Biomedical Engineering Reference
In-Depth Information
5x10
2
10
2
10
1
c (wt%)
8
7
6
5
3
2
1
0.55
10
0
10
-1
10
-2
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
.
(s
-1
)
γ
Steady-state viscosity versus shear rate at various concentrations of HM poly(acrylamide)
(DiHexAm) samples in water: M
w
= 4.2 × 10
5
g mol
−
1
, hydrophobic block length 3.2 units, 9
hydrophobic blocks per chain. Adapted with permission from Regalado et al.(
1999
) © 1999
American Chemical Society.
Figure 6.8
Viscosity increases signi
cantly:
*
with the amount of substitution: for instance, for methyl cellulose, with the percentage
of dodecyl groups to glucose units (Cohen Stuart et al.,
1998
). When this percentage is
varied from 3 × 10
-
3
to 5 × 10
-
2
, the zero-shear viscosity of a 1% solution increases by
three orders of magnitude.
*
even though it is not entirely desirable, with increasing degree of heterogeneity, as in
the composition drift of HM poly(acrylamide)s prepared by micellar copolymerization
(Volpert et al.,
1996
). Here there is an order of magnitude difference between homo-
geneous and non-homogeneous samples.
*
with polymer concentration, as shown in
Figure 6.8
.
*
with the blockiness in random-block copolymerization (Candau et al.,
1998
). The
increase of viscosity with the length of the hydrophobic blocks is related to the lifetime
of the (possibly binary) junctions between hydrophobes. As the lifetime of junctions is
an exponential function of the activation energy for disentanglement (see
Chapter 4
), it
is very sensitive to the length of the blocks. It was also observed that the viscosity of
samples having the same hydrophobic block length, but increasing hydrophobic block
density or increasing molecular mass, follows a scaling law with an exponent close to
4. This exponent suggests an analogy with the entangled regime of the homopolymer,
where chain reptation (Doi and Edwards,
1986
) is hindered by the presence of
intermolecular junctions.
*
with the length of the end cap in solutions of telechelic molecules (as shown in
Figure 6.7
).
In the semi-dilute range of concentrations, HM polymer solutions generally exhibit
shear thinning behaviour consistent with predictions of the Tanaka
-
Edwards model. This
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