Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 212

Chapter 8 ). The authors, however, warn that in their scenario the occurrence of the coil -

helix transition underlies gelation and is responsible for the thermal hysteresis, although
the evidence suggests that the hysteresis re

ects the subsequent aggregation. The coil -

helix transition and their interaction with water. Since double helices

can be approximated as rod-like particles, the phase diagram of Flory

' alters the solutes '

Onsager type
might also be relevant from a qualitative point of view, but the topological constraints
among helices prevent the occurrence of an actual nematic ordering (see also the
discussion in Chapter 3 ).
Ayma rd et al.( 2001 ) re-examined the phase diagram at a

-

fixed concentration (2 wt%),

considered as a by the previous authors, with particular emphasis on

the thermal history and time dependence, particularly in the range 36

' high concentration '

- 43°C. From their

work, it was con rmed that the cooling rate plays a major role on the state of aggregation, as

first established in the early work byDea et al.( 1972 ) and in subsequent rheological studies.
Again, the growth rate of the gel modulus, as followed by small-strain dynamic
measurements, was dramatically reduced above 35°C, so longer isothermal curing was
required for this modulus to tend towards a pseudo-plateau value or to enter a regime of
slower growth (up to 6 days at 43°C). Gelation always took place after the final temper-

ature was reached; however, kinetic effects were not totally suppressed, within the

finite
long-time scale investigated. Rather than a concentration range, the temperature range
35

-

43°C appears as a boundary between different types of gelation. Turbidity measure-
ments performed at various temperatures by quenching and annealing the sample, as for
the rheology, also showed marked differences. Samples quenched above 35°C showed a
dramatic change in turbidity, whose wavelength dependence suggested the formation of
large-scale

fluctuations of concentration in the system, i.e. polymer-rich and polymer-
poor regions ( Figure 7.21 ).

10 10 0

90
80
70
60
50

1

T ( ° C)

43
41.5
38
34.4
20
10

0.1

40
30
20
10
0

0.01

1 10 100 1000 10000

Time (min)

Figure 7.21

Time course of the turbidity measured at 800 nm for an agarose solution at 2 wt%, cured
isothermally. The cooling temperature pro le is indicated on the right-hand scale. Adapted from
Aymard et al.( 2001 ) with permission of John Wiley & Sons.