Optical Characterization and Computational Chemical Evaluation of Electronic Localized States in Polyolefin

Abstract

Optical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet. Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of ultraviolet photons weakened the luminescence intensity. It is assumed from these results that α, β -unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing ultraviolet photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the above-mentioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the above-mentioned assumption of the luminescence center and its decomposition.

Optical Properties of Saturated and Unsaturated Carbonyl Defects in Polyethylene

“…While for absorption most recent results found a threshold around 4.5 eV and further absorption peak above 5.5 eV, emission can occur below 4 eV by fluorescence and even down to less than 2.5 eV by phosphorescence, with a longer time delay. The position of experimental peaks, together with our calculated exciton energies, are summarized in Figure , where we superimpose them to an emission/absorption experimental chart from ref . Experimental resonances show up as shoulders in significantly broadened spectra.…”

“…Some absorption peaks can be identified with α,β-unsaturated carbonyl excitonic peaks, in particular, the absorption/emission feature around 5.5−6.5/4−5 eV labeled α PL. 11 Saturated carbonyls, although their formation is expected to be limited only by diffusion or reaction kinetics in most temperature/pressure regimes, are not easily detectable due to their much lower optical absorption cross section. Phosphorescence emission lines coincide partially with the calculated de-excitations energies from triplet states to the singlet ground state for both defects.…”

“…Another interesting recent work tries to interpret the experimental PL signatures with excitation energies calculated with a configuration-interaction approach, however the comparison of calculations with experiments is not conclusive. 11 Excitonic effects can be, conversely, included either in the Bethe−Salpeter equation (BSE) or in time-dependent DFT (TDDFT). The latter was recently used to exploit the results of reflectivity measurement of degraded paper, 35,36 where cellulose, at variance with PE, is a polymer with aromatic units.…”

“…In the absence of antioxidants, polyethylene in air is known to accumulate a certain amount of groups containing CO double bonds. The amount of those species, is generally assessed through infrared spectroscopy. , Optical spectroscopies have also been used to characterize oxidized polyolephins. − However, absorption/emission of electromagnetic radiation in PE, which is a large band gap insulator, is not only a probe of the material, but can also induce damage. Indeed, aging of polymers is supposed to occur, or to be enhanced, also through electronic excitations induced by photons of sufficiently high energy; in this respect, many qualitative similarities exist between insulating polymers and ionic or ionic–covalent inorganic insulators, in which defect production by electronic excitation has been an active research field, especially in alkali halides − and silicon dioxide. − In this framework, understanding absorption and emission of light or UV radiation from polymers, with or without defects, is of particular interest.…”

“…Although the quasiparticle band structure is here much improved thanks to the fraction of Hartree–Fock exchange, excitonic effects are still out of the scope of the theoretical framework employed. Another interesting recent work tries to interpret the experimental PL signatures with excitation energies calculated with a configuration-interaction approach, however the comparison of calculations with experiments is not conclusive …”