Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 46

measuring in highly viscous low molar mass solvents such as saturated sucrose, is an
active area of interest.
What the above does suggest is the well-known time

temperature superposition (TTS)
effect in polymer materials science, namely that high frequencies and low temperatures
may be regarded as equivalent (Ferry, 1980 ). Very often this works well, but caution
should always be applied. The glass transition itself is related to polymer free volume,
and temperature discontinuities in said free volume should make the approach invalid. If
we are to follow the principles outlined by Ferry

-

- one of the co-devisers of the method,

and its strongest protagonist then TTS should never be applied within 50°C of a phase

transition within the system. This would eliminate all TTS approaches for water-based
(hydro-)gels from

-

50°C to +150°C, i.e. more than the whole regime of potential interest.
This is an extreme view and, provided no deep interpretation is given, the approach may
still be useful. It has been employed by several workers.
Here it is important to point out that many gelation experiments in the literature have
been performed using temperature

−

, i.e. heating, say, from 25°C to 85°C at a rate
of +1°C min − 1 . This applies whether the temperature ramp is positive, as here, in order to
form heat-set gels or melt out cold-set thermoreversible systems, or negative, i.e. a
cooling ramp applied to

' ramps '

cold-set gels such as gelatin. From the theoretical and
interpretation viewpoint this is not ideal, since the rate of heating is con

' set up '

ated with the
temperature rise, so analysing the real kinetics of gelation becomes impossible. For this,
it is far better to perform the experiment as an isothermal temperature jump, although this
approach is experimentally more testing because it obviously requires very fast and
ef

cient heating, cooling and re-equilibration.
In early instruments, where a hot-air heating system was used and cooling involved
boiling off liquid nitrogen, ±20°C min − 1 was easily achieved. Later,

fluid circulation
systems, which are intrinsically slow to change and to re-equilibrate, were used. Then the
only way to obtain fast rates was to have a swappable liquid bath system, with one bath at
a low temperature and the other at a high temperature, and a tap turned to change from
one to the other. Nowadays, Peltier systems are all but ubiquitous even for controlled-
strain instruments, and so there is less of a problem. That said, some of the data discussed
in other chapters has been obtained using the temperature-ramp approach, and the ' ramp '

approach is still widely used, not least because it tends to mirror the DSC technique.

2.5.1.6 Time-dependent systems

The kinetic gelation experiment
Clearly if we are by some physical method converting a solution to a gel,

we will change the initial sol mechanical spectrum ( Figure 2.16 ) to the gel spectrum
( Figure 2.17 ) (Kavanagh and Ross-Murphy, 1998 ). In a typical experiment, following the
progress of gelation using mechanical spectroscopy, the oscillatory frequency is kept
constant at c.1 Hz (6.28 rad s − 1 )

- say, heating -

-

for convenience many workers use a frequency of

10 rad s − 1

-

and the strain is maintained constant and low, say typically 10% or less. The
choice of frequency is always a compromise

we need a high enough value that a single
frequency measurement does not take too long, so we can collect enough data, but not so
high that instrumental artefacts begin to appear. In our experience these can be seen quite

-