The mechanism of chemiluminescence (CL) during thermal oxidation of polypropylene (PP)
was probed by doping PP with an energy acceptor (9,10-dibromoanthracene [DBA]) and a chemiluminescence (CL) activator (9,10-diphenylanthracene [DPA]). Doping PP with DBA had little effect on the
shape of the CL intensity (I
CL)−time profile. This suggests that energy transfer from triplet states is
probably not significant in the scheme of PP CL. However, the CL activator (DPA) had a significant
effect on the shape and intensity of the I
CL−time profile. In the absence of DPA, the I
CL−time profile
matches the profile for the formation of carbonyl-containing oxidation products from FTIR-emission
spectra. In contrast, in the presence of DPA, it was the integrated DPA I
CL−time profile which matched
the oxidation product profile, indicating that now I
CL was proportional to the hydroperoxide concentration.
It is suggested that peroxides formed during PP oxidation are capable of reacting with DPA to produce
chemically induced electron exchange luminescence (CIEEL). It is also suggested that CL from undoped
PP, i.e., direct CL, may also occur by a CIEEL mechanism. This mechanism is believed to involve the
reaction of PP peroxides with an easily oxidizable luminescent oxidation product. For the detector system
used in this study the majority of light emitted is consistent with reactions between acyl peroxides and
α,β-unsaturated carbonyls. The consequences of this mechanism are that the I
CL−time curve measured
during the oxidation of PP may reflect either the hydroperoxide profile or the oxidation product profile
depending on the spectral wavelength analyzed or the state of purity of the polymer.