PHYSICAL REVIEW E 87, 052602 (2013) Influence of polar groups in binary polymer blends on positronium formation P. Ramya, 1 P. Guagliardo, 2 T. Pasang, 1 C. Ranganathaiah, 1,2 S. Samarin, 2 and J. F. Williams 2,* 1 Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore-570006, India 2 Australian Research Council Centre of Excellence for Antimatter-Matter Studies, Centre for Atomic, Molecular and Surface Physics, School of Physics, University of Western Australia, Crawley, WA 6009, Australia (Received 2 March 2013; published 13 May 2013) The present work studied the role of the polar group unconjugated oxygen on the inhibition of positronium (Ps) formation in two binary blends made from a set of chosen constituent polymers with polar and weakly polar groups (nonpolar). The polymer blend samples of PVC-EVA and PVC-SAN were investigated by coincidence Doppler broadening and positron lifetime techniques. The strong polar acetate group in the EVA contributed to positron annihilation with electrons of unconjugated oxygen (–C + =O − ) as revealed by the momentum distribution curves peaking around 17 P L (10 −3 m 0 c). The ortho-Ps intensity indicated the unconjugated oxygen shows about a 28% Ps reduction even in the presence of a strong Ps inhibiting halogen (Cl − ). In contrast, this effect was not seen in the PVC-SAN blends since SAN contains a weakly polar (nonpolar) acrylonitrile group (C≡N). Our results indicate the chlorine of PVC in the blends is a major contributor to Ps inhibition through the formation of a (Cl − -e +) bound state but the unconjugated oxygen in EVA of the PVC-EVA blend also plays a similar, but lesser, role. DOI: 10.1103/PhysRevE.87.052602 PACS number(s): 61.25.hk I. INTRODUCTION This paper concerns the application of positron annihilation techniques to identify the effects of polar groups with a dipole distribution of electric charge in polymer chains and the extent to which those effects may be modified in polymer blends [1–16]. We look specifically at the polar carbonyl group with oxygen double bonded to a carbon atom with a dipole moment (–C=O) to explore how positron trapping by polar groups, that are a negatively charged part of the dipole (–C + =O − ), inhibits Ps formation. The effects of a conjugated oxygen atom in a molecule of the type C=C–C=O are compared with those of an oxygen atom in an isolated –C=O unconjugated group and with those of halogen atoms. Noting that a conjugated system contains overlapping (delocalized) p orbitals across adjacent aligned p orbitals which may bridge adjacent single bonds, two polymer blends were selected with different conjugation groups, in general with lower overall energy and different stability. We chose PVC-EVA (poly vinyl chloride-ethylene vinyl acetate) in which EVA contains a strong polar group of the type –C=O (unconjugated) and PVC-SAN (poly vinyl chloride-styrene acrylonitrile) in which SAN is nearly nonpolar or very weakly polar in character due to the presence of the C≡N group. According to the polar group concept the atoms with a difference in electronegativity less than 1.7 results in a covalent bond of a polar nature and when the difference is less than 0.5 a nonpolar covalent bond is formed. The carbonyl group (–C=O) in the acetate part of EVA shows a difference in the electronegativity between C (2.55) and O (3.44) of 0.89 indicating a strong polar group. For the SAN polymer, the cyanidyl (–C≡N) group is nonpolar or very weakly polar as the difference in the electronegativity between C (2.55) and N (3.04) is 0.49. In both blends of this * Corresponding author: jfw@physics.uwa.edu.au study, the PVC contains chlorine which is a strong inhibitor of Ps formation like other halogens. These polymer blends also have extensive applications in industrial and scientific fields requiring multifunctional and high performance materials without the limitations of single polymeric materials. An understanding at the atomic level has direct significance. Coincidence Doppler broadening spectroscopy (CDBS) [17] and positron lifetime annihilation spectroscopy (PALS) are the preferred techniques for this study. Coincidence Doppler broadening spectroscopy is an established technique to mea- sure and understand the chemical environment of materials at the site of positron annihilation with the measurement of mo- mentum distributions of annihilation events with core/valence electrons. The well-established positron annihilation lifetime spectroscopy, particularly for polymers and polymer-based materials, concerns the free volume size and its distribution [18–26]. The high-momentum part of the Doppler-broadened annihilation spectra is used to distinguish different elements because the core electrons retain their atomic character even when atoms form a complex polymer. For example here, the relationship between the oxygen-containing groups and Ps annihilation can be identified and it also can identify elemental variations around a defect site. The next section considers the details of these experimental approaches. II. EXPERIMENT A. Blend preparation Samples of PVC, SAN (25 wt. % acrylonitrile), and EVA (40 wt. % VA) with densities 1.34, 1.08, and 0.94 g/cc and weight-average molecular weights 43 000, 165 000 and 150 000 g/mol, respectively, were procured from M/s Sigma-Aldrich Chemicals Ltd. Bangalore. Blends of these samples were prepared by the conventional solvent-casting method. The weighed fractions of PVC and EVA were dissolved in the common solvent tetrahydrofuran at 60 ◦ C in three different compositions (80:20, 50:50, 20:80) 052602-1 1539-3755/2013/87(5)/052602(8) ©2013 American Physical Society