Biomedical Engineering Reference
In-Depth Information
Xanthan does not form ionic gels of the type discussed in earlier sections, but the
properties do vary with the associated counterion species (and also the ionic composition
of the solvent). Unfortunately, the order
disorder transition temperature shifts both with
ionic strength and with the amount of pyruvyl and acetyl substituents. The consequence
of this is that in pure water it can pass into the disordered form or vice versa, at or below
room temperature, depending on the polymer (or, more accurately, the associated coun-
terion) concentration. That is mostly likely why studies of xanthan rheology in water, for
example (Choppe et al.,
2010
), while quite numerous, are often contradictory (though, of
course, such a combination may be necessary in applications). For this chapter, we
largely restrict subsequent discussion to solution properties measured fully in the ordered
form, and in electrolyte solution.
As far as small-deformation oscillatory shear measurements for semi-dilute solutions
-
-
say ~ 1% w/w
flat gel-
like mechanical spectrum with G' > G" (Ross-Murphy et al.,
1983
). On the other hand,
this holds only for small deformations
-
are concerned, xanthan
-
electrolyte solutions give a reasonably
since when subjected to
larger shear strains such a system does not fracture and fail like the systems above, but
instead tends to
-
shear strains, say, <5%
-
flow rather like a
'
normal
'
viscoelastic liquid (Ross-Murphy et al.,
1983
).
For this reason it has been called a
'
weak gel
'
system, although we prefer the rheolog-
'
'
ically more accepted term
.
Moreover, under these circumstances, the Cox
structured liquid
-
'
'-
Merz
rule
the superposition of
steady shear viscosity
η
at shear rate
γ
and dynamic viscosity
η
* at frequency
ω -
is not
obeyed as it would be for our normal polymer solution, and
η
* seems always to be >
η
.It
has been suggested that this response is due to more speci
c ion-mediated inter-chain
couplings between xanthan molecules (Frangou et al.,
1982
) in addition to the topolog-
ical restraints, but an element of liquid crystalline behaviour (
Chapter 3
) has also been
advocated. Here, there seems no doubt that there is evidence for some liquid crystallinity,
but at signi
cantly higher concentrations, where polymer molecular solubility itself can
become compromised.
In steady shear, apparent viscosity
shear rate power-law exponents for xanthan
solutions are high (~0.9), and the overall viscosity versus shear rate pro
-
le is more like
that of a so-called Bingham viscoplastic material than a conventional
'
shear thinning
'
polymer solution. Con
rmatory evidence can be obtained from so-called stress overshoot
and start-shear history experiments (Richardson and Ross-Murphy,
1987
). Such work
shows that the stress overshoot recovery time for xanthan extends out to times of at least
10
4
s, whereas at comparable concentrations for
'
simple
'
polysaccharide solutions this
tends to be complete after, say, 10
100 s.
What is important to point out is that such rheological behaviour is not unique to
xanthan solutions, nor is it always seen with these. Indeed, Milas and co-workers have
demonstrated that a xanthan solution prepared directly from the culture broth without
pasteurization or freeze-drying shows only very small deviations from the Cox
-
-
Merz
rule, even at quite high concentrations (Milas et al.,
1990
). Correspondingly, other
workers have published
flow curves for commercial xanthan samples in water above
the overlap concentration c* which are quite as expected for polymers in solution. This
implies that sample history is rather crucial; the
'
typical
'
results above apply to solutions
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