Chemistry Reference
In-Depth Information
characteristic gel point is considered to be a material parameter and so its
value should not be influenced by the method of determination. A commonly
employed technique to follow the gelation of colloidal polymer solutions is
small-amplitude oscillatory rheometry. The time at which the G
0
and G
00
curves cross each other has been proposed as a simple method for locating the
gel point, but in practice it obviously depends on the frequency of the
oscillatory measurement.
7
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
00
(
o
)/G
0
(
o
)
becomes independent of frequency over a wide range of frequencies at the gel
point. From the slopes of G
0
and G
00
versus
o
at the gel point, a relaxation
exponent n may be calculated. Its value has been found to range between 0 and
1 depending on the composition and on the molecular structure of the
material.
10,11
The validity of the Winter-Chambon criterion has been dem-
onstrated in various studies including chemically and physically cross-linked
gels.
11-13
A complication with classical rheometry is that the investigation of
gelation of weak physical gels may be hampered by the fact that the physical
bonds are shear sensitive. Weak physical gels show a significant decrease of
the critical strain,
12
which limits the linear viscoelastic region close to the gel
point. Moreover the determination of the entire relaxation spectrum in a
single run is difficult and time consuming, and in many cases it is not
even feasible for such a time-evolving sample. In contrast, DWS provides
non-invasive access to the linear viscoelastic properties of the gelling
sample at any time during the gelation process. In a recent article
14
we
demonstrated that an accurate determination of the gel point of starch
dispersions is possible using DWS. For a different system here, we demon-
strate that the short time of DWS data acquisition - 1 min or less - turns out
to be a highly advantageous factor. Time-dependent changes in the mechan-
ical properties on a minute scale can be resolved while still covering a
frequency range of up to seven decades (
o
from
10
6
rad s
1
).
Furthermore the technique always works in thermal equilibrium; hence no
structural changes or distortions of the systems arise due to the application of
external shear forces.
10
1
to
B
B
21.3 Sample Preparation
A dispersion of sulfated polystyrene latex spheres (diameter 720 nm) (Interfa-
cial Dynamics, Portland, USA) at a concentration of 8.2 g per 100 g of solution
was added to a gelatin solution to give a final concentration of 0.5 g of latex
particles +2 g gelatin in 100 g of solution. The polystyrene spheres act as tracer
particles to probe the local viscoelastic properties via Brownian motion. The
particle size is chosen such that the typical relevant length scale
x
(several
nanometres) of the biopolymer solutions is much smaller than the particle size
(
x
{
720 nm).
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