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lignosulfonates are much more compact, having low intrinsic viscosity values
(2-10 ml/g) and a very low exponent (0.36).
6.1.7. Using
the MHS equation
to
find molecular weights
Once the MHS parameters (K and a) have been determined for a particular
macromolecule-solvent system, they can be used to find M if the (more easily
accessible) intrinsic viscosity has been measured. The MHS curve then
functions as a ‘standard curve’. This is a very common strategy in polymer
research and development because an intrinsic viscosity measurement is so
simple to perform.
It is important to note that in most cases the MHS parameters are only valid
for a given solvent and temperature. Changing the solvent, for example
changing pH or ionic strength, may induce a change in the overall shape in
the polymer, and thereby in the intrinsic viscosity.
Moreover, the polydispersity plays a pivotal role here. If both standards and
unknown samples are monodisperse then there is no problem, M is M. But
what if the polydispersity is different in the two cases? We will illustrate this by
two cases:
Case 1: Monodisperse standards
Sample
M
w
M
n
[
η
]
Std 1
10 000 10 000
23
Std 2
50 000 50 000
52
Std 3
100 000 100 000
75
Unknown
?
?
46
y = 0.2052x
0.5122
10
100
1 000
10 000 100 000 1 000 000
M (g/mol)
[
η
] (ml/g)
In this case M
w
= M
n
= M (of course) for the standards since they are
monodisperse. The data for the standards gives the following MHS relation for
this polymer (see graph above):
[ ] .
.
η
=
0 2052
0 5122
M