Polymer Science and Technology PDF

 POLYMER SCIENCE AND TECHNOLOGY Robert O. Ebewele Department of Chemical Engineering University of Benin...

 POLYMER SCIENCE AND TECHNOLOGY Robert O. Ebewele Department of Chemical Engineering University of Benin Benin City, Nigeria CRC Press Boca Raton New York Copyright 2000 by CRC Press LLC  8939-frame-discl Page 1 Monday, April 3, 2006 2:40 PM Library of Congress Cataloging-in-Publication Data Ebewele, Robert Oboigbaotor. Polymer science and technology / Robert O. Ebewele. p. cm. Includes bibliographical references (p. - ) and index. ISBN 0-8493-8939-9 (alk. paper) 1. Polymerization. 2. Polymers. I. Title. TP156.P6E24 1996 668.9--dc20 95-32995 CIP This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. © 2000 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-0849-8939-9 Library of Congress Card Number 95-32995 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper Copyright 2000 by CRC Press LLC  PREFACE The book is divided into three parts. The first part covers polymer fundamentals. This includes a brief discussion of the historical development of polymers, basic definitions and concepts, and an overview of the basis for the various classifications of polymers. It also examines the requirements for polymer formation from monomers and discusses polymer structure at three levels: primary, secondary, and tertiary. The relationship between the structure of the monomers and properties of the resulting polymer is highlighted. This section continues with a discussion of polymer modification techniques. Throughout the discussion, emphasis is on the structure-property relationship and several examples are used to illustrate this concept. The second part deals with how polymers are prepared from monomers and the transformation of polymers into useful everyday articles. It starts with a discussion of the various polymer preparation methods with emphasis on reaction mechanisms and kinetics. The control of molecular weight through appropriate manipulation of the stoichiometry of reactants and reaction conditions is consistently empha- sized. This section continues with a discussion of polymer reaction engineering. Emphasis is on the selection of the appropriate polymerization process and reactor to obtain optimal polymer properties. The section terminates with a discussion of polymer additives and reinforcements and the various unit operations in polymer processing. Here again, the primary focus is on how processing conditions affect the properties of the part produced. The third part of the book deals with the properties and applications of polymers. It starts with a discussion of polymer solution properties through the mechanical properties of polymers and concludes with an overview of the various applications of polymer materials solids. The viscoelastic nature of polymers is also treated. This section also includes a discussion of polymer fracture. The effects of various molecular and environmental factors on mechanical properties are examined. The primary focus of the book is the ultimate property of the finished polymer product. Consequently, the emphasis throughout the book is on how various stages involved in the production of the finished product influence its properties. For example, which polymerization process will be preferable for a given monomer? Having decided on the polymerization process, which type of reactor will give optimum product properties? What is the best type of processing technique for a given polymer material? How do processing conditions affect the properties of the part produced and which polymer material is most suitable for a particular application? The book addresses the elements that must be considered to come up with appropriate answers to these types of questions. The distinguishing features of the book are intended to address certain problems associated with teaching an elementary course in polymers: 1. For a vast majority of introductory polymer courses, very frequently the instructor has to rely on several textbooks to cover the basics of polymers as none of the existing textbooks discusses the required materials satisfactorily. Most students find dealing with several textbooks in an introductory course problematic. This book attempts to remedy this problem. A deliberate effort has been made to cover most of the areas normally taught in such an introductory course. Indeed, these areas are typical of existing texts. However, the approach and depth of coverage are different. The book presents various aspects of polymer science and technology in a readily understandable way. Emphasis is on a basic, qualitative understanding of the concepts rather than rote memorization or detailed mathematical analysis. Description of experimental procedures employed in the characterization of polymers has been either completely left out or minimized. I strongly believe that this approach will appeal to those students who will be learning polymer science for the first time. 2. None of the existing texts has worked examples. It is my experience that students feel more comfortable with and generally prefer textbooks that illustrate principles being discussed with examples. I have followed this approach throughout the text. In addition, each chapter has review problems; answers are provided in a Solutions Manual. Both the worked examples and the review problems are designed to provide additional insight to the materials covered. The overall objective of this approach is to enhance the reader’s understanding of the material and build his/her confidence. Emphasis throughout the book is on structure-property relationship and both the worked examples and review problems reflect this basic objective. Robert O. Ebewele Copyright 2000 by CRC Press LLC  ACKNOWLEDGMENT In writing this book, I have had to rely on materials from various sources. These sources have been compiled as references at the end of each chapter. While I express my profound gratitude to publishers for permission to use their materials, I apologize for ideas and materials which I have inadvertently failed to acknowledge. I certainly do not lay claim to these published concepts and ideas. The skeletal framework for this book was initiated during my student days at the University of Wisconsin, Madison and over the years, the material in the book has been constantly refined as it was being developed for use by successive generations of undergraduate and graduate students at the Ahmadu Bello University, Zaria, Nigeria. The final version of the book was written during my sabbatical leave at the Department of Chemical Engineering, University of Wisconsin, Madison, and subsequently during my leave of absence at the Forest Products Laboratory Madison, Wisconsin. I am grateful to the Ahmadu Bello University, Zaria, the University of Wisconsin, Madison and the Forest Products Laboratory, Madison for providing me unlimited access to their library materials and other facilities. Finally, I am indebted to the late Prof. J. A. Koutsky of the University of Wisconsin, Madison; Dr. George E. Myers and Mr. Bryan H. River, formerly of the Forest Products Laboratory, Madison; and a host of others for reviewing various parts of this book. Your contributions have greatly improved the quality of the book. I, however, take full responsibility for any lapses and errors that may be contained in the book. Copyright 2000 by CRC Press LLC  