Polymers Properties and Applications 12 Editorial Board: Prof Hans-Joachim Cantow Institut f1ir Makromolekulare Chemie der Universitiit Stefan-Meier-StraBe 31, 7800 FreiburglFederal Republic of Germany Prof. H. James Harwood Institute of Polymer Science, University of Akron Akron, OH 44325/USA Prof Joseph P. Kennedy Institute of Polymer Science, University of Akron Akron, OH 44325/USA Prof Anthony Ledwith Pilkington Research and Development Laboratories Pilkington Brothers Place Lathom Ormskirk Lancashire L 40 5UF Prof Joachim Meiflner Institut f1ir Polymere, Eidgenossische Technische Hochschule, ETH-Zentrum, CH-8092 ZUrich, Switzerland Prof Seizo Okamura No. 24 Minami-Goshomachi Okazaki Sakyo-ku, 606 Kyoto, Japan Dr. G. Henrici-OlivelProf S. Olive Monsanto Textiles Co. P. O. Box 12830 Pensacola, FL 32575/USA Lev Zlatkevich Radiothermoluminescence and Transitions in Polymers With 60 Illustrations Springer-Verlag New York Berlin Heidelberg London Paris Tokyo Lev Zlatkevich Department of Materials Science and Engineering The Technological Institute Northwestern University Evanston, IL 60201 U.S.A. Library of Congress Cataloging in Publication Data Zlatkevich, L. (Lev) Radiothennoluminescence and transitions in polymers. (Polymers, properties and applications; 12) Includes bibliographies and index. \. Polymers and polymerization-Thennal properties. 2. Polymers and polymerization Radiation effects. 3. Thennoluminescence. I. Title. II. Series. QD38\.9.T54Z53 1987 547.7'045 86-27928 © 1987 by Springer-Verlag New York Inc. Softcover reprint of the hardcover 1st edition 1987 All rights reserved. This work may not be translated or copied in whole or in part without the written pennission of the publisher. (Springer-Verlag, 175 Fifth Avenue, New York, New York 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any fonn of infonnation storage and retrieval, electronic adaptation, computer software, or by similar or dis similar methodology now known or hereafter developed is forbidden. The use of general descriptive names, trade names, trademarks, etc. in this publication, even if the fonner are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. Typeset by The Maple-Vail Book Manufacturing Group, Inc., Binghamton, New York. 9 8 7 6 5 432 I ISBN-13: 978-1-4613-8697-1 e-ISBN-13: 978-1-4613-8695-7 DOl: 10.1007/978-1-4613-8695-7 In memory of my grandmother, Roza Preface This book deals with one of the recently developed methods of the analysis of tem perature transitions in polymers-the radiothermoluminescence method. Although thermoluminescence from irradiated inorganic materials was first found and examined as far back as the beginning of this century, a systematic study of this phenomenon in organic solids was initiated considerably later, in the I 96Os. In the past 25 years, essential achievements have been made both in understanding the mechanism of radiothermoluminescence in organic substances and in practical appli cations of the technique. The results obtained to date are described and discussed in relation to our knowl edge about structure and temperature transitions in polymeric systems. Lev Zlatkevich Contents Preface ........................................................ vii 1 Luminescence ........................................... . 1.1 Luminescence as a Phenomenon ............................ . 1.2 Electronic Excited States .................................. . 2 1.3 Fluorescence and Phosphorescence .......................... . 4 1.4 Delayed Fluorescence ..................................... . 5 1.5 Thermoluminescence ...................................... . 6 1.6 Absorption and Emission Spectra ............................ . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... . 7 2 Interaction of Radiation with Matter ....................... . 8 2.1 Energy Absorption and Trace Structure Produced by Charged Particles. Thermolization of Electrons ........................ . 8 2.2 Transient Intermediates in Chemical Reactions Induced by High- Energy Radiation ......................................... . 11 2.2.1 Behavior of the Excited Molecule ........................... . 11 2.2.2 Fate of the Free Electron .................................. . 13 2.2.3 The Nature of Positive Ions ................................ . 14 2.2.4 Radicals Produced by Irradiation ............................ . 15 2.3 Energy Transfer .......................................... . 17 References .............................................. . 19 3 Thermoluminescence in Polymers Induced by Radiation ...... . 20 3.1 Thermoluminescence Induced by Ionizing and Ultraviolet Radiation: The Notion of a Glow Curve ............................... . 22 3.2 Evidences for the Ionic Nature of Radiothermoluminescence in Organic Substances ....................................... . 23 3.3 Factors Determining Radiothermoluminescence ................ . 25 3.3.1 The Nature of Electron Traps ............................... . 25 3.3.2 Luminescence Centers. Spectral Distribution of Thermoluminesc- ence Emission ........................................... . 27 3.4 Kinetic Aspects of Charge Recombination in Organic Substances .. 30 3.5 Isothermal Luminescence: Tunneling and Diffusion-Type Charge Recombination ........................................... . 35 x Contents 3.6 Variation of Luminescence Intensity with Dose and Dose Rate ... . 38 3.7 Factors Influencing Radiothermoluminescence ................. . 40 3.7.1 Presence of Oxygen ...................................... . 