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

该奖项将使凯斯西储大学物理系John D. McGervey教授和美国同事能够与东京大学工程研究所Yusuke Ujihira教授以及其他日本研究人员合作，通过正电子湮灭技术研究聚合物。这项技术的基础是一个事实，即正电子在各种材料中的寿命已被发现依赖于其周围的电子密度。在电子密度相对较低的区域——例如，在固体的空穴或缺陷中——寿命可测量地比在其他密度较高的区域更长（尽管仍以万亿分之一秒测量）。寿命是通过观察伽马射线来测量的，伽马射线是正电子产生的信号，其次是正电子湮灭时出现的第二束伽马射线。在聚合物中，更长的寿命被认为是正电子“原子”（o-Ps）形成的结果，它比正电子本身更容易被电子湮灭。因此，正电子的寿命是它所处的自由体积元素（“空穴”）大小的量度。在聚合物的情况下，o- p似乎形成或被困在存在于聚合物分子链之间的自由体积元素中。这类元素的数量和体积越大，形成o- p的可能性就越大。从聚合物样品中测量的正电子寿命分布，可以根据其自由体积元素的尺寸分布部分表征样品。这些信息在旨在了解自由体积与诸如粘度、弹性和透气性等聚合物性质之间关系的研究中是有用的；自由体积随聚合物老化的变化（减小）；以及聚合物对温度和压力等变量的响应。合作的科学家在正电子寿命技术方面有相当丰富的经验。在这次合作中，他们计划通过开发样品制备和表征的标准化方法，以及处理可能的系统误差来源的程序，进一步将其应用于聚合物研究。该技术将应用于各种聚合物，包括一些可能用于高级工程塑料的聚合物。

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.