Chemistry Reference
In-Depth Information
recover the original MWD from the frac-
tions, a special analysis was performed in
which the whole sample was injected from
the TREF column to the GPC columns in
the same manner as the fractions were
injected. In this way, a direct GPC analysis
of the whole sample was performed and the
resulting MWD was overlaid in Figure 9
with the recovered MWD. Given the number
of individual fractions that were measured
and integrated independently, it is remark-
able how similar the two results are.
The chemical composition view is
obtained by reconstructing the TREF
profile from the individually recovered
weights of the fractions. This curve is pre-
sented in Figure 10, in which the higher
resolution achieved in the comonomer
dimension is clearly seen, in comparison
with the typical lower resolution in the
molar mass dimension. The high quality of
the recovered TREF curve is quite compar-
able to that directly obtained with analy-
tical TREF.
Figure 10 also plots the
composition distribution. In this case, the
high density material has a lower average
molar mass than the lower density one. At
the same time, it is also seen that there is a
trend of decreasing M
w
with increasing
comonomer content or lower TREF elution
temperatures for both materials.
Similar information can be obtained by
coupling molar mass sensitive detectors
such as laser light scattering or viscometer
detectors to an analytical TREF instru-
ment.
[8-10]
However, this information is
much more evident in the 3D surface and
contour plots, which can only be directly
measured by this cross-fractionation tech-
nique. It is important to emphasize the
tremendous power of having the full MWD
at each elution temperature, which allows
the determination of the polydispersity and
other parameters of interest.
Linear Low Density Polyethylene (LLDPE)
A commercial LLDPE was run in this
cross-fractionation unit using the same
conditions summarized in Table 1. The
3D surface and contour plots are presented
M
w
of the
fractions as a function of the chemical
Figure 9.
Overlay of directly measured MWD of the whole polyethylene blend with the recovered MWD after the
cross-fractionation process.