40 3.7.2 Optical Bleaching ........................................ . 43 3.7.3 Temperature Quenching ................................... . 46 3.7.4 Electric Field Effects ...................................... . 48 3.8 Radiothermoluminescence of Organic and Inorganic Substances: Similarities and Dissimilarities .............................. . 51 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4 Molecular Mobility and Transitions in Polymers ............. . 55 4.1 Phase Transitions ............................ : ............ . 56 4.2 Relaxation Transitions ..................................... . 58 4.2.1 Nomenclature ............................................ . 60 4.2.2 Glass Transition .......................................... . 61 4.2.3 Secondary Transitions ..................................... . 64 4.2.4 Activation Energy of Relaxation ............................ . 65 4.3 Various Methods of Transition Analysis in Polymers ............ . 67 4.3.1 Mechanical Spectroscopy .................................. . 68 4.3.2 Dielectric Spectroscopy .................................... . 71 4.3.3 Nuclear Magnetic Resonance Spectroscopy .................... . 73 4.3.4 Dilatometry and Calorimetry ............................... . 74 4.4 Connection Between Mechanical Properties and Transitions in Polymers ............................................... . 75 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ . 80 5 Radiothermoluminescence as a Method of Analysis of Transitions in Polymers .................................. . 81 5.1 Radiothermoluminescence of Organic Substances and Molecular Motion ................................................. . 81 5.2 Influence of Impurities on the Glow Curve .................... . 85 5.3 Activation Energy and Methods for Its Estimation .............. . 88 5.3.1 Methods Employing Shape Parameters of the Peak ............. . 89 5.3.2 The Initial-Rise Method ................................... . 89 5.3.3 Various Heating Rates .................................... . 91 5.3.4 Isothermal Decay ......................................... . 92 5.4 Quasicontinuous Distribution of Activation Energies. Temperature Dependence of the Frequency Factor ......................... . 92 5.5 The Shape of Thermoluminescence Peaks and Structural Uniformity of a Substance ........................................... . 95 5.6 The Connection Between Thermoluminescence and Electrical Conductivity in Irradiated Materials .......................... . 95 5.7 Radiothermoluminescence in Comparison to the Other Methods of Transition Analysis ....................................... . 96 5.8 Instrumentation .......................................... . 97 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Contents xi 6 Application of Radiothermoluminescence to the Study of Polymer Systems ............................ , ........... . 102 6.1 Evaluation of Processes Accompanied by a Shift in Transition Temperatures ............................................ . 102 6.1.1 Plasticization ............................................ . 102 6.1.2 Cross-Linking ........................................... . 103 6.2 Amorphous Polymers ..................................... . 106 6.2.1 Glass Transition and Secondary Transitions ................... . 106 6.2.2 Changes in Radiothermoluminescence Associated with Induced Crystallization ........................................... . 107 6.2.3 Individual Polymers ...................................... . 111 6.2.3.1 Polybutadienes ........................................... . 111 6.2.3.2 Polymethyl Methacoylate .................................. . 115 6.2.3.3 Polystyrene ............................................. . 116 6.3 Semicrystalline Polymers .................................. . 116 6.3.1 Radiothermoluminescence and Crystallinity ................... . 116 6.3.2 Relaxation Transitions in Sernicrystalline Polymers ............. . 119 6.3.3 Individual Polymers ...................................... . 121 6.3.3.1 Polyethylene ............................................ . 121 6.3.3.2 Polypropylene ........................................... . 143 6.3.3.3 Polytetrafluoroethylene .................................... . 155 6.4 Multicomponent Polymer Systems ........................... . 156 6.4.1 Compatibility of Polymers ................................. . 156 6.4.2 Experimental Methods for Evaluating Compatibility ............ . 158 6.4.3 Polymer Blends .......................................... . 159 6.4.3.1 Homogeneous and Heterogeneous Polymer Blends ............. . 159 6.4.3.2 Dependence of the Glass-Transition Temperature on the Homogeneous System Composition .......................... . 166 6.4.3.3 Effect of Cross-Linking One of the Components on Compatibility .. 169 6.4.3.4 Other Factors Influencing Compatibility ...................... . 172 6.4.3.5 Convulcanization of Elastomers ............................. . 174 6.4.3.5 Covulcanization of Elastomers .............................. . 174 6.4.3.6 Covulcanization of Elastomers with Polyfunctional Unsaturated Compounds ............................................. . 177 6.4.4 Copolymers ............................................. . 177 6.4.4.1 Random Copolymers ...................................... . 178 6.4.4.2 Phase Separation in Block Copolymers ....................... . 183 6.4.4.3 Radiation-Grafted Systems: Distribution of Grafted Polymer Over the Bulk of the Test Specimen .............................. . 