Environmental Engineering Reference
In-Depth Information
the polymer is better characterized by a weight average molecular weight
Mw
.
The weight average molecular weight can be similarly defined as follows:
where
w
i
is the weight of chains of molecular weight
w
i
.
Figure 3.4
shows a smooth curve of the frequency distribution of molecular
weights of individual chains in a polymer sample. In practice, such a
distribution can be determined for polymers using gel permeation
chromatography (GPC). This is essentially a modified column
chromatographyexperimentwhereadilutepolymersolutionisinjectedinto
the top of a column packed with porous (cross-linked PS) beads. Molecules
elute out from the bottom of the column at different times; larger molecules
do not penetrate the “holes” in porous beads and elute relatively faster. The
shorter chains tend to get trapped in the “holes” and are relatively slower
to elute. A detector based on refractive index changes or other properties
(e.g., changes in UV absorption) monitors the concentration of polymer
in solution exiting the column. Multiple detectors are used in the newer
instruments. Polymer fractions with the same molecular size
(hydrodynamic volume) elute at the same time. The column, however, has
to be calibrated using polymers of known average molecular weight.
