Chemistry Reference
In-Depth Information
Table 1.
Standard conditions for cross-fractionation analysis of polyethylene samples.
Solvent
1,2 ortho dichlorobenzene with 300 ppm antioxidant (BHT)
Dissolution
60 minutes at 150
8
C at 200 rpm stirring inside the stainless steel vessels
Sample weight loaded
into TREF column
1.6 mg
Crystallization rate
0.5
8
C/min
Elution
24 fractions from 40
8
C to 120
8
C, temperature step every 3
8
C,
overlapped GPC injections with 19 minutes inter-injection time
GPC analysis flow rate
1 mL/min
GPC column set
3 PLGel 10 micron mixed columns (Polymer Laboratories Inc.)
polydispersity index (
). The weight
fraction at each temperature is also com-
puted by comparing the area of every single
chromatogram to the total area.
That set of MWDs measured at different
temperatures are combined with their respec-
tive weight fractions to generate a 3D
plot representing the bivariate composition-
molar mass distribution of the sample.
Numerical interpolation smoothing spline
algorithms are used in this step, in an
approach similar to that described by
Nakano and Goto.
[12]
The whole sample MWD is also recov-
ered as the weighted sum of the MWD of
individual fractions according to their
weight fractions. The reconstruction of
the TREF profile of the whole sample is
a more complex process. It is based on
constructing a discontinuous TREF cumu-
lative curve with the progressive sum of the
weights of the fractions at each elution
temperature, starting with the soluble
fraction at the initial temperature and
ending with the total weight at the final
temperature. The first derivative of the
smoothed interpolation line of the cumu-
lative curve constitutes the reconstructed
TREF profile.
PDI
resulting in an overall analysis time of
roughly 11 hours per sample for a quite
reasonable number of fractions and very
good resolution.
Blend Analysis
A two component blend of metallocene
polyethylenes was first used to evaluate the
resolution of this method. The description
of the components in the blend is summar-
ized in Table 2.
A view of the 3D plot of the bivariate
distribution of this blend is shown in
Figure 7, together with its contour plot.
The logM axis was obtained with the GPC
calibration curve, while the temperature
axis corresponds to the elution tempera-
tures of the different TREF fractions. The
elution temperature can be related to
comonomer content considering that high
crystalline materials eluting at high tem-
perature have little or no comonomer,
while increasing levels or comonomer re-
duce crystallinity and result in lower TREF
temperatures. The height of the surface
plot represent the relative IR absorbance
signal, directly related to concentration at
every molar mass-temperature point, so it
represents the amount of material having a
given molar mass and eluting at a given
temperature.
Results and Discussion
Table 2.
Description of metallocene polyethylene blend com-
ponents.
In order to demonstrate the operation of
this new cross-fractionation instrument and
to show its capabilities, a series of poly-
ethylene samples have been analyzed and
the results are presented in this section.
The standard cross-fractionation condi-
tions used are summarized in Table 1,
Component
Density
(g/cm
3
)
MI
Weight
percent
A
0.957
19.6
50%
B
0.921
0.25
50%
