Chemistry Reference
In-Depth Information
Figure 15.
3D-surface plot (left) and 2-dimensional contour plot (right) of a HDPE pipe grade resin.
HDPE sample. This HDPE is a reverse
comonomer composition resin, in which a
low level of comonomer was added to the
higher molecular weight components in
order to achieve the desired end product
properties.
Figure 15 clearly shows the heterogene-
ity of this product. The pronounced tailing
towards lower elution temperatures in the
low molecular weight range is explained by
the effect of the chain ends which act as a
source of crystal defects as described by
Nieto et al.
[14]
and produce a depression in
TREF elution temperatures. The fact that
for molecular weights lower than 10,000
there is a rather sharp decrease in TREF
temperatures is also in good agreement
with Nieto's observations based on CRYS-
TAF analysis of narrow-MWD polyethy-
lene fractions and paraffins. This low
molecular weight effect on TREF elution
temperatures can be compensated in the
processing software when translating the
TREF temperature axis into the comono-
mer weight percent in order to produce the
two dimensional distributions of molar
mass and comonomer composition.
On the other hand, the higher molar
mass fractions fromLog M from 5 to 6 show
a broad TREF elution temperature range
produced, in this case, by the addition of
comonomer in that area of the molar mass
distribution.
In the TREF view plot (Figure 16), M
w
values are rather uniform for most all
temperatures, decreasing only at lower
temperatures because of the low molar
mass tail observed in the two dimensional
plots. The different contributions to the
TREF temperature decrease by either
comonomer addition or low molar mass
species are clearly seen in this two-
dimensional plot measured with the cross-
fractionation instrument. They would not
be detected by one-dimensional
techni-
ques, even with multiple detectors.
Figure 16 also shows how the polydis-
persity index depends on the TREF elution
temperature. PDI increases dramatically as
the temperature is reduced, from initial
values of approximately 5 at high tempera-
tures to about 30 at low temperatures, due
to the bimodality of the measured chro-
matograms.
In the case of these HDPE products with
broad MWD it has been of great interest to
study the molar mass view and also to add
the comonomer distribution information to
it. An approach to investigate this problem
using the cross-fractionation information in
the 3D plots is outlined in the following
paragraphs.
The recovered MWD for the whole
sample is shown in Figure 17. The MWD is
clearly bimodal and the question now is to
determine the range across this broad
MWD in which comonomer is present.
In this study we start by realizing that for
every molar mass value, a TREF-like profile
is obtained from the 3D surface plot,
