Practical Aspects of Molecular Weight Measurements - Polymer Science and Engineering - page 131

When a differential refractometer is used as a detector, instrumental broaden-
ing of the GPC chromatogram is compensated to some extent by another effect
due to the tendency for specific refractive indexes of polymer solutions to
decrease with decreasing molecular weight in the low-molecular-weight range.
3.4.3 Universal Calibration for Linear Homopolymers
We consider now how the elution volume axis of the raw chromatogram can be
translated into a molecular weight scale. A series of commercially available
anionically polymerized polystyrenes is particularly well suited for calibration of
GPC columns. These polymers are available with a range of molecular weights
and have relatively narrow molecular weight distributions ( Section 2.5 ). When
such a sample is injected into the GPC column set, the resulting chromatogram is
narrower than that of a whole polymer, but it is not a simple spike because of the
band broadening effects described earlier and because the polymer standard itself
is not actually monodisperse. Since the distribution is so narrow, however, no
serious error is committed by assigning the elution volume corresponding to the
peak of the chromatogram to the molecular weight of the particular polystyrene.
(The molecular weight distributions of most of these samples are sharp enough
that all experimental average molecular weights are essentially equivalent to
within experimental error.) Thus, a series of narrow distribution polystyrene sam-
ples yields a set of GPC chromatograms as shown in Fig. 3.10 . The peak elution
volumes and corresponding sample molecular weights produce a calibration curve
(see Fig. 3.12 later) for polystyrene in the particular GPC solvent and column set.
It turns out that combinations of packing pore sizes which are generally used
result in more or less linear calibration curves when the logarithms of the polysty-
rene molecular weights are plotted against the corresponding elution volumes.
Increasing molecular
weight
Elution volume
FIGURE 3.10
Gel permeation chromatography elution curves for anionic polystyrene standards used for
calibration. The polystyrene standard samples were measured separately; use of a mixture
of polymers may cause elution volumes of very high-molecular-weight standards to be
erroneously low [18] .