Biomedical Engineering Reference
In-Depth Information
Fig. 8
Frequency sweep
test in the sol (10 °C) and
gel (37 °C) phases of the
CS-g-(PAF-PEG) polymer
aqueous solution (6.0 wt%).
Reproduced with permission
from [
91
]
These measurements have been used to determine the sol-gel properties of ther-
mogelling polymers. Jeong et al. studied the sol gel behavior of thermogelling
polymers with this approach [
91
]. The frequency sweep test showed that the sol
and gel phases of the PEG-PAF grafted chitosan (CS-g-(PAF-PEG)) aqueous solu-
tion were characterized by fluid-like behavior and solid-like behavior (Fig.
8
). At
10 °C, the elastic modulus and loss modulus of the aqueous polymer solution were
proportional to
ˉ
2.1
and
ˉ
1.1
, respectively, indicating a typical viscoelastic fluid-
like phase of the sol [
92
-
94
]. In addition, the loss modulus was greater than elastic
modulus at 10 °C. At 37 °C, the elastic modulus was greater than the loss modu-
lus by an order of magnitude at 37 °C. The elastic modulus was nearly independ-
ent of frequency, whereas the loss modulus slightly decreased as the frequency
increased in the investigated frequency range of 0.1-10 rad s
-1
. In the solution
state, the thermogelling system showed viscous fluid-like behavior with G″ > G′
and a frequency-dependent modulus, whereas in the gel state, G′ > G″ and G′ was
independent of the frequency.
Next, it is important to determine if the material requires pre-treatment (such
as pre-shearing) before measurements. This can be determined from the pseudo-
viscosity profile of the material. Pre-shearing will determine a zero-time of shear,
thereby eliminating any structure history prior to loading. This is done by perform-
ing a continuous flow test under the broad torque range. The data can be viewed as
viscosity versus torque/stress and converted to viscosity versus shear rate.
Most food formulations, cosmetics, pharmaceuticals and paints are structured
fluids, containing droplets of an immiscible fluid or particle suspended in a liq-
uid matrix. The viscosity of the liquid matrix in the dispersions plays an impor-
tant role on the flow properties of the material. When there are repulsive forces
between particles they do not settle rapidly, forming a network structure, which
stabilizes the suspension. The delicate network structure can be destroyed by
shearing, resulting in decreased fluid viscosity.
Most structured fluids do not obey a simple linear relationship between applied
stress and flow (Newtonian fluid behavior). Most of these materials have viscosi-
ties, which decrease with increasing stress. Such an observation is known as shear
thinning which becomes progressively significant as the volume concentration of
solid particles increases.
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