Chemistry Reference
In-Depth Information
having similar hydrodynamic volumes and
differing long chain branching topology.
Sample recovery from the MTF column
was also studied. For linear chains, quanti-
tative recoveries were obtained over the
molecular weight range of 10 kg/mol to
5,000 kg/mol. For the 3-arm star sample,
recoveries on the order of 60% were
typical. Because recoveries were quantita-
tive for linear chains, it is assumed that the
lack of recovery was due to incomplete
elution of the star branched molecules.
Even though recovery was below 100%, it is
still clear that MTF separates based on
topology. The estimate of peak M
W
came
from ratioing the light scattering detector
signal to the concentration detector signal.
The accuracy of the estimate, and hence the
interpretation of the fractogram, does not
require complete elution of the sample.
Recent data acquired at the University of
Amsterdam
[9]
with a similar column,
revealed near quantitative elution (recov-
ery
>
95%) of the same 3-arm star sample
studied here. Longer run times were
required to achieve the near quantitative
recovery. Run time has no influence on
MTF resolution. Flow rate and macropore
dimensions within the column are the two
largest factors influencing resolution. The
fact that longer run times were required to
achieve near quantitative recovery suggest
that even higher order stars (more than
4 arms) were present in the sample, and
more time would be needed to elute these.
MTF of Polyethylene Fractions
The molecular weight range of the poly-
ethylene fractions relative to the MTF
reversal molecular weight is an important
consideration. Figure 7, for example, shows
an overlay of all the MTF peak elution
times for the complete set of polyethylene
fractions (full range of M
W
's and LCBf's),
superimposed with those of the linear
polystyrene standards. For the linear poly-
styrene standards, the M
W
's have been
plotted after scaling them down by a factor
of 0.43 in order to adjust for the difference
in the hydrodynamic volume relative to
linear polyethylene. After accounting for
hydrodynamic size differences, the super-
positions are good as was observed pre-
viously for the PS, polybutadiene pair.
[1]
However, the M
W
's for most fractions lie
too close to the reversal or critical mole-
cular weight, M
C
, for it to be differentiated
based on branching (all points in the set lie
on one curve). For each LCBf, at least one
fraction lies above M
C
, by about a factor of
two, as M
C
for polyethylene would appear
to be on the order of
100 Kg/mole. Even
these fractions cannot be distinguished by
their peak elution times.
Figure 7.
MTF reversal curve comparing peak elution times of linear PS standards with those from lightly cross-linked
homogeneous ethylene octene copolymer fractions.
