Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 215

10 6

F5

F4

10 5

F3

F2

F1

10 4

0 10 20 30 40 50 60 70 80

T ( ° C)

Temperature dependence of storage modulus E ' for five agarose fractions with M w from 3.4 × 10 4 g
mol − 1 (F1) to 48.5 × 10 4 g mol − 1 (F5). Reprinted with permission from Nishinari and Watase
( 1993 ) © 1983 Springer.

Figure 7.22

7.3.3.3 Effect of temperature

Nishinari and co-workers (Nishinari et al., 1992a , 1992b ) investigated the temperature
dependence of the modulus s modulus E 0 -

over the range
5°C to 80°C. For lower M w samples, this range tended to exceed the melting temperature,
but generally the modulus showed a slight increase before the expected decrease in the
melting range. Perhaps surprisingly in view of the structure, these moduli showed rubber-
like behaviour, i.e. proportionality to absolute temperature in the lower temperature range
( Figure 7.22 ). These results can be analysed in terms of both entropic and enthalpic
contributions to the elasticity, with the former dominating at lower temperatures and the
latter at higher temperatures. The melting behaviour, particularly as investigated by DSC,
is amenable to treatment by the

- mainly the storage Young '

' zipper model ' ( Chapter 3 ).

7.3.3.4 Other characterization work

In solution, the overlapping concentration c* for agarose was estimated by Aymard et al.
( 2001 ) to be 0.6 wt% and by San Biagio et al.( 1996 ) to be 1.5 wt%; both values are also
in the range of gelatin c* (0.5 wt%). The gelatin chain is more flexible than agarose;

therefore c* should be lower in agarose at equivalent molecular masses. The gel mesh
size determined by Aymard et al.( 2001 ) in the homogeneous state of gels at c = 2 wt% is
about 100 nm, while at these concentration it is only a few nm in gelatin. This indicates
that there are greater void spaces in agarose gels than in gelatin gels.

7.3.3.5 Large-deformation behaviour of agarose gels

The data here is also somewhat limited, and few workers have used a rigorous geometry
set-up so that the actual stress and strain are obtainable. Here we summarize results
obtained by a number of workers (Watase and Nishinari, 1983 ; McEvoy et al., 1985b ;
Normand et al., 2000 ). The Watase results were obtained with the same stress relaxation
apparatus used in the small-deformation work, and they studied the various M w samples,