Chemistry Reference
In-Depth Information
0.005 to 0.7 long branches per thousand
backbone carbon atoms. LCBfs were not
determined for the fractions themselves.
Because a fractionation was carried out, it is
not clear that the average LCBfs still apply
to each fraction in a series. It is expected
that fractions from the LCBf
¼
0.005 sam-
ple would have a
with sensors at 15
. The pairing of
these detectors provided the capability to
observe the molecular weight distribution
and produce an estimate of radius gyration
across the elution profile of a sample.
The TCB eluent was continuously
degassed by sparging the reservoir with
helium and by the operation of the in-line
system degasser. Injection volumes for
these experiments were 0.01 mL. For high
temperature MTF, concentrations were
chosen to keep the product of intrinsic
viscosity and concentration (C
[
h
]) less
than
and 90
8
8
small population of
long chain branched molecules, while
fractions
very
0.70 sample
would be expected to have a large popula-
tion of multiply-branched LCB molecules,
especially in the higher molecular weight
fractions.
from the LCBf
¼
0.15. Samples were prepared directly
in the autosampler vial by weighing an
aliquot of the polyethylene fraction, adding
the appropriate amount of solvent and
shaking for 2 hours at 160
8
C.
Equipment for MTF
The LC system used for MTF and SEC
analyses consisted of a Waters Alliance
2695 pump/autosampler and a Viscotek
model 302 multidetector array. The 2695
pump was used to deliver precise flowrates
in the 0.01 to 0.05 mL/minute regime for
MTF. HPLC grade THF (Omnisolv, HPLC
Grade, EMD Chemicals) was used as the
solvent and eluent. The eluent was con-
tinuously degassed. Injection volumes were
0.01 mL. The multi-detector array included
UV absorbance, low angle (LALLS) and
right angle laser light scattering, differential
refractive index (DRI) and differential
viscometer detectors. The detectors and
columns were held at 50
MTF Columns and Functionalized
Packing Material
The polydisperse silica obtained from
Admatechs was functionalized with PS by
Mel Cabey of the Diazem Corporation
(Midland, MI). Columns for MTF were
packed with this PS functionalized silica by
Diazem. The packing procedure consisted
of slurrying the particles in a proprietary
mixture of solvents and pressure packing
them into the column at 6,000 PSI. Pressure
was held at 6,000 PSI for 8 hours. The
column outlet frit was constructed as
follows. A nominal 0.5 micrometer frit
was placed in the column outlet. Next, a
short (e. g. a few millimeters) layer of
2 micrometer non-porous silica was packed
next to the outlet frit. Finally, the column
was filled with the packing particles. The
column was then flushed overnight with
THF or TCB before MTF experiments
commenced.
C for SEC.
Column dimensions for MTF were 4.6 mm
I. D. X 150 mm L and were packed with PS
functionalized silica (made from SO-C2
described above).
A Waters Alliance Model GPCV2000
was used for high temperature MTF experi-
ments. A flow of 0.01 ml/min was employed,
and like the Waters 2695 pump used in
ambient temperature experiments, the
pump in the high temperature SEC was
able to deliver this low flow rate both
reliably and reproducibly. The carousel,
injector and column regions were all
controlled at 145
8
C. Two detectors were
used for the sample analysis: (1) A Polymer
Char Model IR4 (Valencia, Spain) infrared
concentration detector and (2) A Precision
Detectors, Inc. (Franklin, MA) dual angle
light scattering detector, Model 2040 PDI,
8
SEC Experiments
SEC was carried out using the same liquid
chromatography hardware as was used for
MTF in order to define the polydispersity of
the 3-arm star test sample as precisely as
possible. A set of four mixed-B columns
from Polymer Laboratories (Amherst, MA)
was chosen for the separation. A flowrate of
1.0 ml/minute and an injection volume of
