Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 211

associated with (Watase et al., 1988 ), while other studies have

extended this work to examine agarose gels in sucrose and other solutions. Studies of the
sol

' non-freezable water '

gel transition include the work by Watase and co-workers (Watase et al., 1989 ;
Nishinari et al., 1992a , 1992b ). They noticed an initial endothermic peak at 75°C, shifting
to higher temperatures with increasing concentration of agarose gels. An exothermic peak
was seen at lower temperatures

-

and was attributed to the increase in
ordered regions. More recent data collected for agarose in sucrose solutions is discussed in
Section 7.3.4 below.

- around 30°C -

7.3.2 Phase behaviour

At a molecular level the helix coil transition observed by OR and WAXS is well

established, but the subsequent aggregation mechanism is still not fully clear, although
it has been examined by several authors and a coherent picture is now slowly emerging.
Pines and Prins ( 1973 ) were the

-

first to report small-angle light scattering data for low-

concentration gel systems (0.5 1 wt%). This showed a distinct maximum in the scattered

intensity versus the scattering vector, generally ascribed to phase separation occurring by
spinodal decomposition ( Chapters 8 and 10), and raised the question of the mechanism of
gel formation with phase separation occurring at low concentration. Since then, this point
has been examined by many papers, in particular by San Biagio and co-workers (San
Biagio et al., 1996 ). They noted two domains, at low and high concentrations. When
aqueous agarose solutions at concentration 0.5% w/v were quenched to 47.5°C, their
experiments showed de-mixing of the sol and the sol

-

gel transition as two distinct
phenomena, occurring on very different time scales. (Here it is important to appreciate
that, although the concentration used in this work was above the expected minimum gel
concentration, say, 0.2 wt% at 25°C, the temperature was also higher than normally used
in preparing a macroscopic gel, so the effective minimum concentration was increased.)
Within the

-

200 min, the sol underwent spinodal demixing as detected by
several features ( Chapter 10 ), including the occurrence of a forward-scattered light pattern
(a small-angle scattering ring), the exponential growth of the scattered light intensity and
the linear Cahn plot growth rates. Within this time, the sample remained liquid, with the
appearance of a

rst 100 -

viscosity peak. At later times, between 100 and 1000 min, the
number of free polymer coils in the sol decreased substantially and a population of

' transient '

' objects '

having a diffusion coef cient two orders of magnitude smaller than that of the free polymer

coils was observed. After several days, in sols of various polymer concentrations, the
diffusion coef cients of these species showed an equilibrium size, with no sign of macro-

scopic gelation such as viscoelasticity or turbidity. Macroscopically, the sample remained a
liquid. Samples containing objects of sizes 0.2

0.4 μm could be redissolved by heating,
however, with the same thermal hysteresis as macroscopic gels. Such objects appeared
therefore to be

-

' '

(obtained by spinodal demixing), where local gelation had
occurred. At higher concentrations (c

mesoscopic

≥

2% w/v) direct gelation occurred around 70°C and
no sign of spinodal demixing was observed.
The phase diagram proposed by San Biagio et al.( 1996 ) for the gelation of agarose
solutions is similar to the one proposed by Tan et al. for atactic poly(styrene) (see