Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 216

but at a fixed concentration (2.4 wt%). In all cases failure occurred at comparatively small

strains, but at the highest M w the strain to fail extended to c.20%. This is, of course, much
lower than found for gelatin, con

rming the more brittle nature of this material, but up to

this point the stress - strain curve is close to linear, i.e. there is no indication of yield

behaviour.
The McEvoy work used the

sample method described above for gelatin, for
1 wt% and 2 wt% samples. Results are in good agreement with the Watase results, as far
as the stress

' racetrack '

- strain behaviour and ultimate properties are concerned. From the deforma-

tion rate 30%. Data for

gelatin at approximately the same c/c 0 , the appropriate comparison in this case, shows
failure occurring at much higher strains (40

- dependent failure envelope, strain to break fell in the range 8 -

100%).
The small-deformation work by Normand et al.( 2000 ) was mentioned above. For
large-deformation measurements in compression they used pre-moulded cylinders and
lubricated the test bed; in tension they used dumbbell-shaped cutters applied to gelled
sheets. Data for the two deformation modes gives fair agreement in terms of initial
modulus, but failure of cylindrical samples in compression is always ambiguous. In
tension their results compare reasonably well with those described above, with a linear
stress

-

strain trace up to the point where failure occurs. They comment that strain to fail is
largely invariant with concentration at c.40 wt%, but regrettably they did not report
measurements of failure at various rates, or construct the relevant failure envelope.

-

7.3.4 Solvent effects

The state of aggregation can be effectively modi ed by changes of the solvent, as was

first proposed by Watase et al.( 1990 ): upon addition of sucrose, agarose gels lose their
turbidity and eventually become transparent, presumably partly a refractive index
matching effect and partly structural, as discussed below. These phenomena were

first investigated in detail by Nishinari and co-workers (Watase et al., 1990 ;
Nishinari et al., 1992a , 1992b ). In more recent experiments reported by Normand
et al.( 2003 ), large amounts of sucrose were added in water, and calorimetric and
rheological measurements were performed for gels with various molecular masses and
concentrations. A distinct exothermic peak, generally attributed to double helix for-
mation, was observed during setting (cooling) of 2 wt% solutions. As sucrose was
increased from 0 to 60 wt% the enthalpy remained almost constant at

11 kJ
mol − 1 per residue, but decreased by 20% at 70 wt% of sucrose. This effect can be
ascribed either to a sudden decrease of the enthalpy of the coil

Δ H=10 -

helix transition or, more
likely, to a decrease in the helix fraction. Increasing sucrose concentration in the
solvent up to 60 wt% leads to an increase in the apparent gelation temperature, the
rate of gel formation and the elastic modulus obtained at long times. Within this range
of sucrose concentrations, sucrose seems to accelerate the gelation process. However,
when sucrose concentration exceeds 60% the reverse trend is observed and the gelation
process seems to slow dramatically. A monotonic increase in the loss modulus at long
times is observed with increasing sucrose content, in the binary solvent. This is
consistent with previous observations, which report that sucrose reduces the quantity

-