan produced by lactic acid bacteria. Strictly speaking, the
combination does not form a mixed gel since dextran itself never forms a gel under
normal conditions. On the other hand, as a model system for investigating phase
behaviour it has advantages since both components can be obtained

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quite easily. Because of this the system has been studied quite widely over recent
years. As expected, close to the cooling conditions where gelatin begins to gel, phase
separation begins, but it can be trapped (or to use the parlance common in this area,
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pinned ) in various non-equilibrium states.

The kinetics of segregative phase separation was investigated in dextran

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gelatin
system by Tromp et al.( 1995 ) using time-resolved small-angle light scattering and
phase contrast microscopy. After a quench into the two-phase region of the phase
diagram, at or below the gelling temperature, the resultant two-dimensional scattering
pattern was recorded. This showed a ring corresponding to a peak in radially averaged
scattered intensity, and is characteristic of spinodal decomposition (SD). For the samples
quenched to the higher temperature, the logarithm of the scattered intensity plotted versus
time showed an initial linear increase, as expected from linearized SD theory. At lower
temperatures the initial linear growth regime was of very short duration, making it hard to
observe. When the

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fixed above or below the gelation
temperature, a difference in behaviour was clearly observed by phase contrast micro-
scopy as the morphology coarsened.
As shown in Figure 10.5 , the position of the lower quench temperature scattering peak
moves to lower q values as time progresses. This indicated that for q values close to q m
the system was outside the range of validity of the linear theory of SD. The initial time
measurements in Figure 10.5 showed a maximum at q m ~1μm − 1 , corresponding to a
characteristic length of 6 μm. Cahn

final quench temperature was

Hilliard plots were also drawn and they are seen to be
far from linear, for both gelling and non-gelling systems. A possible explanation for the
overall curvature of the Cahn

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Hilliard plots is that there is more than one time scale
governing the phase separation kinetics in this case (e.g. polydispersity effects) or that the
very early stages were not captured because of experimental limitations.
Direct microscopic observation of this system gave images consistent with the scatter-
ing data. Phase contrast micrographs at two temperatures showed that, for both high- and
low-temperature quenches, the pattern has the same network-like character, consistent
with a common mechanism of phase separation in the early stages. Qualitative differ-
ences appeared at later times: at the higher temperature, droplet-like domains appeared
after about 10 min, whereas such a transition did not take place for the low-temperature

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