The Effects of Radiation on High-Technology Polymers PDF
Preview The Effects of Radiation on High-Technology Polymers
ACS SYMPOSIUM SERIES 381 The Effects of Radiation on High-Technology Polymers Elsa Reichmanis, EDITOR AT& T Bell Laboratories Jame University of Queensland Developed from a workshop sponsored by the Division of Polymer Chemistry, Inc., of the American Chemical Society and the Polymer Division of the Royal Australian Chemical Institute, Queensland, Australia August 16-19, 1987 American Chemical Society, Washington, DC 1989 In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. Library of Congress Cataloging-in-Publication Data The effects of radiation on high-technology polymers Elsa Reichmanis, editor, James H. O'Donnell, editor. p. cm.—(ACS Symposium Series, ISSN 0097-6156; 381) Developed from a workshop sponsored by the Division of Polymer Chemistry, Inc., o Society and the Polymer Divisio Chemical Institute, Queensland 1987. Bibliography: p. Includes index. ISBN 0-8412-1558-8 1. Polymers and polymerization—Effect of radiation on—Congresses. I. Reichmanis, Elsa, 1953- . II. O'Donnell, James H. III. American Chemical Society. Division of Polymer Chemistry. IV. Royal Australian Chemical Institute. Polymer Division. V. Series. QD381.9.R3E42 1989 620.1 '9204228—dc19 88-39298 CIP Copyright © 1989 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per-copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 or the U.S. Copyright Law. 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Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. ACS Symposium Series M. Joan Comstock, Series Editor 1988 ACS Books Advisory Board Paul S. Anderson Vincent D. McGinniss Merck Sharp & Dohme Research Battelle Columbus Laboratories Laboratories Daniel M. Quinn Harvey W. Blanch Universit of Iowa University of California—Berkele James C. Randall Malcolm H. Chisholm Exxon Chemical Company Indiana University Alan Elzerman E. Reichmanis Clemson University AT&T Bell Laboratories John W. Finley C. M. Roland Nabisco Brands, Inc. U.S. Naval Research Laboratory Natalie Foster Lehigh University W. D. Shults Oak Ridge National Laboratory Marye Anne Fox The University of Texas—Austin Geoffrey K. Smith Rohm & Haas Co. Roland F. Hirsch U.S. Department of Energy Douglas B. Walters National Institute of G. Wayne Ivie Environmental Health USDA, Agricultural Research Service Michael R. Ladisch Wendy A. Warr Purdue University Imperial Chemical Industries In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. Foreword The ACS SYMPOSIUM SERIES was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously pub lished papers are no research are acceptable symposi y types of presentation. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. Preface REST IN THE EFFECTS OF RADIATION on polymeric materials is rapidly increasing. Changes in the physical or mechanical properties of a polymer can be induced by small amounts of radiation; for example, even a few scissions or cross-links per molecule can dramatically affect the strength or solubility of a polymer. Such changes determine whether a particular polymer will have an application in industry. The irradiation of polymers is widespread in many industries. For example, microlithograph integrated circuits that y volatility of thin polymer resist films by radiation. The sterilization by radiation of medical and pharmaceutical items, many of which are manufactured from polymeric materials, is increasing. This trend arises from both the convenience of the process and the concern about the tox icity of chemical sterilants. Information about the radiolysis products of natural and synthetic polymers used in the medical industry is required for the evaluation of the safety of the process. UV and high-energy irradiation of polymer coatings on metals and other substrates has been developed for various industrial purposes. Irradiation causes the modification of polymer properties, and these spe cial properties form the basis of major industries in heat-shrinkable films and tubings, cross-linked polymers, and graft copolymers. In the aerospace industry, advanced polymeric materials are used in many applications because of their high strength and low-weight performance. However, these materials are subject to degradation in space by UV and high-energy radiation, as well as bombardment by oxygen atoms. A thorough understanding of the fundamentals of polymer chemistry and the effects of radiation on polymeric materials is therefore critical for the aerospace and other industries. Further technological advances in this field will require the interaction and collaboration between basic and applied researchers. This book emphasizes the technological significance of the effects of radiation on polymers and draws attention to the major interactions between fundamental science and advanced technology. Although the field of polymer radiation chemistry is not exhaustively covered, a sam pling of the ongoing basic and applied research in this area is presented. Review chapters have been included that cover fundamental radiation chemistry, spectroscopic methods, materials for microlithography, and radiation-durable materials. vii In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. The authors and editors are indebted to the National Science Foun dation, the Department of Science, and the IBM Corporation for provid ing financial support to the U.S.