Chemistry Reference
In-Depth Information
Figure 9.
3D-GPC-TREF determination of ''gel or un-melt?'' structure in HDPE film.
sample sizes makes it very difficult for them
to be studied by most other analytical tests.
This makes 3D-GPC-TREF uniquely use-
ful for samples as little as a fraction of a
milligram. Highly cross-linked gels may
exist in samples that may not dissolve in hot
TCB. These gel structures would not be
injected in either the GPC or TREF
column, and would not be detected. The
presence of these gel fractions can be
estimated by the percent mass recovery
data in the GPC or TREF experiment.
In the next study example, we show that
one can distinguish different types of LCB
in polymers and that LCB is not limited to
LDPE. LCB can exist in HDPE made from
ZN slurry process or as a result of thermal
degradation and chain extension processes
in melt extrusion. While the 3D-GPC side
of the
m
-structure plot in Figure 10 shows
the existence of LCB in all three samples,
3D-GPC is not capable of distinguishing the
LCB types. However, the LCB in LDPE is
clearly distinguished from that in the
HDPE samples by the 3D-TREF result
shown in Figure 10. The two HDPE
samples with LCB content elute from
TREF at high temperature, but the LDPE
sample has a lower TREF elution tem-
perature because of its high content of butyl
(C4) branching. These C4 branches are the
result of the chain-end backbiting branching
mechanism in the high-pressure LDPE pro-
cess. It is kind of fascinating to picture that the
basic structure of LDPE
minus the presence of
LCB
would not be very different from a
single-site hexene LLDPE copolymer of very
narrow CCD.
Moderation of Co-Crystallization Effect in
TREF Separation
Co-Crystallization of polymer blends is a
complicating factor that can compromise
the accuracy of the compositional analyses by
TREF and by crystallization analysis fractio-
nation (Crystaf).
[8]
Strong co-crystallization
distorts the elution profile of TREF and
Crystaf separations. Evidence has been
reported that the co-crystallization problem
is less severe in TREF than in Crystaf.
These reports conclude that TREF is more
appropriate for analyzing samples with
complex CCDs because it provides better
peak resolution of blends. Nevertheless,
