Chemistry Reference
In-Depth Information
Using Solvents to Improve the Chemical Shift
Differences Between Short-Chain Branch Methines
and Long-Chain Branch Methines in
Polyethylene Copolymers
Dan Baugh,
*
1
O. David Redwine,
2
Angela Taha,
1
Ken Reichek,
1
Janece Potter
1
Summary: Detection and quantification of long-chain branches in some polyethylene
copolymers is challenging due to the near coincidence of the chemical shifts for the
carbons at the short-chain and long-chain branches present in these copolymers. The
small chemical shift difference can be enhanced by changes in solvent and tempera-
ture. This allows one to use lower field magnets for some copolymers. Results are
presented comparing several solvents and blends at a variety of temperatures using
500, 600 and 750 MHz spectrometers.
Keywords: branched; LLDPE; NMR; polyethylene (PE); solvent
Introduction
Solvent screening experiments have
shown that solvent and temperature effects
on the shift difference between the short
and long-chain branch methines are quite
significant. The small chemical shift differ-
ence observed in samples prepared using
high boiling chlorinated aromatic solvents
can be enhanced by changes in solvent and
temperature. This allows one to use lower
field magnets for some copolymers. A
combination of resolution enhancement,
solvent selection, sample temperature and
high magnetic field (188 MHz, 150 MHz
and 125 MHz
13
C) have been used to
achieve enhanced resolution for the respec-
tive methine carbons of these two branch
types. Quantification of LCB was validated
by the measurement of a controlled sample
that contained a known amount of long-
chain branches.
This work is a continuation of the
extensive application of NMR, rheology,
and solution property methods to charac-
terize polyolefins.
[1]
This capability will
allow extended characterization of compe-
titive copolymers and enhanced materials
science understanding of new polyolefin
materials.
Ethylene-octene and ethylene-hexene
copolymers are common linear low density
polyethylene (LLDPE) polymers, repre-
senting over 75% of the total LLDPE
market. As new materials are developed
and commercialized for this growing market
(6% global annual growth rate), it is expec-
ted that ethylene-octene and ethylene-
hexene copolymers will continue to con-
stitute a major portion of it. To fully
evaluate these new materials, it is of critical
value to expand and improve the available
analytical methods for long-chain branch-
ing (LCB) analysis. Detection and quanti-
fication of LCB in these copolymers is
challenging due to the near coincidence of
the chemical shifts for the carbons at the
short-chain branches (SCB) and long-chain
branches present
in these copolymers
(Figure 1).
