U.S.-Japan Cooperative Research: Study of Polymers by Positron Annihilation

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This award will enable Professor John D. McGervey, Department of Physics, Case Western Reserve University, and U.S. colleagues to collaborate with Professor Yusuke Ujihira, Engineering Research Institute, The University of Tokyo, and other Japanese researchers in a study of polymers by the technique of positron annihilation. The basis of this technique is the fact that the lifetime of a positron in a variety of materials has been found to be dependent upon the electron density in its surroundings. In regions of relatively low electron density--in holes or defects in a solid, for example--the lifetime is measurably longer (though still measured in trillionths of a second) than in other, higher density regions. The lifetime is measured by observing the gamma ray which signals the creation of a positron, followed by a second gamma ray which appears when the positron is annihilated. In polymers, the longer lifetime is thought to be the result of the formation of an "atom" of ortho-positronium (o-Ps), which is more stable toward annihilation by an electron than is a positron itself. The lifetime of positronium is thus a measure of the size of the free- volume element ("hole") in which it finds itself. In the case of polymers the o-Ps appears to be formed or trapped in the free- volume elements that exist between the strands of polymer molecules. The greater the number and volume of such elements, the greater is the probability of forming o-Ps. From the measured positron lifetime distribution in a polymer sample it is possible to partially characterize the sample in terms of the size distribution of its free-volume elements. Such information is useful in studies aimed at understanding the relationship between free volume and such polymer properties as viscosity, elasticity, and gas permeability; the change (decrease) in free volume with polymer aging; and the response of polymers to variables such as temperature and pressure. The cooperating scientists have had considerable experience in the positron lifetime technique. In this cooperative effort they plan to pursue further its application to polymer studies by developing standardized methods for sample preparation and characterization, as well as procedures for dealing with possible sources of systematic error. The technique will be applied to various polymers, including some that may be used in advanced engineering plastics.