Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 213

Solutions were transparent at high temperature. Upon cooling to T < 35°C, a sharp
increase in turbidity was observed, due to double helix formation and aggregation of
helices into bundles. Major differences were observed when curing occurred at
35°C < T < 43°C. First, a marked delay in the onset of turbidity was apparent, increasing
with increasing temperature. Second, the rate of increase in turbidity was much lower
than for elasticity. Both effects were qualitatively similar to the apparent gelation time
observed in rheology. Finally, much higher turbidity values were observed in this
temperature range. The wavelength dependence of turbidity with time, at 41°C and
43°C, was interpreted in terms of heterogeneities. Assuming that the contribution of
the heterogeneities dominates the scattering at the end of curing at the high temperature,
the use of the Debye Bueche expression in light scattering experiments yielded an

average size of about 350 nm at 38°C and larger than 5 μm at 41.5°C and 43°C. For
further con

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fine structures of agarose gels with various thermal histories
were also evaluated by TEM. Gels formed at 20°C appeared homogeneous at distances
larger than the correlation length, similar to previous reports in the literature. At 43°C,
micrographs showed a truly phase separated microstructure with polymer-rich and
polymer-poor domains.
These experiments, together with previous investigations, show that gelation and
phase separation are competing processes. It is likely that complete demixing would
represent the thermodynamic equilibrium for agarose helices, but this is prevented by
gelation, which traps the system away from equilibrium. On this basis, the existence of
two temperature domains could be explained in terms of a competition between osmotic
forces (in favour of a phase separation of the helical conformation) with elastic forces
(which tend to prevent this). When solutions were quenched below 35°C, the rapid
development of the network of aggregated double helices stopped the demixing process
and a pseudo-equilibrium state was reached.

rmation, the

7.3.3 Mechanical properties

Gel mechanical properties have been investigated under both small and large strains or
stresses. Agarose gels are easily fractured, and the linear region can only be accessed at
lower strains. The vast majority of studies have been carried out on gels with concen-
trations below, say, 3 wt% (for high M w starting materials), since producing homoge-
neous materials at higher concentrations is very dif cult. That said, a recent paper on agar

(note, not agarose) gels (Ayyad et al., 2010 ) has used a novel

method to
produce homogeneous materials of up to 30 wt%. However, this seems to simply involve
heating the sealed sample in an oil bath up to 145°C. The authors claim that this does not
degrade M w , and the high values of moduli obtained tend to support this. Extension to
higher-quality agarose samples seems to be worthwhile.

' hydrothermal '

7.3.3.1 Effect of concentration

Just as for other physical gelling systems, increasing the concentration of agarose above
the minimum required for gelation produces a higher-modulus product. The published
data is discussed in more detail below, with both M w and c varied. To summarize here, the