TABLE OF CONTENTS PART I: FUNDAMENTALS Chapter One Introduction I. Historical Development II. Basic Concepts and Definitions III. Classification of Polymers A. Natural vs. Synthetic B. Polymer Structure 1. Linear, Branched, or Cross-Linked Ladder vs. Functionality, 2. Amorphous or Crystalline 3. Homopolymer or Copolymer 4. Fibers, Plastics, or Elastomers C. Polymerization Mechanism D. Thermal Behaviour E. Preparative Technique F. End Use IV. Problems References Chapter Two Polymerization Mechanisms I. Introduction II. Chain-Reaction Polymerization A. Initiation B. Propagation C. Termination D. Chain Transfer E. Diene Polymerization III. Ionic and Coordination Polymerizations A. Cationic Polymerization B. Anionic Polymerization C. Coordination Polymerization IV. Step-Growth Polymerization A. Typical Step-Growth Polymerizations 1. Polyesters 2. Polycarbonates 3. Polyamides 4. Polyimides 5. Polybenzidazoles and Polybenzoxazoles 6. Aromatic Ladder Polymers 7. Formaldehyde Resins 8. Polyethers 9. Polysulfides 10.Polysulfones V. Ring-Opening Polymerization A. Poly(Propylene Oxide) B. Epoxy Resins C. Polycaprolactam (Nylon 6) VI. Problems References Copyright 2000 by CRC Press LLC  Chapter Three Chemical Bonding and Polymer Structure I. Introduction II. Chemical Bonding A. The Ionic Bond B. The Covalent Bond C. Dipole Forces D. Hydrogen Bond E. Induction Forces. F. van der Waals (Dispersion) Forces III. Primary Structure A. Polarity of Monomers IV. Secondary Structure. A. Configuration 1. Diene Polymerization 2. Tacticity B. Conformation C. Molecular Weight V. Tertiary Structure A. Secondary Bonding Forces (Cohesive Energy Density) B. Crystalline and Amorphous Structure of Polymers 1. Crystallization Tendency 2. Structural Regularity 3. Chain Flexibility 4. Polarity C. Morphology of Crystalline Polymers 1. Crystal Structure of Polymers 2. Morphology of Polymer Single Crystals Grown from Solution 3. Morphology of Polymers Crystallized from the Melt VI. Crystallinity and Polymer Properties VII. Problems References Chapter Four Thermal Transitions in Polymers I. Introduction II. The Glass Transition Temperature A. Molecular Motion and Glass Transition B. Theories of Glass Transition and Measurement of the Glass Transition Temperature 1. Kinetic Theory 2. Equilibrium Theory 3. Free Volume Theory C. Factors Affecting Glass Transition Temperature 1. Chain Flexibility 2. Geometric Factors 3. Interchain Attractive Forces 4. Copolymerization 5. Molecular Weight 6. Cross-Linking and Branching 7. Crystallinity 8. Plasticization III. The Crystalline Melting Point A. Factors Affecting the Crystalline Melting Point, TM 1. Intermolecular Bonding. 2. Effect of Structure Copyright 2000 by CRC Press LLC  3. Chain Flexibility 4. Copolymerization IV. Problems References Chapter Five Polymer Modification I. Introduction II. Copolymerization A. Styrene-Butadiene Copolymers 1. Styrene-Butadiene Rubber (SBR) (Random Copolymer) 2. Styrene-Butadiene Block Polymers B. Ethylene Copolymers C. Acrylonitrile-Butadiene-Styrene Copolymers (ABS) D. Condensation Polymers 1. Acetal Copolymer 2. Epoxies 3. Urea-Formaldehyde (UF) Resins III. Postpolymerization Reactions A. Reactions of Polysaccharides 1. Cellulose Derivations 2. Starch and Dextrins B. Cross-Linking 1. Unsaturated Polyesters 2. Vulcanization D. Block and Graft Copolymer Formation 1. Block Copolymerization 2. Graft Copolymerization E. Surface Modification IV. Functional Polymers A. Polyurethanes B. Polymer-Bound Stabilizers 1. Antioxidants 2. Flame Retardants 3. Ultraviolet Stabilizers C. Polymers in Drug Administration 1. Controlled Drug Release, Degradable Polymers 2. Site-Directed (Targeted) Drug Delivery V. Problems References PART II: POLYMER PREPARATION AND PROCESSING METHODS Chapter Six Condensation (Step-Reaction) Polymerization I. Introduction II. Mechanism of Condensation Polymerization III. Kinetics of Condensation Polymerization IV. Stoichiometry in Linear Systems V. Molecular Weight Control VI. Molecular Weight Distribution in Linear Condensation Systems VII. Molecular Weight Averages VIII. Ring Formation vs. Chain Polymerization IX. Three-Dimensional Network Step-Reaction Polymers Copyright 2000 by CRC Press LLC  X. Prediction of the Gel Point XI. Morphology of Cross-Linked Polymers XII. Problems References Chapter Seven Chain-Reaction (Addition) Polymerization I. Introduction II. Vinyl Monomers III. Mechanism of Chain Polymerization A. Initiation 1. Generation of Free Radicals B. Propagation C. Termination D. Chain Transfer IV. Steady-State Kinetics of Free-Radical Polymerization A. Initiation B. Propagation C. Termination V. Autoacceleration (Trommsdorff Effect) VI. Kinetic Chain Length VII. Chain-Transfer Reactions A. Transfer to Undiluted Monomer B. Transfer to Solvent VIII. Temperature Dependence of Degree of Polymerization IX. Ionic and Coordination Chain Polymerization A. Nonradical Chain Polymerization B. Cationic Polymerization 1. Mechanism 2. Kinetics C. Anionic Polymerization 1. Mechanism 2. Kinetics D. Living Polymers E. Coordination Polymerization 1. Mechanisms X. Problems References Chapter Eight Copolymerization I. Introduction II. The Copolymer Equation III. Types of Copolymerization A. Ideal Copolymerization (r1r2 = 1) B. Alternating Copolymerization (r1 = r2 = 0) C. Block Copolymerization (r1 > 1, r2 > 1 IV. Polymer Composition Variation with Feed Conversion V. Chemistry of Copolymerization A. Monomer Reactivity B. Radical Reactivity C. Steric Effects D. Alternation-Polar Effects VI. The Q-e Scheme Copyright 2000 by CRC Press LLC  VII. Problems References Chapter Nine Polymer Additives and Reinforcements I. Introduction II. Plasticizers III. Fillers and Reinforcements (Composites) IV. Alloys and Blends V. Antioxidants and Thermal and UV Stabilizers A. Polymer Stability 1. Nonchain-Scission Reactions 2. Chain-Scission Reactions 3. Oxidative Degradation 4. Hydrolysis and Chemical Degradation B. Polymer Stabilizers VI. Flame Retardants VII. Colorants VIII. Antistatic Agents (Antistats). IX. Problems References Chapter Ten Polymer Reaction Engineering I. Introduction II. Polymerization Processes A. Homogeneous Systems 1. Bulk (Mass) Polymerization B. Solution Polymerization C. Heterogeneous Polymerization 1. Suspension Polymerization 2. Emulsion Polymerization 3. Precipitation Polymerization 4. Interfacial and Solution Polycondensations III. Polymerization Reactors A. Batch Reactors B. Tubular (Plug Flow) Reactor C. Continuous Stirred Tank Reactor (CSTR) IV. Problems References Chapter Eleven Unit Operations in Polymer Processing I. Introduction II. Extrusion A. The Extruder B. Extrusion Processes. III. Injection Molding A. The Injection Unit B. The Plasticizing Screw C. The Heating Cylinder D. The Clamp Unit E. Auxiliary Systems F. The Injection Mold Copyright 2000 by CRC Press LLC  IV. Blow Molding A. Process Description B. Extrusion Blow Molding C. Injection Blow Molding V. Rotational Molding A. Process Description B. Process Variables VI. Thermoforming A. Process Description 1. Vacuum Forming 2. Mechanical Forming 3. Air Blowing Process B. Process Variables VII. Compression and Transfer Molding A. Compression Molding B. Transfer Molding VIII. Casting A. Process Description B. Casting Processes 1. Casting of Acrylics 2. Casting of Nylon IX. Problems References PART III: PROPERTIES AND APPLICATIONS Chapter Twelve Solution Properties of Polymers I. Introduction II. Solubility