Chemistry Reference
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and the ICF develops a plateau whose height can directly be interpreted in
terms of the elastic modulus of the system. Figure 2(b) shows the plateau height
recorded with the echo technique at a lag time of
t
¼
0.2 s (corresponding to the
time evolution of the elastic modulus G
0
). In our measurement, the onset of a
detectable non-zero plateau value is visible after 40-50 min (Figure 3). The
measurement at a single relaxation time, however, does not clearly indicate the
location of the gel point. As mentioned above, a better way to detect the gel
point is the method of Winter and Chambon,
8,9
where the criterion is that the
ratio G
0
(
o
)/G
00
(
o
) becomes constant and independent of frequency over a wide
range of frequencies at the gel point. Taking advantage of the large window of
frequencies accessible with DWS, we have plotted this ratio in Figure 4 for
several waiting times. We find that the data set measured at a time of 24 min
time =12min
time = 0min
10
3
10
2
10
1
10
0
10
-1
time = 24min
time = 1h
10
3
10
2
10
1
10
0
10
-1
time = 4h
time = 2h
10
3
10
2
10
1
10
0
10
-1
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
ω [rad/s]
ω [rad/s]
Figure 3 Frequency dependence of the storage modulus G
0
(filled circles) and the loss
modulus G
00
(open circles) of a 2 wt.% gelatin solution kept at 201C for various
times as determined by optical microrheology
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