Chemistry Reference
In-Depth Information
often a differential refractometer, as mentioned) with detectors that measure the
molecular weights to the polymers in the SEC eluant. These are continuous
viscometers and light-scattering detectors. The former are used to measure the
intrinsic viscosity of the eluting polymer at each GPC retention time. The univer-
sal calibration relation of
Eq. (3-94) or (3-99)
is equivalent to
½η
M
5 ½η
lin
M
lin
(3-108)
where the unsubscripted values refer to the branched polymer and the subscript
lin refers to its linear counterpart, which appears at the same elution volume (or
to the narrow distribution polystyrene or other polymer used as a standard for uni-
versal calibration). When [
] of each fraction is measured, the molecular weight
of the branched polymer which elutes at any given retention volume is available
from the relation of
Eq. (3-108)
. This procedure is also applicable to copolymers,
if the variable copolymer composition does not affect the response of the concen-
tration detector that is used along with the viscometer.
With a light-scattering photometer and a concentration detector such as a
differential refractometer, the molecular weight distribution of the unknown poly-
mer is obtained directly without need for the universal calibration procedure
of the preceding section. This is by application of
Eq. (3-53)
to each succes-
sive “slice” of the GPC chromatogram. The virial coefficient terms in this equa-
tion are best set equal to zero, since their molecular weight dependence
(
Section 3.1.4
) is not known
a priori
. Various designs of light-scattering detec-
tors are now available, differing primarily in the number and magnitude of view-
ing angles used. Low-angle light scattering (using laser light) eliminates the need
for angular correction of the observed turbidity (
Section 3.2.3
), whereas
photometers operating at right angles to the incident light beam are less sensitive
to adventitious dust.
The three SEC detector types in common use at the time of writing—differen-
tial refractometer, continuous viscometer, and light-scattering photometer—differ
in sensitivity. The differential refractometer signal scales as the concentration,
c
,of
the polymer solute. The viscometer signal is proportional to
cM
a
(
Section 3.3.2
),
with the exponent
a
about equal to 0.7 for most polymer solutions used in this anal-
ysis. The light-scattering signal scales as
cM
(
Eq. 3-56
). When all three detectors
are employed simultaneously, the light-scattering device is most sensitive to large
species and relatively insensitive to low-molecular-weight polymer, while the
reverse selectivity applies to the differential refractometer. Current continuous
viscometers are intermediate in performance and are the most generally useful
detectors. The analytical technique should, however, be tailored to the specific
characteristics of the polymer of interest.
In a multidetector SEC apparatus, it is necessary to match the output of the
detector that senses eluant concentration with the signals of the detectors that
sense molecular weight directly. To do this, the analyst should match the different
signals at equal hydrodynamic volumes in the different detectors
[24]
.
η