Parameter (Cohesive Energy Density) III. Conformations of Polymer Chains on Solution A. End-to-End Dimensions B. The Freely Jointed Chain C. Real Polymer Chains 1. Fixed Bond Angle (Freely Rotating) 2. Fixed Bond Angles (Restricted Rotation) 3. Long-Range Interactions IV. Thermodynamics of Polymer Solutions A. Ideal Solution B. Liquid Lattice Theory (Flory-Huggins Theory) 1. Entropy of Mixing 2. Heat and Free Energy of Mixing C. Dilute Polymer Solutions (Flory–Krigbaum Theory) D. Osmotic Pressure of Polymer Solutions V. Solution Viscosity A. Newton’s Law of Viscosity B. Parameters for Characterizing Polymer Solution Viscosity C. Molecular Size and Intrinsic Viscosity D. Molecular Weight from Intrinsic Viscosity VI. Problems References Copyright 2000 by CRC Press LLC  Chapter Thirteen Mechanical Properties of Polymers I. Introduction II. Mechanical Tests A. Stress–Strain Experiments B. Creep Experiments C. Stress Relaxation Experiments D. Dynamic Mechanical Experiments E. Impact Experiments III. Stress–Strain Behavior of Polymers A. Elastic Stress–Strain Relations IV. Deformation of Solid Polymers V. Compression vs. Tensile Tests VI. Effects of Structural and Environmental Factors on Mechanical Properties A. Effect of Molecular Weight B. Effect of Cross-Linking C. Effect of Crystallinity D. Effect of Copolymerization E. Effect of Plasticizers F. Effect of Polarity G. Steric Factors H. Effect of Temperature I. Effect of Strain Rate J. Effect of Pressure VII. Polymer Fracture Behaviour A. Brittle Fracture B. Linear Elastic Fracture Mechanics (LEFM) VIII. Problems References Chapter Fourteen Polymer Viscoelasticity I. Introduction II. Simple Rheological Responses A. The Ideal Elastic Response B. Pure Viscous Flow C. Rubberlike Elastic III. Viscoelasticity IV. Mechanical Models for Linear Viscoelastic Response A. Maxwell Model 1. Creep Experiment 2. Stress Relaxation Experiment 3. Dynamic Experiment B. The Voight Element 1. Creep Experiment 2. Stress Relaxation Experiment 3. Dynamic Experiment C. The Four-Parameter Model V. Material Response Time — The Deborah Number VI. Relaxation and Retardation Spectra A. Maxwell-Weichert Model (Relaxation) B. Voight-Kelvin (Creep) Model VII. Superposition Principles A. Boltzmann Superposition Principle B. Time-Temperature Superposition Principle Copyright 2000 by CRC Press LLC  IX. Problems References Chapter Fifteen Polymer Properties and Applications I. Introduction II. The Structure of the Polymer Industry A. Polymer Materials Manufacturers B. Manufacturers of Chemicals, Additives, and Modifiers C. Compounding/Formulating D. The Processor E. The Fabricator F. The Finisher III. Raw Materials for the Polymer Industry. IV. Polymer Properties and Applications.. A. Polyethylene B. Polypropylene (PP) C. Polystyrene D. Poly(Vinyl Chloride) (PVC) V. Other Vinyl Polymers. A. Poly(Vinyl Acetate) PVAC) B. Poly(Vinyl Alcohol) (PVAL) VI. Acrylics A. Poly(Methyl Methacrylate) (PMMA) B. Polyacrylates C. Polyacrylonitrile (PAN)—Acrylic Fibers VII. Engineering Polymers. A. Acrylonitrile-Butadiene-Styrene (ABS) B. Polyacetal (Polyoxymethylene — POM C. Polyamides (Nylons) D. Polycarbonate (PC) E. Poly(Phenylene Oxide (PPO) F. Poly(Phenylene Sulfide) (PPS) G. Polysulfones H. Polyimides I. Engineering Polyesters J. Fluoropolymers K. Ionomers VIII. Elastomers A. Diene-Based Elastomers 1. Polybutadiene (Butadiene Rubber, BR) 2. Syrene-Butadiene Rubber (SBR) 3. Acrylonitrile-Butadiene Rubber (Nitrile Rubber, NBR) 4. Polyisoprene 5. Polychloroprene (Neoprene) 6. Butyl Rubber B. Ethylene-Propylene Rubbers C. Polyurethanes D. Silicone Elastomers E. Thermoplastic Elastomers (TPE) 1. Styrene Block Copolymers (Styrenics) 2. Thermoplastic Polyurethane Elastomers (TPUs) 3. Polyolefin Blends 4. Thermoplastic Copolyesters (COPE) 5. Thermoplastic Polyamides Copyright 2000 by CRC Press LLC  IX. Thermosets A. Phenolic Resins B. Amino Resins C. Epoxy Resins D. Network Polyester Resins X. Problems References Appendix I Polymer Nomenclature Appendix II Answers to Selected Problems Appendix III Conversion Factors Solutions to Problems Copyright 2000 by CRC Press LLC  Chapter 1 Introduction I. HISTORICAL DEVELOPMENT Before we go into details of the chemistry of polymers it is appropriate to briefly outline a few landmarks in the historical development of what we now know as polymers. Polymers have been with us from the beginning of time; they form the very basis (building blocks) of life. Animals, plants — all classes of living organisms — are composed of polymers. However, it was not until the middle of the 20th century that we began to understand the true nature of polymers. This understanding came with the development of plastics, which are true man-made materials that are the ultimate tribute to man’s creativity and ingenuity. As we shall see in subsequent discussions, the use of polymeric materials has permeated every facet of our lives. It is hard to visualize today’s world with all its luxury and comfort without man-made polymeric materials. The plastics industry is recognized as having its beginnings in 1868 with the synthesis of cellulose nitrate. It all started with the shortage of ivory from which billiard balls were made. The manufacturer of these balls, seeking another production method, sponsored a competition. John Wesley Hyatt (in the U.S.) mixed pyroxin made from cotton (a natural polymer) and nitric acid with camphor. The result was cellulose nitrate, which he called celluloid. It is on record, however, that Alexander Parkes, seeking a better insulating material for the electrical industry, had in fact discovered that camphor was an efficient plasticizer for cellulose nitrate in 1862. Hyatt, whose independent discovery of celluloid came later, was the first to take out patents for this discovery. Cellulose nitrate is derived from cellulose, a natural polymer. The first truly man-made plastic came 41 years later (in 1909) when Dr. Leo Hendrick Baekeland developed phenol–formaldehyde plastics (phenolics), the source of such diverse materials as electric iron and cookware handles, grinding wheels, and electrical plugs. Other polymers — cellulose acetate (toothbrushes, combs, cutlery handles, eyeglass frames); urea–formaldehyde (buttons, electrical accessories); poly(vinyl chloride) (flooring, upholstery, wire and cable insulation, shower curtains); and nylon (toothbrush bristles, stockings, surgical sutures) — followed in the 1920s. Table 1.1 gives a list of some plastics, their year of introduction, and some of their applications. It is obvious that the pace of development of plastics, which was painfully slow up to the 1920s, picked up considerable momentum in the 1930s and the 1940s. The first generation of man-made polymers was the result of empirical activities; the main focus was on chemical composition with virtually no attention paid to structure. However, during the first half of the 20th century, extensive organic and physical developments led to the first understanding of the structural concept of polymers — long chains or a network of covalently bonded molecules. In this regard the classic work of the German chemist Hermann Staudinger on polyoxymethylene and rubber and of the American chemists W. T. Carothers on nylon stand out clearly. Staudinger first proposed the theory that polymers were composed of giant molecules, and he coined the word macromolecule to describe them. Carothers discovered nylon, and his funda- mental research (through which nylon was actually discovered) contributed considerably to the elucida- tion of the nature of polymers. His classification of polymers as condensation or addition polymers persists today. Following a better understanding of the nature of polymers, there was a phenomenal growth in the numbers of polymeric products that achieved commercial success in the period between 1925 and 1950. In the 1930s, acrylic resins (signs and glazing); polystyrene (toys, packaging and housewares industries); and melamine resins (dishware, kitchen countertops, paints) were introduced. The search for materials to aid in the defense effort during World War II resulted in a profound impetus for research into new plastics. Polyethylene, now one of the most important plastics in the world, was developed because of the wartime need for better-quality insulating materials for such applications as radar cable. Thermosetting polyester resins (now used for boatbuilding) were developed for military use. The terpolymer acrylonitrile-butadiene-styrene (ABS), (telephone handsets, luggage, 0-8493-????