Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 307

The value of E 0 for single-component gels with the same concentration is signi

cantly

-carrageenan. The increase in E 0 with temperature observed for
gellan or gellan-rich mixtures has also been found for agarose (Watase et al., 1989 ) and
polyvinyl alcohol systems (Nishinari et al., 1985 ), and was interpreted by the reel-chain
model discussed in Chapter 4 . From this data, Nishinari et al.( 1996 ) concluded that the
gellan and

higher for gellan than for κ

κ -carrageenan mixed gel is a phase separated gel.

10.4.4.2 Gellan agarose

Mixtures of gellan and agarose also show separated endothermic peaks in heating DSC
curves (Nishinari et al., 1993 ). A lower-temperature peak is mainly due to the agarose
component and is shifted to still lower temperatures with increasing concentration, while
a higher-temperature peak is caused mainly by gellan and is shifted to higher temper-
atures with increasing concentration. This is consistent with rheological observation:
again, E 0 for agarose gels is larger than that of gellan gels in the concentration region
1

-

4%. This suggests that,
at concentrations higher than c.2%, the additional agarose does not contribute to the
network structure effectively, and so that the mixture is a phase separated gel.
Amici et al.( 2000 , 2001 ) studied the same gellan

- 2%w/v, but this effect is reversed at higher concentrations of 3 -

agarose mixture by rheology, DSC,
transmission electron microscopy (TEM) and turbidity measurements. They observed two
separated exothermic peaks in cooling DSC, originating from gellan and agarose, but they
attributed these not to the usual phase separation

-

- which, judging from their images, they

could safely exclude but to individual networks of each component. They concluded that

this mixture forms a molecular interpenetrating network (IPN). Hitherto the evidence for
IPN physical gels was equivocal, but here it is more convincing, at least from the structural
evidence. For example, they did not observe any phase separation in TEM; instead images,
including those employing the immuno-staining technique to label one component (in this
case agarose), appear to support the

-

IPN hypothesis. It
may be impossible to eliminate the possibility of cross-network interactions.
Deducing IPN behaviour from either small- or large-deformation rheology is much
more complex. Amici et al.( 2000 , 2001 ) have argued this in terms of the mutual effective
concentrations of the two components and their contribution to the modulus, but this
remains an indirect approach. In compression measurements, adding agarose to gellan
increased the stress at failure, although the strain at failure shifted to lower values.
Although the TEM evidence appears very convincing, it is fair to say that there is still
no consensus on this system. Allowing for sample differences, it is still dif

' separate network superposition '

cult to

establish the detailed structure of gellan - agarose mixed gels, although it is hoped this

will be established by further studies.

10.4.4.3 κ agarose

Interestingly, Amici et al.( 2002 ) also showed evidence for IPN formation in the
κ

-carrageenan -

-

agarose system. Again they were able to exclude substantial phase
separation, although they conceded that a process of limited microphase separation
during gelation had to be considered more seriously. However, the density

-carrageenan

fluctuations

seen in images of the κ -carrageenan in the mixed system appeared very similar to those in