Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 53

6000

0.5

5000

0.4

4000

0.3

3000

0.2

2000

0.1

1000

0.0

0

− 5 0 5 10 15 20 25 30 35 40 45

T ( ° C)

Figure 1.50 1/ K values from fluorescence ( • )andCD( ◦ ) data and tan δ ( ,fromrhe-
ology measurements) versus temperature using 1.0 wt% 79 in n -octane or n -dodecane.
Reprinted with permission from Ref. [15]. Copyright 2005 American Chemical Society.

temperatures of the supersaturated sols (i.e., below the macroscopically measured
gelation temperatures) is plotted in Figure 1.50. The values of 1/ K from the
different techniques are consistent, showing one temperature regime in which 1/ K
has a very small temperature dependence and another, approaching the gelation
temperature, with a very large temperature dependence. The transition between
the two regimes is close to the temperature at which the SAFIN morphology

changes from spherulites to rods. It suggests that the mode of formation

changes from being dominated by thermodynamic considerations (such as the
degree of supersaturation) to kinetic considerations (such as rates of diffusion
of

SAFIN

molecules to and from growing 1D objects). More detailed descriptions of
thermodynamic versus kinetic control of aggregation, nucleation, and growth of
1D objects are described below.
The kinetic data for gelation of

79

in ethyl acetate using Equations 1.4 and 1.5 lead
to an even more complicated picture. As seen in Figure 1.51a, the D f values remain
nearly constant (

80

1.1-1.3) throughout the temperature range explored. However,
the Avrami exponent n undergoes a precipitous jump from

∼

2 near 2.5
◦ C! Because there is no discernible differences between the optical micrographs
of

∼ 1to ∼

in the temperature regimes just below and above the point of
changing n , a change in the nucleation mechanism (rather than in the growth
and structure of the fibers) must be responsible. At incubation temperatures in

SAFIN sof 80