Chemistry Reference
In-Depth Information
different site types:
shows the GPC-IR plot for a linear low
density polyethylene (LLDPE) resin. We
immediately notice the fingerprint mark of
a heterogeneous Ziegler-Natta catalyst: as
the molecular weight increases, the fraction
of 1-butene in the sample decreases.
It is possible to use Modeling Principle 1
to interpret this profile. Figure 4 shows that
the MWD can be represented as a super-
position of five Flory's distributions. If we
assume that each distribution is associated
with an active site type that produces
LLDPE with a distinct average molar
fraction of 1-butene (
Wð
log
MWÞ¼
X
n
m
j
w
j
ð
log
MWÞ
j
¼
1
¼
2
:
3056
MW
2
X
n
2
j
exp
m
j
t
ð
MWt
j
Þ
(13)
j¼
1
Equation (13) is a statement of our first
modeling principle:
The microstructural distri-
bution of a polymer made with a multiple-
site catalyst can be represented as a weighted
superposition of distributions for single-site
catalysts.
Principle 1:
F
j
), we can say that
the overall 1-butene fraction measured by
the IR detector in a given molecular weight
(
D
MW
) interval is,
Fð
D
MWÞ¼
X
5
This principle must be used with care: we
must keep in mind that it only provides a
convenient way to represent microstruc-
tural distributions of polymers made with
multiple site catalysts.
1
D
w
j
ð
D
MWÞF
j
(14)
j
¼
where
D
w
j
is the mass fraction of polymer
made on site type j eluting from the GPC
column set in the interval
. The mass
fractions
D
w
j
are obtained from the inte-
gration of the Flory distribution associated
with each site type:
D
MW
Hyphenated Techniques: GPC-IR
The use of a infrared detector (IR) with
GPC is becoming increasingly popular for
polyolefin characterization. This relatively
simple combination permits the detection
of the average chemical composition (gen-
erally reported as molar fraction of
a
-olefin
or short chain branching frequency) as a
function of molecular weight. Figure 3
MW
Z
þ
D
MW
D
w
j
ðMWÞ¼m
j
w
j
ðMWÞ
d
MW
MW
<
:
=
;
ð
1
þMWt
j
Þ
exp
ðMWt
j
Þ
½
1
þðMW þ
D
MWÞt
j
exp
¼ m
j
Þt
j
½ð
MW
þ
D
MW
(15)
Figure 3.
GPC-IR plot of a LLDPE resin.
