Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 242

these two signals (tt/tg or I 860 /I 843) has been used as a quantitative measure of the degree
of coil

helix transformation.
This IR transition occurs during cooling in the same temperature domain as that
observed in DSC experiments. The IR data indicates that the amount of conformational
change increases with decreasing gelation temperature and reaches an equilibrium value
at every gelation temperature (during cooling). Consequently, the proportion of regular
sequences available for network formation increases correspondingly. Network forma-
tion represents the second step in the gelation: it is responsible for the hysteresis observed
between cooling and heating in both DSC and IR measurements, and it occurs at a

-

nite

rate which depends on the concentration of ordered sequences. This is further con rmed

by rheological measurements.

8.4.1 Rheology of the stereo-complex gels

Aggregation is a very complex process, depending on many factors, but the effect of time
is of great importance. The formation of

aggregates, which is relatively rapid
and which is preceded by an even more rapid change of conformational structure of the
stereo-regular sequences, is followed by the very slow process of so-called

' primary '

' secondary

'

aggregation , with steadily increasing particle size. This process has been detected by

various physical methods, albeit with different sensitivities.
The rate of gelation increases with decreasing temperature because of the increasing
number of regular sequences present. Instantaneous gelation occurs at temperatures
where at least 50% of all the transformable sequences have a regular conformation, for
instance room temperature. At higher temperatures, gels are formed at a measurable rate,
as deduced from rheological observations. The

first step is apparently dominated by an
equilibrium constant with a position that changes with temperature. The second step is
not at equilibrium since, as a consequence of intermolecular associations, a temperature
hysteresis is observed.
The viscoelastic behaviour of gels of the PMMA stereo-complexes in o-xylene (c ~5 -

12% w/w) was studied by Pyrlik and Rehage ( 1975 ) using rheological techniques. They
showed that G 0 increases with time and depends strongly on temperature. After pro-
longed ageing, G 0 and G 00 depend only weakly on frequency. The results also depend on
the thermal history of the solutions: fast gelation occurs when the solutions are heated at a
temperature not suf ciently high to destroy the primary nuclei of stereo-complexation.

(It was found necessary to heat the gels at 145°C, the boiling point of o-xylene, to fully
erase the thermal history.) G 0 values can reach values of the order of 10 4 or 10 5 Pa at lower
temperatures, from room temperature down to 20°C (Pyrlik and Rehage, 1975 ).

In Figure 8.13 the macromolecular component of the gel was a mixture of 5 parts
s-PMMA and 4 parts i-PMMA, giving a ratio of triads of about 1:1. Using o-xylene as
solvent, the polymer concentration was 12.6% w/w (Pyrlik and Rehage, 1975 ). The
authors also observed that syneresis accompanies the gelation process of these thermor-
eversible gels. Stress relaxation at constant strain is observed after long periods of time,
indicating that the physical network can rearrange slowly (10 3

−

to 10 4 s), as shown in

Figure 8.14 for a low initial modulus (~15 Pa) gel in s-PMMA.