Chemistry Reference
In-Depth Information
to tailor-make materials for specific appli-
cations.
[3]
Figure 1 provides an example of the
hypothetical microstructure of an OBC
produced by chain shuttling catalysis. This
type of block copolymer can be termed a
''linear statistical multiblock'' or LSMB
and is quite different than the conventional
block copolymers made by living polymer-
ization chemistry as well as commercially
available polyolefins. In a LSMB the chain
lengths, comonomer distribution, and block
distribution of block sizes and lengths are
polydisperse. By contrast, the traditional
block copolymers have structures that are
well-controlled to be nearly mono-disperse
(equal chain lengths) and the lengths of
each type of block are essentially equal
from chain to chain as dictated by the
polymerization chemistry and conditions.
For typical random polyolefins, the
molecular structure distributions such as
the molecular weight distribution and/or
the short chain branching distribution can
be measured by GPC and ATREF/CRYS-
TAF, respectively. However, neither of
these techniques is able to measure the
blockiness of a copolymer (without prior
knowledge of its structure), as reflected in
the intrachain monomer distribution. To
our knowledge, there are no analytical tools
in the literature to specifically characterize
an olefin block copolymer.
In this paper, we describe some of the
unique analytical characteristics of an olefin
block copolymer. In addition, based on
deviations from the observed relationship
for structure and ATREF elution tempera-
ture for random copolymers, we outline a
''Block Index'' method to quantify the
observed microstructure and distinguish it
from traditional random copolymers.
Experimental Part
Materials
An olefin block copolymer synthesized by
the chain-shuttling method was produced in
a continuous polymerization process as
described by Arriola et al.
[2]
The olefin
block copolymer was produced using ethyl-
ene and 1-octene comonomer as per the
design shown in Table 1.
Material Analysis
Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry was per-
formed on a TA Instruments Q1000 DSC
equipped with an RCS cooling accessory
and an auto sampler. A nitrogen purge gas
flow of 50 ml/min was used. The sample was
pressed into a thin film and melted in the
press at about 190
C and then air-cooled to
room temperature (25
8
C). About 3-10 mg
of material was then cut, accurately
weighed, and placed in a light aluminum
pan (ca. 50 mg) which was later crimped
shut. The thermal behavior of the sample
was investigated with the following tem-
perature profile: the sample was rapidly
heated to 190
8
C and held isothermal for
3 minutes in order to remove any previous
thermal history. For these octene-based
polymers, the sample was then cooled to
40
8
Cat10
8
C/min cooling rate and held at
8
C for 3 minutes. The sample was then
heated to 150
8
Cat10
8
C/min heating rate.
The cooling and second heating curves
were recorded.
40
8
Figure 1.
A pictorial example of a linear statistical multiblock copolymer.
