Chemistry Reference
In-Depth Information
used. The most common type and the first to be employed widely with synthetic
polymers consists of polystyrene gels. These porous beads are highly cross-linked
so that they can be packed firmly without clogging the columns when the solvent
flows through them under pressure. They are also highly porous and are made
with controlled pore sizes. This combination of properties is achieved by copoly-
merization of styrene and divinylbenzene in mixed solvents, which are good
solvents for the monomers but have marginal affinity for polystyrene
[15]
.
The most common packings for GPC in aqueous systems (also called gel filtration
and gel chromatography) are cross-linked dextran or acrylamide polymers and
porous glass.
A dilute solution of polymer in the GPC solvent is injected into the flowing
eluant. In the column, the molecules with smaller hydrodynamic volumes can dif-
fuse into and out of pores in the packing, while larger solute molecules are
excluded from many pores and travel more in the interstitial volume between the
porous beads. As a result, smaller molecules have longer effective flow paths
than larger molecules and their exit from the GPC column set
is relatively
delayed.
The initial pulse of polymer solution which was injected into the column entry
becomes diluted and attenuated as the different species are separated on the gel
packing. The column effluent is monitored by detectors that respond to the weight
concentration of polymer in the flowing eluant. The most common detector is a
differential refractometer. Spectrophotometers, which operate at fixed frequencies,
are also used as alternative or auxiliary detectors. Some special detectors which
are needed particularly for branched polymers or copolymers are mentioned in
Section 3.4.4
.
It is also necessary to monitor the volume of solvent that has passed through
the GPC column set from the time of injection of the sample (this is called the
elution volume or the retention volume). Solvent flow is conveniently measured
by means of elapsed time since sample injection, relying implicitly on a constant
solvent pumping rate. As an added check on this assumption, flow times may be
ratioed to those of a low-molecular-weight marker that provides a sharp elution
peak at long flow times.
The raw data in gel permeation chromatography consists of a trace of detector
response, proportional to the amount of polymer in solution, and the correspond-
ing elution volumes. A typical SEC record is depicted in
Fig. 3.9
. It is normal
practice to use a set of several columns, each packed with porous gel with a dif-
ferent porosity, depending on the range of molecular sizes to be analyzed.
3.4.2
Data Interpretation
The differential refractive index detector response on the ordinate of the SEC
chromatogram in
Fig. 3.9
can be transformed into a weight fraction of total poly-
mer while suitable calibration permits the translation of the elution volume axis
into a logarithmic molecular weight scale.