-Australia Workshop on Radiation Effects on Polymeric Materials, from which this book was developed. Our sincerest thanks are extended to Robin Giroux and the production staff of the ACS Books Department for their efforts in publishing this volume. ELSA REICHMANIS AT&T Bell Laboratories Murray Hill, NJ 07974 JAMES H. O'DONNELL University of Queenslan St. Lucia, Brisbane 406 Australia August 24, 1988 viii In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. Chapter 1 Radiation Chemistry of Polymers James H. O'Donnell Polymer and Radiation Group, Department of Chemistry, University of Queensland, St. Lucia, Brisbane 4067, Australia Changes in the properties of polymer materials caused by absorption of high-energy radiation result from a variety of chemical reactions subsequent to the initial ionizatio experimental procedures may be used to measure, directly or indirectly, the radiation chemical yields for these reactions. The chemical structure of the polymer molecule is the main determinant of the nature and extent of the radiation degradation, but there are many other parameters which influence the behaviour of any polymer material when subjected to high-energy radiation. Development of new applications of radiation modifications of the properties of polymers in high technology industries such as electronics and the exposure of polymer materials to radiation environments as diverse as medical sterilization and the Van Allen belts of space have resulted in a renewed interest in fundamental radiation chemistry of polymers. The main features of the chemical aspects of radiation -induced changes in polymers, which are responsible for changes in their material properties are considered in this chapter. TYPES OF RADIATION High-energy radiation may be classified into photon and particulate radiation. Gamma radiation is utilized for fundamental studies and for low-dose rate irradiations with deep penetration. Radioactive isotopes, particularly cobalt-60, produced by neutron irradiation of naturally occurring cobalt-59 in a nuclear reactor, and caesium-137, which is a fission product of uranium-235, are the main sources of gamma radiation. X -radiation, of lower energy, is produced by electron bombardment of suitable metal targets with electron beams, or in a 0097-6156/89A)381-0001$06.00A) « 1989 American Chemical Society In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 2 EFFECTS OF RADIATION ON HIGH-TECHNOLOGY POLYMERS synchrotron. Photon radiation has a large half-distance for absorption compared to the range of particulate radiation. Electron irradiation is normally obtained from electron accelerators to give beams with energies in the MeV range. The corresponding penetration depths are then a few mm. Much lower energy electron beams, e.g. 10-20 keV, are used in electron microscopy and in electron beam lithography. A large proportion of the energy is then deposited in urn thick polymer films. The electron beams may be programmed to transfer a circuit pattern from a computer to a resist film of radiation-sensitive polymer. Nuclear reactors are a source of high radiation fluxes. This comprises mainly neutrons and gamma rays, and large ionized particles (fission products) close to the fuel elements. The neutrons largely produce protons in hydrocarbon polymers by "knock-on" reactions, so that the radiation chemistry of neutrons is similar to that of proton beams, which may alternatively be produced using positive-io The effects of th materials is now of considerable importance due to the increasing use of communications satellites. Geosynchronous orbit corresponds to the second Van Allen belt of radiation, which comprises mainly electrons and protons of high energy. Alpha particles cause intense ionization and excitation due to their large mass and consequently produce substantial surface effects. Larger charged particles may be produced in positive ion accelerators. ABSORPTION OF RADIATION Photon radiation undergoes energy absorption by pair production (high energies, > 4 MeV), Compton scattering and the photoelectric effect (low energies, < 0.2 MeV). In the photoelectric effect all of the energy of the incident photon is transferred to an electron ejected from the valence shell, whereas in Compton scattering there is also a scattered photon (of lower energy). Thus, the radiation chemistry of photons occurs mainly through interaction of secondary electrons with the polymer molecules. Electrostatic repulsion between high-energy electrons - produced from an accelerator, or by photon interaction with substrate atoms - and valency electrons in the polymer cause excitation and ionization. The chemical reactions result from these species. The absorption of high-energy radiation depends only on the electron density of the medium. Mass density is a reasonable first approximation to electron density. More accurately, and conveniently, the average value of the ration of Z/A for the atoms, where Z is the atomic number and A is the atomic mass, can be used to calculate relative dose. