Biomedical Engineering Reference
In-Depth Information
prepared from commercial samples usually, but not exclusively, ion-exchanged to a
single or restricted cation form, and with added electrolyte. Under these conditions
there is almost a power-law behaviour of
η
0
plateau.
Among the most extensive and precise studies of xanthan rheology are those by Lee
and Brant (
2002a
,
2002b
, 2002c), who improved upon almost all earlier work by
using molecular mass (or
η
ðÞ
, and no indication of an
, MW) fractionated samples. They demon-
strated, with their fractionated lowest-MW sample (M
w
~ 1.5 × 10
5
g mol
−
1
, contour
length ~ 2/3 × l
p
), a de
'
molecular weight
'
nitive transition (maximum) in the viscosity at c.10% w/w (Lee
and Brant,
2002a
), as expected at, or close to, an isotropic
nematic transition
(Wissbrun,
1981
). At lower concentrations, the behaviour was largely as described
above, with no Cox
-
biphasic
-
-
MW sample, where L ~2l
p
, the transition, which now occurred at c.7% w/w, was less
pronounced (Lee and Brant,
2002b
). They argued that this re
-
Merz superposition except for dilute solutions. For their
'
medium
'
ects the increased tendency
of the higher-MW sample to form chain entanglements.
In the third paper of their series, Lee and Brant (
2002c
) investigated the temperature
dependence of viscosity for their low- and medium-MW samples. First they mapped out
the xanthan order
disorder transition temperature, but then were able to compare their
results from viscosity measurements to a Flory chimney type phase diagram (
Chapter 3
).
For the higher-MW sample, and at the lowest ionic strength (0.01M, where the xanthan
chain order
-
-
disorder transition occurs at c.60°C), they demonstrated how the extent of
both isotropic and biphasic boundaries increased with concentration. However, it appears
that the interesting liquid crystalline phenomena lie well above concentrations of prac-
tical interest for higher-MW commercial samples.
For these high-MW xanthans, preparation of a solution at above, say, 2% w/w
becomes increasingly dif
cult. Other factors also begin to take over; liquid crystalline
transitions tend to be
'
smeared out
'
by polydispersity, and most such samples have M
w
/
M
n
≳
2(M
n
is the number average molecular mass). Also, as pointed out by Lee and
Brant, as chain length increases, entanglement effects begin to dominate. Overall, then,
this suggests that liquid crystallinity will tend to be observed only under conditions rather
far from those of practical interest.
Interestingly, the failure of the Cox
-
Merz rule, and the observation of power-law
(log
log) viscosity versus shear rate curves, have also been reported for a number of
other polysaccharides, including a chemically modi
-
ed carboxymethylcellulose (Westra,
1989
).
5.7
Chitin and chitosan
Chitin is one of the most commonly occurring materials on earth because it is the main
component of the exoskeleton of crustaceans such as shrimps, and more particularly krill.
The attached pendant N-acetyl groups of natural chitin are partially removed under
alkaline conditions to yield a water-soluble and consequently more reactive derivative,
called chitosan. Chitosan itself can be regarded formally as a copolymer of
4)-
linked 2-acetamido-2-deoxy-d-glucopyranose and 2-amino-2-deoxy-d-glucopyranose.
β
(1
→
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