-?/97/$0.00+$.50 © 1997 by CRC Press LLC Copyright 2000 by CRC Press LLC  2 POLYMER SCIENCE AND TECHNOLOGY Table 1.1 Introduction of Plastics Materials Date Material Typical Use 1868 Cellulose nitrate Eyeglass frames 1909 Phenol–formaldehyde Telephone handsets, knobs, handles 1919 Casein Knitting needles 1926 Alkyds Electrical insulators 1927 Cellulose acetate Toothbrushes, packaging 1927 Poly(vinyl chloride) Raincoats, flooring 1929 Urea–formaldehyde Lighting fixtures, electrical switches 1935 Ethyl cellulose Flashlight cases 1936 Polyacrylonitrile Brush backs, displays 1936 Poly(vinyl acetate) Flashbulb lining, adhesives 1938 Cellulose acetate butyrate Irrigation pipe 1938 Polystyrene Kitchenwares, toys 1938 Nylon (polyamide) Gears, fibers, films 1938 Poly(vinyl acetal) Safety glass interlayer 1939 Poly(vinylidene chloride) Auto seat covers, films, paper, coatings 1939 Melamine–formaldehyde Tableware 1942 Polyester (cross-linkable) Boat hulls 1942 Polyethylene (low density) Squeezable bottles 1943 Fluoropolymers Industrial gaskets, slip coatings 1943 Silicone Rubber goods 1945 Cellulose propionate Automatic pens and pencils 1947 Epoxies Tools and jigs 1948 Acrylonitrile-butadiene-styrene copolymer Luggage, radio and television cabinets 1949 Allylic Electrical connectors 1954 Polyurethane Foam cushions 1956 Acetal resin Automotive parts 1957 Polypropylene Safety helmets, carpet fiber 1957 Polycarbonate Appliance parts 1959 Chlorinated polyether Valves and fittings 1962 Phenoxy resin Adhesives, coatings 1962 Polyallomer Typewriter cases 1964 Ionomer resins Skin packages, moldings 1964 Polyphenylene oxide Battery cases, high temperature moldings 1964 Polyimide Bearings, high temperature films and wire coatings 1964 Ethylene–vinyl acetate Heavy gauge flexible sheeting 1965 Polybutene Films 1965 Polysulfone Electrical/electronic parts 1970 Thermoplastic polyester Electrical/electronic parts 1971 Hydroxy acrylates Contact lenses 1973 Polybutylene Piping 1974 Aromatic polyamides High-strength tire cord 1975 Nitrile barrier resins Containers safety helmets, etc.) owes its origins to research work emanating from the wartime crash program on large-scale production of synthetic rubber. The years following World War II (1950s) witnessed great strides in the growth of established plastics and the development of new ones. The Nobel-prize-winning development of stereo-specific catalysts by Professors Karl Ziegler of Germany and Giulio Natta of Italy led to the ability of polymer chemists to “order” the molecular structure of polymers. As a consequence, a measure of control over polymer properties now exists; polymers can be tailor-made for specific purposes. The 1950s also saw the development of two families of plastics — acetal and polycarbonates. Together with nylon, phenoxy, polyimide, poly(phenylene oxide), and polysulfone they belong to the group of plastics known as the engineering thermoplastics. They have outstanding impact strength and thermal and dimensional stability — properties that place them in direct competition with more conventional materials like metals. Copyright 2000 by CRC Press LLC  INTRODUCTION 3 The 1960s and 1970s witnessed the introduction of new plastics: thermoplastic polyesters (exterior automotive parts, bottles); high-barrier nitrile resins; and the so-called high-temperature plastics, includ- ing such materials as polyphenylene sulfide, polyether sulfone, etc. The high-temperature plastics were initially developed to meet the demands of the aerospace and aircraft industries. Today, however, they have moved into commercial areas that require their ability to operate continuously at high temperatures. In recent years, as a result of better understanding of polymer structure–property relationships, intro- duction of new polymerization techniques, and availability of new and low-cost monomers, the concept of a truly tailor-made polymer has become a reality. Today, it is possible to create polymers from different elements with almost any quality desired in an end product. Some polymers are similar to existing conventional materials but with greater economic values, some represent significant improvements over existing materials, and some can only be described as unique materials with characteristics unlike any previously known to man. Polymer materials can be produced in the form of solid plastics, fibers, elastomers, or foams. They may be hard or soft or may be films, coatings, or adhesives. They can be made porous or nonporous or can melt with heat or set with heat. The possibilities are almost endless and their applications fascinating. For example, ablation is the word customarily used by the astronomers and astrophysicists to describe the erosion and disintegration of meteors entering the atmosphere. In this sense, long-range missiles and space vehicles reentering the atmosphere may be considered man-made meteors. Although plastic materials are generally thermally unstable, ablation of some organic polymers occurs at extremely high temperatures. Consequently, selected plastics are used to shield reentry vehicles from the severe heat generated by air friction and to protect rocket motor parts from hot exhaust gases, based on the concept known as ablation plastics. Also, there is a “plastic armor” that can stop a bullet, even shell fragments. (These are known to be compulsory attire for top government and company officials in politically troubled countries.) In addition, there are flexible plastics films that are used to wrap your favorite bread, while others are sufficiently rigid and rugged to serve as supporting members in a building. In the years ahead, polymers will continue to grow. The growth, from all indications, will be not only from the development of new polymers, but also from the chemical and physical modification of existing ones. Besides, improved fabrication techniques will result in low-cost products. Today the challenges of recycling posed by environmental problems have led to further developments involving alloying and blending of plastics to produce a diversity of usable materials from what have hitherto been considered wastes. II. BASIC CONCEPTS AND DEFINITIONS The word polymer is derived from classical Greek poly meaning “many” and meres meaning “parts.” Thus a polymer is a large molecule (macromolecule) built up by the repetition of small chemical units. To illustrate this, Equation 1.1 shows the formation of the polymer polystyrene. n CH2 CH CH2 CH (1.1) n styrene (monomer) polystyrene (polymer) (1) (2) The styrene molecule (1) contains a double bond. Chemists have devised methods of opening this double bond so that literally thousands of styrene molecules become linked together. The resulting structure, enclosed in square brackets, is the polymer polystyrene (2). Styrene itself is referred to as a monomer, which is defined as any molecule that can be converted to a polymer by combining with other molecules of the same or different type. The unit in square brackets is called the repeating unit. Notice that the structure of the repeating unit is not exactly the same as that of the monomer even though both possess identical atoms occupying similar relative positions. The conversion of the monomer to the polymer involves a rearrangement of electrons. The residue from the monomer employed in the preparation of a Copyright 2000 by CRC Press LLC  4 POLYMER SCIENCE AND TECHNOLOGY polymer is referred to as the structural unit. In the case of polystyrene, the polymer is derived from a single monomer (styrene) and, consequently, the structural unit of the polystyrene chain is the same as its repeating unit. Other examples of polymers of this type are polyethylene, polyacrylonitrile, and polypro- pylene. However, some polymers are derived from the mutual reaction of two or more monomers that are chemically similar but not identical. For example, poly(hexamethylene adipamide) or nylon 6,6 (5) is made from the reaction of hexamethylenediamine (3) and adipic acid (4) (Equation 1.2). H H O O H2N (CH2)6 NH2 + HOOC (CH2)4 COOH H N (CH2)6 N C (CH2)4 C OH (1.2) n hexamethylenediamine adipic acid poly(hexamethylene adipamide) (3) (4) (5) H H | | The repeating unit in this case consists of two structural units: – N–(CH2)6 – N–, the residue from hexam- O O \ \ ethylenediamine; and – C–(CH 2) 4– C–, the residue from adipic acid. Other polymers that have repeating units with more than one structural unit include poly(ethyleneterephthalate) and proteins. As we shall see later, the constitution of a polymer is usually described in terms of its structural units. The subscript designation, n, in Equations 1.1 and 1.2 indicates the number of repeating units strung together in the polymer chain (molecule). This is known as the degree of polymerization (DP). It specifies the length of the polymer molecule. Polymerization occurs by the sequential reactions of monomers, which means that a successive series of reactions occurs as the repeating units are linked together. This can proceed by the reaction of monomers to form a dimer, which in turn reacts with another monomer to form a trimer and so on. Reaction may also be between dimers, trimers, or any molecular species within the reaction mixture to form a progressively larger molecule. In either case, a series of linkages is built between the repeating units, and the resulting polymer molecule is often called a polymer chain, a description which emphasizes its physical similarity to the links in a chain. Low-molecular-weight polymerization products such as dimers, trimers, tetramers, etc., are referred to as oligomers. They generally possess undesirable thermal and mechanical properties. A high degree of polymerization is normally required for a material to develop useful properties and before it can be appropriately described as a polymer. Polystyrene, with a degree of polymerization of 7, is a viscous liquid (not of much use), whereas commercial grade polystyrene is a solid and the DP is typically in excess of 1000. It must be emphasized, however, that no clear demarcation has been established between the sizes of oligomers and polymers. The degree of polymerization represents one way of quantifying the molecular length or size of a polymer. This can also be done by use of the term molecular weight (MW). By definition, MW(Polymer) = DP × MW(Repeat Unit). To illustrate this let us go back to polystyrene (2). There are eight carbon atoms and eight hydrogen atoms in the repeating unit. Thus, the molecular weight of the repeating unit is 104 (8 × 12 + 1 × 8). If, as we stated above, we are considering commercial grade polystyrene, we will be dealing with a DP of 1000. Consequently, the molecular weight of this type of polystyrene is 104,000. As we shall see later, molecular weight has a profound effect on the properties of a polymer. Example 1.1: What is the molecular weight of polypropylene (PP), with a degree of polymerization of 3 × 104? Solution: Structure of the repeating unit for PP CH2 CH (Str. 1) CH3 Molecular weight of repeat unit = (3 × 12 + 6 × 1) = 42 Molecular weight of polypropylene = 3 × 104 × 42 = 1.26 × 106 Copyright 2000 by CRC Press LLC  INTRODUCTION 5 Figure 1.1 Molecular weight distribution curve. So far, we have been discussing a single polymer molecule. However, a given polymer sample (like a piece of polystyrene from your kitchenware) is actually composed of millions of polymer molecules. For almost all synthetic polymers irrespective of the method of polymerization (formation), the length of a polymer chain is determined by purely random events. Consequently, any given polymeric sample contains a mixture of molecules having different chain lengths (except for some biological polymers like proteins, which have a single, well-defined molecular weight [monodisperse]). This means that a distribution of molecular weight exists for synthetic polymers. A typical molecular weight distribution curve for a polymer is shown in Figure 1.1. The existence of a distribution of molecular weights in a polymer sample implies that any experimental measurement of molecular weight in the given sample gives only an average value. Two types of molecular weight averages are most commonly considered: the number-average molecular weight rep- resented by Mn, and the weight-average molecular weight Mw. The number-average molecular weight is derived from measurements that, in effect, count the number of molecules in the given sample. On the other hand, the weight-average molecular weight is based on methods in which the contribution of each molecule to the observed effect depends on its size. In addition to the information on the size of molecules given by the molecular weights Mw and Mn, their ratio Mw /Mn is an indication of just how broad the differences in the chain lengths of the constituent polymer molecules in a given sample are. That is, this ratio is a measure of polydispersity, and conse- quently it is often referred to as the heterogeneity index. In an ideal polymer such as a protein, all the polymer molecules are of the same size (Mw = Mn or Mw /Mn = 1). This is not true for synthetic polymers – the numerical value of Mw is always greater than that of Mn. Thus as the ratio Mw /Mn increases, the molecular weight distribution is broader. Example 1.2: Nylon 11 has the following structure H O (Str. 2) N (CH2)10 C n If the number-average degree of polymerization, Xn, for nylon is 100 and Mw = 120,000, what is its polydispersity? Solution: We note that Xn and n(DP) define the same quantity for two slightly different entities. The degree of polymerization for a single molecule is n. But a polymer mass is composed of millions of molecules, each of which has a certain degree of polymerization. Xn is