Chemistry Reference
In-Depth Information
0.05 ml were used. Both conventional SEC
results (relative to linear PS) and absolute
molecular weight results (from LALLS)
were determined. Data were reduced using
OmniSEC software from Viscotek.
The cross-linked polyethylene whole
polymers and fractions were characterized
by triple detector high temperature SEC.
The system consisted of a PL-220 with light
scattering (PDI 2040 as described above),
differential viscometry (Viscotek 4 capil-
lary bridge design), and concentration
detectors (IR4 and DRI). The column set
consisted of 3 mixed-B columns from
Polymer Laboratories. Sample concentra-
tions were 1 to 2 mg/ml and injection
volume was 0.100 ml. Data were reduced
using custom written software.
that there are at least three components
in this sample, even though they are not
well resolved. Based upon calibration with
linear polystyrene standards, these three
components have apparent peak molecular
weights of approximately 1,250, 2,500, and
3,500 kg/mole, as annotated on the dis-
tribution. It is arguable that there is a fourth
component at apparent molecular weight
above 3,500 kg/mole because the shape of
the distribution in that regime appears
slightly non-Gaussian. But if there were
material eluting in that region, its molecular
weight would not be known from this
analysis.
The linear PS equivalent M
W
and
number-average molecular weight (M
N
)
of the 3-arm star determined from the
distribution shown in Figure 3 were, 3,260
and 2,700 kg/mol, respectively. The M
W
value from LALLS was, 3,760 kg/mol. The
fact that the absolute M
W
was larger than
the linear equivalent M
W
is consistent with
the presence of branched molecules in the
distribution. The Mark - Houwink plot
from SEC-LALLS-DV, shown in Figure 4,
also provides strong evidence for the
presence of branched species in the dis-
tribution.
Results and Discussion
SEC Characterization of
the 3-Arm Star Sample
SEC and SEC-LALLS-DV were used to
determine the molecular weight distribu-
tion (MWD) of the 3-arm star in order to
compliment and aid in the analysis of MTF
results. The linear PS equivalent MWD
is shown in Figure 3. It is readily apparent
Figure 3.
Linear PS apparent molecular weight distribution of the 3-arm star sample as determined by SEC. The numbers
next to the arrows represent the linear PS equivalent molecular weight at the apices of the individual peaks.