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 1. O'DONNELL Radiation Chemistry of Polymers 3 The depth-profile of photon absorption is analogous to that for UV-visible light, i.e. I = Io exp(-Ad), where the mass energy absorption coefficient, u/g is used instead of the extinction coefficient. Particulate energy absorption can be described by relative stopping powers. The interaction of neutrons with organic molecules occurs mainly through knock-on of protons. Thus, the radiation chemistry is similar to proton irradiation. Radiation chemistry by positive ions is of increasing importance on account of ion implantation technology, plasma development and deposition processes, and cosmic irradiation. UNITS. Energy absorption has been traditionally expressed as dose rate in rad, corresponding to 10-2 J/kg. The SI unit is the gray (Gy), which is 1 joule per kg. Radiation chemical values for numbers of molecule eV) of energy absorbed. DEPTH PROFILE. The secondary electrons produced by ionization processes from an incident beam of high-energy electrons are randomly directed in space. Spatial "equilibrium" is achieved only after a minimum distance from the surface of a polymer in contact with a vacuum or gaseous environment (of much lower density). Consequently, the absorbed radiation dose increases to a maximum at a distance from the surface (2 mm for 1 MeV electrons) which depends on the energy of the electrons. The energy deposition then decreases towards zero at a limiting penetration depth. TEMPERATURE RISE DURING IRRADIATION. The chemical reactions which result from irradiation of polymers consume only a small fraction of the absorbed energy, which is mainly dissipated in the form of heat. Thus, 0.1 MGy of energy absorbed in water will produce a temperature rise of 24 °C - and more in a polymer. PRIMARY PROCESSES. Absorption of high-energy radiation by polymers produces excitation and ionization and these excited and ionized species are the initial chemical reactants. The ejected electron must lose energy until it reaches thermal energy. Geminate recombination with the parent cation radical may then occur and is more likely in substrates of low dielectric constant. The resultant excited molecule may undergo homolytic or heterolytic bond scission. Alternatively, the parent cation radical may undergo spontaneous decomposition, or ion-molecule reactions. The initially ejected electron may be stabilized by interaction with polar groups, as a solvated species or as an anion radical. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 4 EFFECTS OF RADIATION ON HIGH-TECHNOLOGY POLYMERS P-VWPt + e" -AV»P* e- —> e- th Pt + e" P th P* —> Ri- + R- 2 P* —> A+ + B" pt —> C+ + D • P+ + P-»PX + E+ e"+ S ^ e " s o l v , S T The radiation chemistry of polymers is therefore the chemistry of neutral, cation and anion radicals, cations and anions, and excited species. SECONDARY REACTIONS. The reactions of the free radicals include (1) abstractions (of H atoms, with preference for tertiary H, and of halogen atoms), (2) addition to double bonds, which are very efficient scavengers for radicals, (3) decompositions to give both small molecule products, such as CO2, and (4) chain scission and crosslinking of molecules. R • + R'H —> RH + R'. R • + R'Cl -4 R CI + R'- R • + CH = CHR'-*RCH-CHR 2 2 CAGE EFFECTS. When main-chain bond scission occurs in polymer molecules in the solid state to form two free radicals, the limited mobility of the resultant chain fragments must mitigate against permanent scission. This concept is supported by the increased yields of scission in amorphous compared with crystalline polymers. Similarly, the scission yields ar increased above, the glass transition, Tg, and melting, Tm, temperatures. There is also evidence from NMR studies of the changes in tacticity in poly(methyl methacrylate) that racemization occurs at a higher rate than permanent scission of the main chain, consistent with initial main-chain bond scission, rotation of the newly formed chain-end radical, and geminate recombination. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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