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The molecular weight may be determined by experimental methods such as
mass spectrometry (MS), light scattering or osmometry. We will study also
those methods later in the course.
The overall shape is usually determined indirectly by comparing experimental
data reflecting the shape and the extension such as R
G
(radius of gyration),
intrinsic viscosity (symbol [η])
29
or others, and the molecular weight (M)
30
with
a model.
The models described below are based on obtaining experimental data for
the same polymer type
at different molecular weights
, a homologous series.
Such samples may be produced by fractionating a mixture, or by controlled
(partial) degradation starting with the largest molecule.
Here are some examples taken from the literature (some are actually obtained
in our laboratory). For each sample both the molecular weight and (in this
case) the intrinsic viscosity was measured.
Figure 36. Measurements of the intrinsic viscosity for different molecular weight provides
information about the shape of the polymers.
Note first that the data are plotted on a double-logarithmic scale. This means
straight lines correspond to an exponential equation: y = ax
b
.
Note further that both the slope (b) and the intercept (a) for the fitted straight
lines in the log-log plot vary between the different biopolymers. Analysing
these data gives us information about the shape of the molecules in solution.
Instead of intrinsic viscosity, we could also use data for the radius of gyration
(R
G
). Such data do indeed data exist (shown for pullulan and xanthan only),
and we will start using such data first:
29 Chapter 11.1 in textbook
30 Chapter 9 in textbook
1
10
100
1 000
10 000
10 000
100 000
1 000 000
10 000 000
M (g/mol)
[h] (ml/g)
Pullulan
Xanthan
Alginate
Lignosulfonate
Intrinsic
viscosity
(ml/g)