the average of these. Thus, Copyright 2000 by CRC Press LLC  6 POLYMER SCIENCE AND TECHNOLOGY N ∑n M i r Xn = i =1 N where N = total number of molecules in the polymer mass Mr = molecular weight of repeating unit ni = DP of molecule i. Now Mn = XnMr = 100 (15 + 14 × 10 + 28) = 18,300 Mw 120, 000 Polydispersity = = = 6.56 Mn 18, 300 III. CLASSIFICATION OF POLYMERS Polymers can be classified in many different ways. The most obvious classification is based on the origin of the polymer, i.e., natural vs. synthetic. Other classifications are based on the polymer structure, polymerization mechanism, preparative techniques, or thermal behavior. A. NATURAL VS. SYNTHETIC Polymers may either be naturally occurring or purely synthetic. All the conversion processes occurring in our body (e.g., generation of energy from our food intake) are due to the presence of enzymes. Life itself may cease if there is a deficiency of these enzymes. Enzymes, nucleic acids, and proteins are polymers of biological origin. Their structures, which are normally very complex, were not understood until very recently. Starch — a staple food in most cultures — cellulose, and natural rubber, on the other hand, are examples of polymers of plant origin and have relatively simpler structures than those of enzymes or proteins. There are a large number of synthetic (man-made) polymers consisting of various families: fibers, elastomers, plastics, adhesives, etc. Each family itself has subgroups. B. POLYMER STRUCTURE 1. Linear, Branched or Cross-linked, Ladder vs. Functionality As we stated earlier, a polymer is formed when a very large number of structural units (repeating units, monomers) are made to link up by covalent bonds under appropriate conditions. Certainly even if the conditions are “right” not all simple (small) organic molecules possess the ability to form polymers. In order to understand the type of molecules that can form a polymer, let us introduce the term functionality. The functionality of a molecule is simply its interlinking capacity, or the number of sites it has available for bonding with other molecules under the specific polymerization conditions. A molecule may be classified as monofunctional, bifunctional, or polyfunctional depending on whether it has one, two, or greater than two sites available for linking with other molecules. For example, the extra pair of electrons in the double bond in the styrene molecules endows it with the ability to enter into the formation of two bonds. Styrene is therefore bifunctional. The presence of two condensable groups in both hexamethyl- enediamine (–NH2) and adipic acid (–COOH) makes each of these monomers bifunctional. However, functionality as defined here differs from the conventional terminology of organic chemistry where, for example, the double bond in styrene represents a single functional group. Besides, even though the interlinking capacity of a monomer is ordinarily apparent from its structure, functionality as used in polymerization reactions is specific for a given reaction. A few examples will illustrate this. A diamine like hexamethylenediamine has a functionality of 2 in amide-forming reactions such as that shown in Equation 1.2. However, in esterification reactions a diamine has a functionality of zero. Butadiene has the following structure: CH2› CH–CH› CH2 1 2 3 4 (Str. 3) (6) Copyright 2000 by CRC Press LLC  INTRODUCTION 7 From our discussion about the polymerization of styrene, the presence of two double bonds on the structure of butadiene would be expected to prescribe a functionality of 4 for this molecule. Butadiene may indeed be tetrafunctional, but it can also have a functionality of 2 depending on the reaction conditions (Equation 1.3). CH2 CH 1,2 or n CH2 CH CH CH2 CH 3,4 1 2 3 4 CH2 n (7) (1.3) 1,4 CH2 CH CH CH2 n (8) Since there is no way of making a distinction between the 1,2 and 3,4 double bonds, the reaction of either double bond is the same. If either of these double bonds is involved in the polymerization reaction, the residual or unreacted double bond is on the structure attached to the main chain [i.e., part of the pendant group (7)]. In 1,4 polymerization, the residual double bond shifts to the 2,3 position along the main chain. In either case, the residual double bond is inert and is generally incapable of additional polymerization under the conditions leading to the formation of the polymer. In this case, butadiene has a functionality of 2. However, under appropriate reaction conditions such as high temperature or cross- linking reactions, the residual unsaturation either on the pendant group or on the backbone can undergo additional reaction. In that case, butadiene has a total functionality of 4 even though all the reactive sites may not be activated under the same conditions. Monomers containing functional groups that react under different conditions are said to possess latent functionality. Now let us consider the reaction between two monofunctional monomers such as in an esterification reaction (Equation 1.4). O R COOH + R´ OH R C O R´ (1.4) acid alcohol ester (9) (10) (11) You will observe that the reactive groups on the acid and alcohol are used up completely and that the product ester (11) is incapable of further esterification reaction. But what happens when two bifunctional molecules react? Let us use esterification once again to illustrate the principle (Equation 1.5). O HOOC R COOH + HO R´ OH HOOC R C O R´ OH (1.5) bifunctional bifunctional bifunctional (12) (13) (14) The ester (14) resulting from this reaction is itself bifunctional, being terminated on either side by groups that are capable of further reaction. In other words, this process can be repeated almost indefinitely. The same argument holds for polyfunctional molecules. It is thus obvious that the generation of a polymer through the repetition of one or a few elementary units requires that the molecule(s) must be at least bifunctional. Copyright 2000 by CRC Press LLC  8 POLYMER SCIENCE AND TECHNOLOGY Figure 1.2 Linear, branched, and cross-linked polymers. The structural units resulting from the reaction of monomers may in principle be linked together in any conceivable pattern. Bifunctional structural units can enter into two and only two linkages with other structural units. This means that the sequence of linkages between bifunctional units is necessarily linear. The resulting polymer is said to be linear. However, the reaction between polyfunctional molecules results in structural units that may be linked so as to form nonlinear structures. In some cases the side growth of each polymer chain may be terminated before the chain has a chance to link up with another chain. The resulting polymer molecules are said to be branched. In other cases, growing polymer chains become chemically linked to each other, resulting in a cross-linked system (Figure 1.2). The formation of a cross-linked polymer is exemplified by the reaction of epoxy polymers, which have been used traditionally as adhesives and coatings and, more recently, as the most common matrix in aerospace composite materials. Epoxies exist at ordinary temperatures as low-molecular-weight viscous liquids or prepolymers. The most widely used prepolymer is diglycidyl ether of bisphenol A (DGEBA), as shown below (15): O CH3 O CH2 CH CH2 O C O CH2 CH CH2 (Str. 4) CH3 diglycidyl ether of bisphenol A (DGEBA) (15) The transformation of this viscous