Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 206

d

Fully developed gelatin network made of rigid rods of a typical length L separated by a
distance d. The rods are connected by flexible junctions in the coil conformation. This structure is
similar to TEM images, far from the gelation threshold. Adapted with permission from
Joly-Duhamel et al.( 2002a ) © 2002 American Chemical Society.

Figure 7.17.

network. The model supposes that the connections between rods are very flexible

compared to the rigidity of the triple helix, and therefore gels can support large deforma-
tions which are reversible, even if the frame of rods is rigid by itself. The model also
assumes that the length and the distance between strands are related to each other, so that
there is only one independent length related to c helix .

7.2.9 Mechanical properties including high strains

The vast majority of data in the literature, including that of te Nijenhuis, was obtained
using the oscillatory shear method, although creep results are also interesting. One of the
commonly observed features of a gelatin is that there is a

pronounced minimum in the G 00 versus frequency spectrum for gelatin gels, but not for
most other gelling systems, with a few exceptions, e.g. the E 00 minimum found in agar
gels (Nishinari, 1976). This is well illustrated in te Nijenhuis ( 1997 ), and this minimum
suggests that, at lower frequencies, there is a maximum which reveals a long time

' mechanical spectrum '

ow
process occurring at frequencies ~10 - 5 s − 1 . This corresponds quite accurately to the
retardation times measured in a series of creep experiments. Equilibrium compliances
from these experiments can be estimated reasonably accurately; very high apparent creep
phase viscosities (c.10 8 Pa s) are derived.
There are surprisingly few reliable measurements of the large-deformation or failure
properties of gelatin gels. This is because even a typical 20 wt% gelatin gel is not easily
self-supporting, and the commonly used but less rigorous cylindrical compression
method is far more popular than the more easily analysed tensile mode. Also, relatively
few papers have considered the actual stress - strain behaviour instead of more empirical

(Bloom-like) units.
The work by McEvoy et al.( 1985a ) and its later extension by Bot et al.( 1997 ) have
concentrated on the tensile technique, however, and analysed the resultant data in terms
of a rubber-like rupture method. Both groups employed the phenomenological equation