Urbach Rule in the Red-Shifted Absorption Edge of PET Films Irradiated with Swift Heavy Ions.

Adil Z Tuleushev,Fiona E Harrison,Maxim V Zdorovets,Artem L Kozlovskiy

doi:10.3390/polym14050923

Abstract

This paper presents a new analysis of the experimental transmission spectra of polyethylene terephthalate (PET) films before and after irradiation with swift heavy ions (SHI) films, as reported previously by the authors. It is shown that the absorption edge red shift for irradiated films contains two regions of exponential form, one of which is located in the UV region and the other at lower energy, mainly in the visible part of the spectrum. The behaviour of the transmission curves under different irradiating fluences demonstrates that these two regions reflect respectively the electron-enriched core of the latent track and its electron-depleted peripheral halo. The focal point method yields a bandgap energy of 4.1 eV for the electron-enriched core of the latent track, which is similar to n-doped semiconductors, and a bandgap of about 1.3–1.5 eV for the electron-depleted halo, similar to p-doped semiconductors. The boundary between the latent track cores and halos corresponds to a conventional semiconductor p-n junction. The values of the characteristic Urbach energy determined from experimental data correspond to the nonradiative transition energy between the excited singlet and triplet levels of benzene-carboxyl complexes in repeat units of the PET chain molecule. A parallel is drawn between the SHI-induced redistribution of electrons held in structural traps in the PET film and chemical redox reactions, which involve the redistribution of electrons in chemical bonds. It is suggested that alkali etching triggers the release of excess electrons in the latent track cores, which act as a catalyst for the fragmentation of PET chain molecules along the latent tracks of the SHI irradiation.

Highlights

Irradiation of polymer films with swift heavy ions (SHI) is of ongoing interest [1–3], due to the possibilities it offers to modify the properties of films in useful ways
We explained our observation of irradiation-induced ordering and subsequent spiralization of the molecular structure of polyethylene terephthalate (PET) as due to the rotation of the benzene-carboxyl complexes in the PET repeat units in a residual electric field created by the movement of more numerous slow electrons from the outer periphery into the latent track core to replace those lost through δ-electron processes [20]
Our analysis shows that the ‘nontransmittance’ functions a*(ν) for PET film irradiated with various fluences of SHI have an exponential form in two photon energy intervals within the region of the absorption edge—so exhibiting two Urbach edges

Summary

Introduction

Irradiation of polymer films with swift heavy ions (SHI) is of ongoing interest [1–3], due to the possibilities it offers to modify the properties of films in useful ways. Attention has mainly focused on the effects of the fast δ-electrons (E ~ 103 eV) excited by the SHIs, and their cascades of secondary electrons [5,8–11,13–19] These studies have concluded that the observed effects are the result of irreversible carbonization of the polymer film due to stochastic destruction of covalent bonds in the polymer chain molecules and subsequent disorderly radiation cross-linking. In [12,21–25], we used optical and X-ray methods to show that the effects of SHI irradiation on PET films depend on the energy and fluence of the SHI ions, and their charge. This charge dependence can only be understood by the consideration of slow electrons. A study of the optical spectra of irradiated PET film after thermally stimulated discharge (annealing) showed that electrons were released from the deepest traps (benzene-carboxyl complexes) during thermal relaxation from the irradiation-induced ordered state to a more thermodynamically stable disordered state [12,21]

Methods

Discussion

Conclusion