Ž . Applied Surface Science 149 1999 116–124 Free volumes and holes near the polymer surface studied by positron annihilation H. Cao a , J.-P. Yuan a , R. Zhang a , C.S. Sundar a,1 , Y.C. Jean a, ) , R. Suzuki b , T. Ohdaira b , B. Nielsen c a Department of Chemistry, UniÕersity of Missouri-Kansas City, Kansas City, MO 64110, USA b Electrotechnical Laboratory, Tsukuba, Ibaraki 305, Japan c Department of Applied Science, BrookhaÕen National Laboratory, Upton, NY 11972, USA Abstract Positron annihilation lifetime and Doppler broadening of annihilation radiation experiments have been performed in a polystyrene film using the mono-energetic slow positron probe as a function of implantation energy. Significant variations of ˚ Ž . positron annihilation signals are observed at a short distance from the surface F200 A . The ortho-positronium lifetime in the polymer increases near the surface, while its intensity decreases. The intensity results are interpreted in terms of a free-volume hole model for positronium formation. The lifetime results are interpreted as an expansion of local hole volume near the polymer surface. The free-volume and hole distribution near the surface is found to be broader than in the bulk. Applications of slow positrons to industrial problems, such as degradation of polymer coatings and membrane separations, are promising. q 1999 Elsevier Science B.V. All rights reserved. PACS: 64.70pf; 68.60Bs; 78.35 Keywords: Free volume; Hole distribution; Surface; Polymer; Polystyrene 1. Introduction In recent years, positron annihilation spectroscopy Ž . PAS has been developed as a useful tool in probing wx the microscopic and local properties of polymers 1 . One of the great successes in this line of research is the determination of mirco-structural properties of defects, such as free volumes and holes, on an ) Corresponding author. Tel.: q1-816-235-2295; Fax: q1-816- 235-5502; E-mail: jeany@umkc.edu 1 Visiting from Material Science Division, IGCAR, Kalpakkam, India. ˚ Ž . atomic scale 2–20 A in polymers. It has been demonstrated that positron annihilation lifetime Ž . PAL spectroscopy is capable of determining size, distribution, fraction, and anisotropic structure of w x holes and free volumes in polymers 2–9 . The high sensitivity of PAS in probing defect properties arises Ž from the fact that the positronium atom Ps, an atom . consisting of a positron and an electron is preferen- Ž . tially trapped localized in atomic-scale free vol- umes and holes. Most existing PAS studies have emphasized the bulk properties of polymers using radioisotope w x positron sources and the lifetime technique 2–9 . Recently, there have been several reports on results 0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-4332 99 00185-3