Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 137

Fig. 3.64 Experimental E ʱ
versus T dependence ( circles )
for gelation of 40 wt. % solu-
tion of gelatin in water. The
solid line is a fit by Eq. 3.90

-200

-250

-300

-350

-400

280 285 290 295 300 305

T / K

As seen from Fig. 3.64 , this equation fits quite well the temperature dependence of
the effective activation energy for gelation of 40 wt. % solution of gelatin [ 164 ]. The
dependence has been derived from the E ʱ versus ʱ dependence by replacing ʱ with
the average temperature corresponding to this conversion at different cooling rates.
T gel for this solution is ~ 317 K as estimated by extrapolating the gel melting data of
Godard et al. [ 158 ].
Although the application of the Turnbull-Fisher and Hoffman-Lauritzen models
to the process of gelation is rather empirical, it can still be used to extract mean-
ingful information, especially for comparative purposes. One such example is the
effect of the concentration on the kinetics of physical gelation of aqueous solutions
of methylcellulose [ 169 ].
Aqueous solutions of methylcellulose gel on heating. The mechanism of the pro-
cess has been examined in a number of studies, the results of which are briefly
summarized by Kobayashi et al. [ 173 ]. Methylcellulose has the inverse solubility in
water, i.e., it is soluble in cold but not in hot water. Dissolution occurs via hydration
of methoxyl groups. When the solution temperature is increased, hydrogen bonds
break, causing dehydration of hydrated methoxyl groups. The latter then undergo
hydrophobic association forming a network, i.e., a gel.
Breakage of hydrogen bonds is accompanied by an endothermic effect that can be
used to monitor gelation by using DSC. Typical DSC curves of gelation are shown
in Fig. 3.65 . An increase in the concentration of the methylcellulose solution causes
some small shift of the process to lower temperatures. The heat of gelation per gram
of methylcellulose is about − 7 J g −1 for both 2 and 4 % solutions that indicates that
both samples have reached similar extent of cross-linking. For 8 % solution, the heat
of gelation is − 5 J g −1 ; that means, that the extent of cross-linking is about 30 %
smaller than in the two other samples. This is not surprising because the molecular
mobility of the methylcellulose chains in the highly viscous 8 % solution should be
dramatically slowed down, therefore, limiting the process of cross-linking.