Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 135

2.5
5
7.5
10
12.5

-10 0 10 0 0 0

T / o C

Fig. 3.62 DSC curves for gelation of 17 wt. % solution of gelatin in water. The numbers by the line
types are cooling rates in ﾰC min −1 . (Reproduced from Chen and Vyazovkin [ 165 ] with permission
of Wiley)

by multiple hydrogen bonds. Gelatin dissolves readily in hot water. Decreasing the
temperature of an aqueous gelatin solution increases the stability of the hydrogen
bond cross-links that allow the polypeptide chains to restore a helical structure. This
process takes place in dilute gelatin solutions [ 163 , 170 , 171 ]. In concentrated solu-
tions, cross-linking becomes predominantly intermolecular, so that instead of form-
ing separate helices, the polypeptide chains form an infinite network (i.e., a gel), in
which partially restored helices serve as cross-link centers (i.e., network junctions).
The formation of hydrogen bonds during gelation of a gelatin solution produces
sufficient amount of heat to follow the process by DSC (Fig. 3.62 ) [ 165 ]. On cooling,
gelation becomes detectable below 40 ﾰC. The gelation temperature depends on the
concentration of the solution as seen from the phase diagrams (Figs. 3.58 , 3.59 ).
According to rheological measurements by Michon et al. [ 172 ], a decrease in the
concentration of a gelatin solution from 20 to 1 wt. % causes a drop in T gel from 33
to 26 ﾰC. Just as in the case of crystallization, the DSC peaks shift to lower tempera-
ture with increasing the cooling rate (Fig. 3.62 ).
The application of an isoconversional method to DSC data on gelation of a gelatin
solution is illustrated in Fig. 3.63 . The obtained E ʱ on ʱ dependencies demonstrate
negative values of the effective activation energy. The E ʱ values tend to increase with
the extent of the sol to gel conversion that is explicable by the departure from the
equilibrium temperature (i.e., T gel ). In accord with either Turnbull-Fisher (Eq. 3.45)
or Hoffman-Lauritzen (Eq. 3.60) equation, the effective activation energy turns to
− ∞ at the equilibrium temperature but increases toward zero as temperature departs
from the critical value. A similar type of dependencies (i.e., negative E ʱ increasing
with ʱ ) is found for crystallization of polymer melts measured on continuous cool-
ing, i.e., when an increase in the extent of the melt to crystal conversion reflects the
departure from the melting point. An example of such behavior is seen in Fig. 3.37 .