Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 144

Fig. 3.71 E ʱ dependen-
cies obtained for melting of
40 wt. % gels obtained in one
and two 30-min annealing
steps performed at tem-
peratures shown by the point
types. Arrow shows respec-
tively comparable parts of
two- and one-step annealing
processes. (Reproduced from
Dranca and Vyazovkin [ 178 ]
with permission of Elsevier)

200

180

160

140

120

15 o C
25 o C
25, 15 o C

100

80

0.0 .2 0.4 .6 0.8 .0

α

larger by 10-30 kJ mol −1 than those for the gel produced at 15 ﾰC that indicates that the
former gel has a few more hydrogen bonds per junction than the latter.
Another instructive experiment that illuminates the effect of the structure on gel
melting involves two successive 30-min steps of annealing at 25 and 15 ﾰC. A DSC
curve for melting of the resulting gel is displayed in Fig. 3.70 . The curve reveals
two distinct melting peaks that apparently reflect the existence of two gel structures,
which differ markedly in their thermal stabilities. The higher-temperature melting
peak is found at the same temperature as the melting peak for the gel produced
by single-step annealing at 25 ﾰC. This is in agreement with the rheological mea-
surements [ 179 ] that demonstrate that the structures formed at higher annealing
temperatures are conserved during annealing at lower temperatures. In contrast,
the lower temperature peak is detected at about 2-3 ﾰC lower than the melting peak
temperature for the gel produced by single-step annealing at 15 ﾰC. That is, the gel
produced on annealing at 15 ﾰC in the two-step procedure is less thermally stable
than the one formed in a single-step annealing at 15 ﾰC. It means that the existence
of the gel structures formed at 25 ﾰC hinders the growth of the network junctions
being formed at 15 ﾰC. Apparently, the hindrance is due to the fact that in partially
cross-linked gel, the conformational degrees of freedom of the polypeptide chains
are largely restricted. Therefore, it is more difficult for new cross-links to form and
for the network junctions to grow.
The E ʱ dependence determined for melting of the gel annealed consecutively at
25 and 15 ﾰC is shown in Fig. 3.71 . The dependence is easy to understand by com-
parison with the individual dependencies estimated for melting of the gels produced
by the respective single-step annealings at 15 and 25 ﾰC. The initial descending
portion of the dependence is similar to the initial portion of the E ʱ dependence
for melting of the gel annealed at 15 ﾰC. For both dependencies, the E ʱ values at
the lowest conversions are ~ 170 kJ mol −1 that suggests that the energy barriers
to melting in both gels are similar. According to DSC (Fig. 3.70 ), melting of the
gel structures formed on annealing at 15 ﾰC contributes less to the total heat of