Positronium?oxygen interactions in polytrimethylsilylpropine membranes

Abstract

The shortening of triplet positronium (Ps) lifetime by oxygen in membranes of polytrimethylsilylpropine has been studied by using the lifetime spectroscopy and the Zeeman effect on Ps: the mixing of the m = 0 Ps sublevels induced by the magnetic field produces a quenching of orthoPs lifetime, which can be used to discriminate between the chemical reactions of Ps by oxygen (ortho–para conversion or oxidation). Our data can be interpreted in terms of an ortho– para conversion; the reaction of Ps oxidation is negligible for the system trapped in the holes forming the free volume of the membrane. PACS: 36.10.Dr; 71.60.+z; 78.70.Bj The bound positron–electron system of positronium (Ps), trapped in the free volume of a polymer, has a generally complex history because of the interactions with the surrounding molecules. In particular, when gases are introduced into the polymer, Ps can react through different processes, which can be broadly classified into three categories: (1) conversion of one Ps state to another; (2) oxidation by electron transfer, leaving a bare positron; (3) formation of a positron compound [1]. For example, the interaction of Ps with a paramagnetic centre, such as a transition metal ion [2], consists of ortho–para conversion due to paramagnetic molecules being in competition with Ps oxidation; each process is characterised by different cross-sections. As far as oxygen is concerned, its interaction with Ps causes a shortening of ortho-Ps lifetime. As a result of an investigation into different media (silica gels, zeolites, organic liquids), a model was proposed several years ago [3], according to which conversion from ortho-Ps to para-Ps should play a role only for Ps trapped in cavities having a mean diameter higher than 2 nm. Conversely, for Ps trapped in smaller cavities oxidation by oxygen should be the most important reaction. In the present work, we report the results of positronannihilation measurements in polytrimethylsilylpropine (PTMSP) in the presence of oxygen. A previous paper pointed out a strong reduction of ortho-Ps lifetime induced by this gas [4]. PTMSP is a membrane that has been the subject of thorough studies in recent years because of its high permeability [5–8], a property that makes the material of the utmost interest for many applications, particularly those related to the problem of separating different gas mixtures by polymer films. The polymer has been studied by means of different techniques; in particular, positron annihilation has been used to investigate free volume and permeability of PTMSP [9, 10]. The aim of the work in the present paper was to discriminate between the two possible processes (conversion and oxidation) able to produce the observed shortening of Ps lifetime and to perform an experimental test of the abovementioned model. To this purpose, we carried out positron-annihilation lifetime measurements coupled with magnetic fields. In this regard, the magnetic quenching of Ps has already revealed itself a powerful tool to shed light on the nature of the interactions between Ps and paramagnetic centres in aqueous solutions [11, 12]. 1 Experimental method PTMSP membranes were prepared as films having a uniform thickness of about 100 μm; details of the materials, the polymerization reaction, and the membrane preparation can be found elsewhere [4]. The synthesis was carried out in the Dipartimento di Chimica Industriale e Ingegneria Chimica “G. Natta” of the Politecnico di Milano. The positron source consisted of a droplet of 22Na from a carrier-free neutral solution, dried onto two identical kapton foils (thickness 1.08 mg cm−2) that were afterwards glued together by a thin layer of a cianoacrylate glue (Loctite 407 by Loctite Corporation). Care was taken to prevent the glue from reaching the region involved in annihilation. The activity of the source was about 3 ×105 Bq. The sample was formed by placing a sufficient number of films, one on another, in order to reach a thickness larger than the positron range in matter, about 170 mg cm−2 [13]. The source-sample assembly was inserted into a glass container, suitably designed in order to fill the gap of an electromagnet. The lifetime spectra were obtained by a con-