Analysis of Hydroperoxides in solid Polyethylene by NMR and EPR Spectroscopy Page: 4 of 5
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increases rapidly for short aging times, but then levels off and decreases
significantly between 11 and 20 days. This behavior is characteristic of a
In addition to observing the formation of hydroperoxides, we are able
to investigate the decomposition chemistry of these reactive species during
subsequent thermal exposure. Figure 3 shows the hydroperoxide region of the
hydroperoxide secondary alcohols
I 10 min
... .. 5 80. ... ... . .65 ppm
FIgure 3. The "3C MAS NMR spectra of "3C-polyethylene annealed at 110*C0
in air for the times indicated. The sample had originally been exposed to y-
radiation for 7 days at 65 *C in air.
sample radiatively aged for 7 days at 25 *C. The sample was then heated at
110 *C in air for the times indicated, returned to room temperature and its
spectrum recorded. The intensity of the hydroperoxide resonance decreases
rapidly as a function of annealing time. Additional samples that had also been
radiatively aged for 7 days at 25 *C were annealed for various times at 95, 80
and 65 *C. The hydroperoxide concentrations were difficult to accurately
quantify because of the broadness of the resonances and the uneven baselines.
The hydroperoxide resonance also overlaps somewhat with a smaller
unidentified resonance at 81 ppm. For these reasons, each spectrum was
phased identically so that its baseline features were similar. Peak heights
rather than peak areas of the hydroperoxide resonances were then measured.
The annealing process was continued until the spectrum exhibited no further
change. The normalized peak heights versus annealing times for the various
annealing temperatures are shown in Figure 4a.
The best approach to determine the activation energy of the
hydroperoxide composition is achieved by a time-temperature superposition.0
The advantage of this approach is that it utilizes all of the data from each
temperature experiment and does not require definitive knowledge of the
underlying kinetic behavior. We first select the lowest temperature, 65 *C, as
the reference temperature, T,,r. If increasing the temperature to T equally
accelerates all of the reactions underlying the oxidation, then the time
behavior of the decomposition will be accelerated by a constant multiplicative
shift factor, aT. For each higher temperature, we empirically determine the
value of aT that results in the best superposition with the data at Taor. Figure 4b
shows the superimposed results for the decomposition of the hydroperoxide
species. If the shift factors follow Arrhenius behavior, then log(aT) should be
a linear function of inverse absolute temperature. Figure 5 shows that such a
plot is consistent with Arrhenius behavior with an activation energy of 98
kJ/mol (25.7 keal/mol) calculated from the slope. This result agrees quite well
with the value of 105 kJ/mol measured by Chien using indirect methods."
We have examined non-enriched, 25 *C y-irradiated PE samples by EPR
in order to determine the type and relative amounts of radicals that exist at the
time of analysis, and to determine the environment of the hydroperoxide
species based upon the radical species observed after UV photolysis. Several
months after irradiation, spectra taken at room temperature show a large
100 1000 10' 10' 1
.8 o O 110C
.6 O 65C
10d 10T 10
Shifted Time at 65 00 (s)
Figure 4. (a) Normalized peak heights of the hydroperoxide resonance as a
function of annealing time at temperatures ranging from 65 to 110 *C. The
sample had originally been exposed to y-radiation for 7 days at 25 *C. (b) The
time-temperature superposition of the normalized peak heights at a reference
temperature of 22 *C.
E = 98 kJ/mol
2.8 2.65 2.7 2.75 2.8 2.85 2.9 2.95 3
Figure 5. Arrhenius plot of the shift factors for loss of hydroperoxide species
during thermal annealing. The activation energy is 98 kJ/mol.
singlet (.g= 2.0045), assignable to the polyenyl radical.'2 K-band room
temperature spectra reveal a small peak assignable to the peroxy radical (g ~
2.009)." The ratio of polyenyl to peroxy radicals is 1000 to I.
During UV-light cleavage experiments, we employ high power (20
mW) to saturate the polyenyl radicals so that we can see the peroxy radicals
that are at lower concentration.0 In Figure 6, we show consecutive EPR
spectra taken at 50 K. Upon exposure to UV light, the hydroperoxides are
cleaved to alkoxy and hydroxy radicals that react further to create more
peroxy radicals9 Subtraction of the spectrum taken before UV-irradiation
from that taken immediately after UV-irradiation shows the rhombic nature of
the nonrotating peroxy radical's g-values, with g, = 2.026 g2 = 2.011, and
ga= 2.003.'" Note that the gi value for the peroxy radicals created by
photolysis is slightly higher than the gi = 2.025 of the peroxy radicals that
already exist in the polymer. A one hour room temperature anneal is
sufficient to cause most of the UV-cleaved peroxides to decay. After a three-
day room temperature anneal, the signal is nearly the same as before UV
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ASSINK,ROGER A.; CELINA,MATHIAS C.; DUNBAR,TIMOTHY D.; ALAM,TODD M.; CLOUGH,ROGER LEE & GILLEN,KENNETH T. Analysis of Hydroperoxides in solid Polyethylene by NMR and EPR Spectroscopy, article, June 12, 2000; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc723380/m1/4/: accessed December 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.