Positron annihilation lifetime study of irradiated and deformed low density polyethylene E.E. Abdel-Hady* Physics Department, Faculty of Science, El-Minia University, B.O. 61519 El-Minia, Egypt Received 21 September 2002; received in revised form 3 December 2002; accepted 15 December 2002 Abstract Positron annihilation lifetime spectroscopy has been used to study the free volume hole properties of unirradiated and irradiated low density polyethylene (LDPE) as a function of deformation in the range from 0 to 110%. Deconvolution of the lifetime spectra into three components reflected three different morphologies of the polymer structure. The variation of positron annihilation parameters with deformation shows three stages; the elastic range, strain-softening region, and plastic region. The free volume was probed using ortho-positronium (o-Ps) pick-off annihilation lifetime parameters as a measure of electron density and the mean free volume hole radius. For unirradiated sample, the average of the free volume hole increases with deformation in the elastic region, remains unchanged within the strain softening region, and finally tends to increase in the plastic region. A reduction in the size of the free volume holes and concentrations has been observed in the case of irradiated sample which is due to a structural change. The distribution of the free volume for unirradiated sample shifts from a small to large size as the deformation increases and has very similar Gaussian-like distribution. For irradiated one, the distribution becomes broader and has an asymmetric appearance. The results show that positron lifetime is a sensitive means to probe the microstructural change of polymers during deformation. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Positron annihilation in LDPE; Fractional free volume; Free volume distribution; Deformation 1. Introduction The mechanical properties and the deformation mechanism of semicrystalline polymers have been of great interest for years [1]. Several deformation mechanisms have been proposed in the past to describe the morphological changes during drawing of flexible semicrystalline polymers [2,3]. The effect of plastic deformation on microstructure is one of the interesting topics in polymer science [4,5]. In the amorphous region of a polymer, structural changes can be characterized in statistical terms such as density fluctuation, or concen- tration and average size of free volume holes [6]. Posi- tron annihilation lifetime spectroscopy (PALS) has been developed to probe the variations of average size and concentration of free volume holes [7,8]. Positrons from a 22 Na source are allowed to penetrate the polymer sample and are thermalized by inelastic collisions with the surrounding medium. The positrons can annihilate as a free positron with an electron in the material or combine with the electron to form a metastable state known as a positronium (Ps) atom. If the spins of the positron and the electron are antiparallel, the species is called para-positronium (p-Ps) and has an intrinsic life- time ( p-Ps of 0.125 ns. When the spins are parallel an ortho-positronium (o-Ps) is formed, which has a mean lifetime ( o-Ps =142 ns in a vacuum. The o-Ps lifetime ( o- Ps in polymers is shortened to a few nanoseconds because the positron can pick-off an electron with opposite spin from the surrounding medium and anni- hilate with it. The most important component is the o-Ps lifetime ( o-Ps which gives us information about the free volume holes in which annihilation of positrons takes place. The o-Ps lifetime ( o-Ps and the radius of the free volume hole R are related through the equation [9,10], ( o-PS ¼ 0:51  R R o þ 1 2% sin 2%R R o     1 ns; ð1Þ 0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00022-3 Polymer Degradation and Stability 80 (2003) 363–368 www.elsevier.com/locate/polydegstab * Tel.: +20-86-36-0250; fax: +20-86-34-2601. E-mail address: esamhady@link.net (E.E. Abdel-Hady).