185 6.5 Latex Systems ........................................... . 187 6.6 Oriented Systems ......................................... . 189 6.7 Filled Systems ........................................... . 191 References .............................................. . 192 Index. .......... . . . . . . .... . . ........ .... . .. . .... . . . ..... 197 Chapter 1 Luminescence 1.1 Luminescence as a Phenomenon Light is a form of energy, and in accordance with the fundamental concept of energy conservation, energy must be supplied to every material system emitting light. There are two processes by which the material can become a generator of light after ab sorbing suitable energy [I]. In one process, the absorbed energy is converted into heat. The thermal agitation of all molecules within the system increases, and simul taneously, more and more of the molecules are transfered into excited states. The higher the temperature, the greater is ~he number of excited molecules and the greater is the intensity of the emitted light. In the other process, the molecules are brought into excited states without increasing their average kinetic energy and without heating the system. An appreciable part of the absorbed energy is temporarily localized as excitation of atoms or small groups of atoms which then emit light; this process is called luminescence. Luminescence is characterised by emission of light in excess of the thermal radiation produced by heat in a given material. The basic rule for distin guishing between thermal and light radiations can be formulated as follows: If the intensity of the emitted light exceeds the intensity of the radiation of the same wave length from a black body of the same temperature, the radiation is a case of lumi nescence [2]. Luminescence occurs when an excited molecule returns to the ground state with the emission of a quantum of light. There are many ways by which excited molecules are produced, and where luminescence is subsequently observed, the mode of exci tation is often referred to in the term used to describe the phenomenon. For example, photoluminescence implies the excitation by prior absorption of light, whereas ra dioluminescence presumes the excitation by irradiation with X-rays, y-rays, elec trons, or fast particles. In addition, in such phenomena as triboluminescence and sonoluminescence, excitation is accomplished by shock waves, and in chemilumi- 2 1: Luminescence nescence and bioluminescence, the emission of light betrays the existence of excited molecules produced by a chemical process. The appearance of luminescence, in fact, always implies the presence of excited molecules. Consequently, luminescence is a valuable tool for studying the chemistry of excited states, and since all chemical reactions involve excited states of some kind or another, luminescence in its broadest sense illuminates the whole field of chemistry [3]. This is not to imply that all excited states luminesce. As a general rule, luminescence increases in efficiency as the mo tion of a molecule is restricted, since the competing processes of radiationless energy transfer require coupling between the excited molecule and the molecules which sur round it and this coupling becomes greater with increased amplitude and diversity of molecular motions. Lowering the temperature, therefore, usually increases the prob ability of luminescent processes. Since the process of luminescence is the de-excita tion of excited molecules by re-emission of absorbed (or otherwise obtained) energy as light quanta, it is in direct competition with chemical reactions and every quantum emitted is wasted from the chemical point of view. However, the way in which luminescence changes in different media or with temperature or is quenched or en hanced by the addition of other molecules or otherwise can tell much about the processes of energy transfer and the chemical reactions which may be taking place. 1.2 Electronic Excited States The internal energy of a monatomic molecule is defined exclusively by the config uration of its electrons. The corresponding energy levels of the atom are in general separated from each other by relatively large intervals. In diatomic and polyatomic molecules, energy is also contained in the vibrations of the atomic nuclei relative to their center of gravity and in the rotation of the molecule around the main axis of inertia. The spacings between the corresponding energy levels which are superim posed on the electronic levels are much narrower than those between the electronic levels themselves. The total energy of a given state is the sum of electronic (Ee), vibrational (Ev) , and rotational (Er) energies: In the normal configuration of most molecules, the shared electrons in a given bond between atoms are paired with regard to electron spin, and the spin effects (resultant spin) cancel. The multiplicity of a state with regard to the resultant spin s is 2s + 1. This implies that there are 2s + 1 ways in which the resultant spin can couple with the orbital angular momentum along the molecular axis to yield the total angular momentum along that axis. Thus, when the electrons are paired, the resultant spin is 0, and the multiplicity of the state is 1. This is the singlet state. In a triplet state, the electrons are unpaired, the resultant spin is 1, and the multiplicity is 3. The electronic excited states and the most important processes occurring in these