liquid into a hard, cross-linked three-dimensional molecular network involves the reaction of the prepolymer with reagents such as amines or Lewis acids. This reaction is referred to as curing. The curing of epoxies with a primary amine such as hexamethylene- diamine involves the reaction of the amine with the epoxide. It proceeds essentially in two steps: 1. The attack of an epoxide group by the primary amine O H OH H2N R NH2 + CH2 CH H2N R N CH2 CH (1.6) 1°amine 1°amine epoxide 1°amine 2°amine (16) (17) (18) Copyright 2000 by CRC Press LLC  INTRODUCTION 9 2. The combination of the resulting secondary amine with a second epoxy group to form a branch point (19). CH OH H OH O CH2 OH H2N R N CH2 CH + CH2 CH H2N R N CH2 CH (1.7) 1°amine 2°amine epoxide branch point (19) The presence of these branch points ultimately leads to a cross-linked infusible and insoluble polymer with structures such as (20). CH OH OH CH2 OH CH CH2 N R N CH2 CH (Str. 5) CH2 CH OH (20) In this reaction, the stoichiometric ratio requires one epoxy group per amine hydrogen. Consequently, an amine such as hexamethylenediamine has a functionality of 4. Recall, however, that in the reaction of hexamethylenediamine with adipic acid, the amine has a functionality of 2. In this reaction DGEBA is bifunctional since the hydroxyl groups generated in the reaction do not participate in the reaction. But when the curing of epoxies involves the use of a Lewis acid such as BF3, the functionality of each epoxy group is 2; that is, the functionality of DGEBA is 4. Thus the curing reactions of epoxies further illustrate the point made earlier that the functionality of a given molecule is defined for a specific reaction. By employing different reactants or varying the stoichiometry of reactants, different structures can be produced and, consequently, the properties of the final polymer can also be varied. Polystyrene (2), polyethylene (21), polyacrylonitrile (22), poly(methyl methacrylate) (23), and poly(vinyl chloride) (24) are typical examples of linear polymers. CH3 CH2 CH2 CH2 CH CH2 C CH2 CH (Str. 6) n CN n C O Cl n (21) (22) O (24) CH3 n (23) O\ Substituent groups such as –CH3 , –O– C–CH3, –Cl, and –CN that are attached to the main chain of skeletal atoms are known as pendant groups. Their structure and chemical nature can confer unique properties on a polymer. For example, linear and branched polymers are usually soluble in some solvent at normal temperatures. But the presence of polar pendant groups can considerably reduce room tem- perature solubility. Since cross-linked polymers are chemically tied together and solubility essentially Copyright 2000 by CRC Press LLC  10 POLYMER SCIENCE AND TECHNOLOGY involves the separation of solute molecules by solvent molecules, cross-linked polymers do not dissolve, but can only be swelled by liquids. The presence of cross-linking confers stability on polymers. Highly cross-linked polymers are generally rigid and high-melting. Cross-links occur randomly in a cross-linked polymer. Consequently, it can be broken down into smaller molecules by random chain scission. Ladder polymers constitute a group of polymers with a regular sequence of cross-links. A ladder polymer, as the name implies, consists of two parallel linear strands of molecules with a regular sequence of cross- links. Ladder polymers have only condensed cyclic units in the chain; they are also commonly referred to as double-chain or double-strand polymers. A typical example is poly(imidazopyrrolone) (27), which is obtained by the polymerization of aromatic dianhydrides such as pyromellitic dianhydride (25) or aromatic tetracarboxylic acids with ortho-aromatic tetramines like 1,2,4,5-tetraaminobenzene (26): O O C C H2N NH2 O O + C C H2N NH2 O O (25) (26) O (Str. 7) C N N C C N N C O n (27) The molecular structure of ladder polymers is more rigid than that of conventional linear polymers. Numerous members of this family of polymers display exceptional thermal, mechanical, and electrical behavior. Their thermal stability is due to the molecular structure, which in essence requires that two bonds must be broken at a cleavage site in order to disrupt the overall integrity of the molecule; when only one bond is broken, the second holds the entire molecule together. Example 1.3: Show the polymer formed by the reaction of the following monomers. Is the resulting polymer linear or branched/cross-linked? i. OCN (CH2)x NCO + HO CH2 CH (CH2)n CH2OH (Str. 8) OH ii. CH2 CH CN + CH2 CH (Str. 9) H2N NH2 COOH iii. CH CH2 CH + HOOC CH2 CH2 CH (Str. 10) H2N NH2 COOH Copyright 2000 by CRC Press LLC  INTRODUCTION 11 O OH iv. H2N C NH2 + HO CH2 CH2OH (Str. 11) CH2OH CH CH v. O C C O + HO (CH2)n OH (Str. 12) O Solution: i. OCN (CH2)x NCO + HO CH2 CH (CH2)n CH2 OH (Str. 13) OH bifunctional polyfunctional O H H O C N (CH2)x N C O CH2 CH (CH2)nCH2 O O branched/cross-linked ii. CH2 CH + CH2 CH (Str. 14) CN polyfunctional polyfunctional CH2 CH CH2 CH CN linear Copyright 2000 by CRC Press LLC  12 POLYMER SCIENCE AND TECHNOLOGY H2N NH2 COOH iii. CH CH2 CH + HOOC CH2 CH2 CH (Str. 15) H2N NH2 COOH polyfunctional polyfunctional H H N N O CH CH2 CH C N N C CH2CH2 CH H H O C O branched/cross-linked O iv. H2N C NH2 + HOCH2 CH2OH (Str. 16) CH2OH bifunctional polyfunctional H O N C O CH2 CH2O CH2O branched/cross-linked The resulting secondary hydrogens in the urea linkages are capable of additional reaction depending on the stoichiometric proportions of reactants. This means that, in principle, the urea molecule may be polyfunctional (tetrafunctional). CH CH v. O C C O + HO (CH2)n OH (Str. 17) O bifunctional bifunctional O C CH CH C O (CH2)n O linear Even though the resulting polymer is linear, it can be cross-linked in a subsequent reaction due to the unsaturation on the main chain – for example, by using radical initiators. Copyright 2000 by CRC Press LLC  INTRODUCTION 13 Example 1.4: Explain the following observation. When phthalic acid reacts with glycerol, the reaction leads first to the formation of fairly soft soluble material, which on further heating yields a hard, insoluble, infusible material. If the same reaction is carried out with ethylene glycol instead of glycerol, the product remains soluble and fusible irrespective of the extent of reaction. Solution: O O HO CH2CHCH2 OH C C O CH2CHCH2 O HOOC COOH + OH O glycerol (Str. 18) phthalic acid + O O HO CH2CH2 OH C C O CH2CH2 O ethylene glycol Phthalic acid and ethylene glycol are both bifunctional. Consequently, only linear polymers are produced from the reaction between these monomers. On the other hand, the reaction between phthalic acid and glycerol leads initially to molecules that are either linear, branched, or both. But since glycerol is trifunctional, cross-linking ultimately takes place between these molecules leading to an insoluble and infusible material. 