Table Of ContentACS 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
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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.
