Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 302

100 μ

1% 1.5% 2% 2.5% 3% 5%

Detail from 1% Detail from 3%

0.5 μ

Light microscope (top row) and electron microscopy (bottom row) results for a series of agarose -
gelatin gels (1% w/w agarose). Gelatin appears as the darkly staining material, the maximum
concentration being 5% w/w. From Clark et al.( 1983b ) © 1983 American Chemical Society.

Figure 10.7

Stress relaxation measurements were made on aqueous gels (Watase and Nishinari, 1980 )
and the observed values of elastic modulus were compared with those calculated using a
formula for composite systems proposed by Okano ( 1962 ), as mentioned above. Since the
observed values were much smaller than the calculated values, the gelling process of the
agarose was considered to be not a simple dispersion of agarose in a gelatin medium, but
rather the hindering of agarose gelation by gelatin. In calculating the elastic moduli of the
mixed gels, no allowance was made for the redistribution of water after mixing the solutions
of agarose and gelatin, whereas this was allowed for in the work of Clark et al.( 1983 ).
These workers also examined the same co-gels by electron and light microscopy
( Figure 10.7 ) and mechanical spectroscopy. The microscopy evidence showed phase
separation of the two polymer networks, with phase inversion occurring at speci c

mixture compositions for example in Figure 10.7 at around 1% agarose, 2.5% w/w

gelatin. Mechanical characterization of the gels was achieved by measurement of the
shear modulus at small deformations, introduction of the solvent partition parameter p a
into the two phases and use of the polymer blending law proposed by Takayanagi et al.
( 1963 ); see Equations ( 10.7 ) and ( 10.8 ).
This approach and variations thereof have been employed for a wide range of phase
separating mixed-gel systems over the succeeding years, although its basis is still
essentially empirical. Applications of this approach have included papers on gelatin

-

-

- β -

low methoxy pectin, sodium caseinate maltodextrin

(Kasapis, 2008 ). Morris ( 1992 ) has even suggested a way to determine self-consistent
values of p a by analysing modulus behaviour.
As far as large-deformation measurements on mixed gels are concerned, the vast
majority of workers have used compression measurements, which remain dif

barley -glucan and gelatin

cult to
interpret in terms of fundamental parameters. However, McEvoy and co-workers
(McEvoy et al., 1985 ) extended their work on the failure of agarose and gelatin gels,