Peroxide-initiated cross-linking of high-density polyethylene
(HDPE) was studied using an
on-line electron spin resonance (ESR) technique. Peroxides were
used to provide primary radicals upon
thermal decomposition at elevated temperatures for the generation of
polymer backbone radicals.
Recording of the ESR spectra during the reaction process verified
the free radical mechanisms involved
in the modification. The determining variables for the reaction
kinetics were the reaction temperature,
peroxide type, and concentration level. Cross-linking via the
recombination of backbone radicals was
the major mechanism for high-density polyethylene modification. It
was shown in the ESR spectra that
some backbone radicals were trapped into the cross-linked polymer
network and were still detectable
after several months. The termination of backbone radicals was
diffusion controlled. The rate constants
were estimated using the measured radical concentration profiles.
Significant changes in the signal
hyperfine structure were also observed, indicating the formation of
allyl radicals and the effects of melting
and cross-linking on the mobility of the backbone
radicals.
Chemical modification of isotactic polypropylene via a
free-radical mechanism was studied using
an on-line electron spin resonance spectrometer. Peroxides were
used to provide primary radicals
upon thermal decomposition at elevated temperatures for the generation
of polymer backbone
radicals. Four types of peroxides, tert-butyl
perbenzoate, dicumyl peroxide, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexyne-3, and benzoyl
peroxide, at various concentration levels were
employed. The ESR measurement demonstrated that the modification
was accomplished via a
free-radical mechanism. Both radical generation and termination
were clearly observed.
Peroxide type, concentration, temperature, and reaction time were
the major factors that affected
the radical concentration profiles. The radical concentration
variations were used to estimate
the radical termination rate constants.
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