Chemistry Reference
In-Depth Information
Fig. 2.22
Molecular
weight calibration curve
for gel permeation
chromatography
From the earlier statement it should be clear that polymer fractionation by gel permeation chroma-
tography depends upon the spaces the polymer molecules occupy in solution. By measuring, experi-
mentally, the molecular weights of polymer molecules as they are being eluted one obtains the
molecular weight distribution. To accomplish this, however, one must have a chromatograph equipped
with dual detectors. One must detect the presence of polymer molecules in the effluent. The other one
must measure their molecular weights. Such detectors might be, for instance, a refractive index
monitor and a low angle laser light scattering photogoniometer to find the absolute value of
.
Many molecular weight measurements, however, are done on chromatographs equipped with only
one detector that monitors the presence of the solute in the effluent. The equipment must, therefore, be
calibrated prior to use. The relationship of the ordinate of the chromatogram, commonly represented
by
M
), must be related to the molecular weight. This relationship varies with the polymer type and
structure. There are three methods for calibrating the chromatograph. The first, and most popular one,
makes use of narrow molecular weight distribution reference standards. The second one is based upon
a polydisperse reference material. The third one assumes that the separation is determined by
molecular size. All three methods require that an experimentally established calibration curve of
the relationship between the molecular size of the polymer in solution and the molecular weight be
developed. A chromatogram is obtained first from every standard sample. A plot is then prepared
from the logarithms of the average weights against the peak retention volumes (
F
(
V
V
R
are measured from the points of injection to the appearances of the maximum values of the
chromatograms. Above
V
R
). The values of
M
4
there is no effective fractionation because of total exclusion
in the first place and total permeation in the second case. These are the limits of separation by the
packing material.
To date the standard samples of narrow molecular weight distribution polymers that are available
commercially are mainly polystyrenes. These samples have polydispersity indexes that are close to
unity and are available over a wide range of molecular weights. For determining molecular weights of
polymers other than polystyrene, however, the molecular weights obtained from these samples would
be only approximations. Sometimes they could be in error. To overcome this difficulty a
universal
calibration method
is used. The basis for universal calibration is the observation [
51
] that the
multiplication products of intrinsic viscosities and molecular weights are independent of the polymer
types. Thus, [
M
1
and below
]
M
is the
universal calibration parameter
. As a result, a plot of log ([
]
M
) vs. elution
volume yields a curve that is applicable for many polymers. The log ([
a given column (or
columns), temperature, and elution volume
is assumed to be a constant for all polymers. This is
illustrated in Fig.
2.22
.
]
M
)
for
