Positron Chemistry in Polymers

Abstract

Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron. Ps is formed predominantly by fast intra-track radiation chemical processes. In polymers it tends to be localized in intra/inter-molecular open space in the sparsely packed amorphous structure. Whilst short-lived singletpara-positronium (p-Ps) undergoes self-annihilation, the positron in long-lived tripletortho-positronium (o-Ps) annihilates with one of the spin opposite electrons bound in the surrounding polymer molecules. This process is called pick-off annihilation. The pick-off annihilation lifetime reflects the polymer chain packing through the size of the volume, where Ps is localized. Positrons are used to probe the amorphous structure of various polymeric systems. In this article, basic concepts and experimental techniques of positron chemistry in polymers as well as applications to the characterization of functional polymeric materials are overviewed.

Perspective of Positron Annihilation Spectroscopy in Polymers

“…In recent years, PAS has been successfully used to determine the free-volume, void, and layer properties in polymeric systems. − The basic principle of using PAS in polymers is based on the fact that the positron and Ps are preferentially localized in pre-existing defects, including free volume (∼0.1–1 nm) and voids (>1 nm) in polymeric systems. The annihilation parameters (e.g., positron annihilation lifetime) are directly related to the integrals of positron and electron wave functions. , Uses and advancements of PAS in polymer science rely on a good understanding of the fundamental properties of Ps and of molecules and their interactions, the chemical and physical aspects of materials, and advances in instrumentation.…”

“…In PAS technology, four major techniques have been developed and used during the past five decades: positron annihilation lifetime spectroscopy (PALS), variable monoenergy slow positron beam (VMSPB), Doppler broadening energy spectroscopy (DBES), and angular correlation of annihilation radiation spectroscopy (ACAR). This article is not intended to review and account for those techniques, which can be found elsewhere. ,,,,,,, In the following, we will focus on a few aspects for better techniques and instruments with a hope to advance more applications in a variety of polymeric systems and with better accuracy in PAS technology.…”

Free Volume of PVA/SSA Proton Exchange Membrane Studied by Positron Annihilation Lifetime Spectroscopy

“…(2) This model is successfully applied to investigate the free volume of many polymer materials [12]. A lot of experiments have been carried out to study the diffusion of gas and methanol molecules through the polymer matrix [13][14][15].…”

Free Volume in Poly(ether imide) Membranes Measured by Positron Annihilation Lifetime Spectroscopy and Doppler Broadening of Annihilation Radiation

“…In addition, in polymers, the decay of p-Ps exhibits an even narrower energy distribution, because of its intrinsic low electron-positron momentum. Since Ps is formed more readily in regions with low electron density, such a narrow distribution can suggest the presence of an open volume. − …”