2. Amorphous or Crystalline Structurally, polymers in the solid state may be amorphous or crystalline. When polymers are cooled from the molten state or concentrated from the solution, molecules are often attracted to each other and tend to aggregate as closely as possible into a solid with the least possible potential energy. For some polymers, in the process of forming a solid, individual chains are folded and packed regularly in an orderly fashion. The resulting solid is a crystalline polymer with a long-range, three-dimensional, ordered arrangement. However, since the polymer chains are very long, it is impossible for the chains to fit into a perfect arrangement equivalent to that observed in low-molecular-weight materials. A measure of imperfection always exists. The degree of crystallinity, i.e., the fraction of the total polymer in the crystalline regions, may vary from a few percentage points to about 90% depending on the crystallization conditions. Examples of crystalline polymers include polyethylene (21), polyacrylonitrile (22), poly(ethyl- ene terephthalate) (28), and polytetrafluoroethylene (29). O O O CH2CH2 O C C (Str. 19) n (28) CF2 CF2 (Str. 20) n (29) Copyright 2000 by CRC Press LLC  14 POLYMER SCIENCE AND TECHNOLOGY In contrast to crystallizable polymers, amorphous polymers possess chains that are incapable of ordered arrangement. They are characterized in the solid state by a short-range order of repeating units. These polymers vitrify, forming an amorphous glassy solid in which the molecular chains are arranged at random and even entangled. Poly(methyl methacrylate) (23) and polycarbonate (30) are typical examples. CH3 O O C O C (Str. 21) CH3 n (30) From the above discussion, it is obvious that the solid states of crystalline and amorphous polymers are characterized by a long-range order of molecular chains and a short-range order of repeating units, respectively. On the other hand, the melting of either polymer marks the onset of disorder. There are, however, some polymers which deviate from this general scheme in that the structure of the ordered regions is more or less disturbed. These are known as liquid crystalline polymers. They have phases characterized by structures intermediate between the ordered crystalline structure and the disordered fluid state. Solids of liquid crystalline polymers melt to form fluids in which much of the molecular order is retained within a certain range of temperature. The ordering is sufficient to impart some solid- like properties on the fluid, but the forces of attraction between molecules are not strong enough to prevent flow. An example of a liquid crystalline polymer is polybenzamide (31). O H C N (Str. 22) n (31) Liquid crystalline polymers are important in the fabrication of lightweight, ultra-high-strength, and temperature-resistant fibers and films such as Dupont’s Kevlar and Monsanto’s X-500. The structural factors responsible for promoting the above classes of polymers will be discussed when we treat the structure of polymers. 3. Homopolymer or Copolymer Polymers may be either homopolymers or copolymers depending on the composition. Polymers com- posed of only one repeating unit in the polymer molecules are known as homopolymers. However, chemists have developed techniques to build polymer chains containing more than one repeating unit. Polymers composed of two different repeating units in the polymer molecule are defined as copolymers. An example is the copolymer (32) formed when styrene and acrylonitrile are polymerized in the same reactor. The repeating unit and the structural unit of a polymer are not necessarily the same. As indicated earlier, some polymers such as nylon 6,6 (5) and poly(ethylene terephthalate) (28) have repeating units composed of more than one structural unit. Such polymers are still considered homopolymers. Copyright 2000 by CRC Press LLC  INTRODUCTION 15 n CH2 CH + mCH2 CH CH2 CH CH2 CH (Str. 23) CN CN m n (32) The repeating units on the copolymer chain may be arranged in various degrees of order along the backbone; it is even possible for one type of backbone to have branches of another type. There are several types of copolymer systems: Random copolymer — The repeating units are arranged randomly on the chain molecule. It we represent the repeating units by A and B, then the random copolymer might have the structure shown below: AABBABABBAAABAABBA (Str. 24) Alternating copolymer — There is an ordered (alternating) arrangement of the two repeating units along the polymer chain: ABABABABABAB (Str. 25) Block copolymer — The chain consists of relatively long sequences (blocks) of each repeating unit chemically bound together: AAAAA BBBBBBBB AAAAAAAAA BBBB (Str. 26) Graft copolymer — Sequences of one monomer (repeating unit) are “grafted” onto a backbone of the another monomer type: B B B B B B AAAAAAAAAAAA AAAAAAAA (Str. 27) B B B B B B B B B B Copyright 2000 by CRC Press LLC  16 POLYMER SCIENCE AND TECHNOLOGY 4. Fibers, Plastics, or Elastomers Polymers may also be classified as fibers, plastics, or elastomers. The reason for this is related to how the atoms in a molecule (large or small) are hooked together. To form bonds, atoms employ valence electrons. Consequently, the type of bond formed depends on the electronic configuration of the atoms. Depending on the extent of electron involvement, chemical bonds may be classified as either primary or secondary. In primary valence bonding, atoms are tied together to form molecules using their valence electrons. This generally leads to strong bonds. Essentially there are three types of primary bonds: ionic, metallic, and covalent. The atoms in a polymer are mostly, although not exclusively, bonded together by covalent bonds. Secondary bonds on the other hand, do not involve valence electrons. Whereas in the formation of a molecule atoms use up all their valence bonds, in the formation of a mass, individual molecules attract each other. The forces of attraction responsible for the cohesive aggregation between individual molecules are referred to as secondary valence forces. Examples are van der Waals, hydrogen, and dipole bonds. Since secondary bonds do not involve valence electrons, they are weak. (Even between secondary bonds, there are differences in the magnitude of the bond strengths: generally hydrogen and dipole bonds are much stronger than van der Waals bonds.) Since secondary bonds are weaker than primary bonds, molecules must come together as closely as possible for secondary bonds to have maximum effect. The ability for close alignment of molecules depends on the structure of the molecules. Those molecules with regular structure can align themselves very closely for effective utilization of the secondary intermolecular bonding forces. The result is the formation of a fiber. Fibers are linear polymers with high symmetry and high intermolecular forces that result usually from the presence of polar groups. They are characterized by high modulus, high tensile strength, and moderate extensibilities (usually less than 20%). At the other end of the spectrum, there are some molecules with irregular structure, weak intermolecular attractive forces, and very flexible polymer chains. These are generally referred to as elastomers. Chain segments of elastomers can undergo high local mobility, but the gross mobility of chains is restricted, usually by the introduction of a few cross-links into the structure. In the absence of applied (tensile) stress, molecules of elastomers usually assume coiled shapes. Consequently, elastomers exhibit high extensibility (up to 1000%) from which they recover rapidly on the removal of the imposed stress. Elastomers generally have low initial modulus in tension, but when stretched they stiffen. Plastics fall between the structural extremes represented by fibers and elastomers. However, in spite of the possible differences in chemical structure, the demarcation between fibers and plastics may sometimes be blurred. Polymers such as polypropylene and pol