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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
New edition brings classic text up to date with the latest science, techniques, and applications
With its balanced presentation of polymer chemistry, physics, and engineering applications, the Third Edition of this classic text continues to instill readers with a solid understanding of the core concepts underlying polymeric materials. Both students and instructors have praised the text for its clear explanations and logical organization. It begins with molecular-level considerations and then progressively builds the reader's knowledge with discussions of bulk properties, mechanical behavior, and processing methods.
Following a brief introduction, Fundamental Principles of Polymeric Materials is divided into four parts:
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Part 1: Polymer Fundamentals
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Part 2: Polymer Synthesis
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Part 3: Polymer Properties
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Part 4: Polymer Processing and Performance
Thoroughly Updated and Revised
Readers familiar with the previous edition of this text will find that the organization and style have been updated with new material to help them grasp key concepts and discover the latest science, techniques, and applications. For example, there are new introductory sections on organic functional groups focusing on the structures found in condensation polymerizations. The text also features new techniques for polymer analysis, processing, and microencapsulation as well as emerging techniques such as atom transfer radical polymerization.
At the end of each chapter are problems—including many that are new to this edition—to test the reader's grasp of core concepts as they advance through the text. There are also references leading to the primary literature for further investigation of individual topics.
A classic in its field, this text enables students in chemistry, chemical engineering, materials science, and mechanical engineering to fully grasp and apply the fundamentals of polymeric materials, preparing them for more advanced coursework.
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Veröffentlichungsjahr: 2012
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Contents
Cover
Title Page
Copyright
Preface
Preface to the Second Edition
Acknowledgements
Chapter 1: Introduction
References
Part I: Polymer Fundamentals
Chapter 2: Types of Polymers
2.1 Reaction to Temperature
2.2 Chemistry of Synthesis
2.3 Structure
2.4 Conclusions
Reference
Chapter 3: Molecular Structure of Polymers
3.1 Types of Bonds
3.2 Bond Distances and Strengths
3.3 Bonding and Response to Temperature
3.4 Action of Solvents
3.5 Bonding and Molecular Structure
3.6 Stereoisomerism in Vinyl Polymers
3.7 Stereoisomerism in Diene Polymers
3.8 Summary
References
Chapter 4: Polymer Morphology
4.1 Amorphous and Crystalline Polymers
4.2 The Effect of Polymer Structure, Temperature, and Solvent on Crystallinity
4.3 The Effect of Crystallinity on Polymer Density
4.4 The Effect of Crystallinity on Mechanical Properties
4.5 The Effect of Crystallinity on Optical Properties
4.6 Models for the Crystalline Structure of Polymers
4.7 Extended Chain Crystals
4.8 Liquid Crystal Polymers 11
References
Chapter 5: Characterization of Molecular Weight
5.1 Introduction
5.2 Average Molecular Weights
5.3 Determination of Average Molecular Weights
5.4 Molecular Weight Distributions
5.5 Gel Permeation (Or Size-Exclusion) Chromatography (GPC, SEC) [7]
5.6 Summary
References
Chapter 6: Thermal Transitions in Polymers
6.1 Introduction
6.2 The Glass Transition
6.3 Molecular Motions in an Amorphous Polymer
6.4 Determination of Tg
6.5 Factors that Influence Tg [4]
6.6 The Effect of Copolymerization on Tg
6.7 The Thermodynamics of Melting
6.8 The Metastable Amorphous State
6.9 The Influence of Copolymerization on Thermal Properties
6.10 Effect of Additives on Thermal Properties
6.11 General Observations About Tg AND Tm
6.12 Effects of Crosslinking
6.13 Thermal Degradation of Polymers
6.14 Other Thermal Transitions
References
Chapter 7: Polymer Solubility and Solutions
7.1 Introduction
7.2 General Rules for Polymer Solubility
7.3 Typical Phase Behavior in Polymer–Solvent Systems
7.4 The Thermodynamic Basis of Polymer Solubility
7.5 The Solubility Parameter
7.6 Hansen's Three-Dimensional Solubility Parameter
7.7 The Flory–Huggins Theory
7.8 Properties of Dilute Solutions
7.9 Polymer–Polmyer-Common Solvent Systems
7.10 Polymer Solutions, Suspensions, and Emulsions
7.11 Concentrated Solutions: Plasticizers
Problems
References
Part II: Polymer Synthesis
Chapter 8: Step-Growth (Condensation) Polymerization
8.1 Introduction
8.2 Statistics of Linear Step-Growth Polymerization
8.3 Number-Average Chain Lengths
8.4 Chain Lengths on a Weight Basis
8.5 Gel Formation
8.6 Kinetics of Polycondensation
Problems
References
Chapter 9: Free-radical Addition (Chain-Growth) Polymerization
9.1 Introduction
9.2 Mechanism of Polymerization
9.3 Gelation in Addition Polymerization
9.4 Kinetics of Homogeneous Polymerization
9.5 Instantaneous Average Chain Lengths
9.6 Temperature Dependence of Rate and Chain Length
9.7 Chain Transfer and Reaction Inhibitors
9.8 Instantaneous Distributions in Free-Radical Addition Polymerization
9.9 Instantaneous Quantities
9.10 Cumulative Quantities
9.11 Relations Between Instantaneous and Cumulative Average Chain Lengths for A Batch Reactor
9.12 Emulsion Polymerization
9.13 Kinetics of Emulsion Polymerization in Stage II, Case 2
9.14 Summary
Problems
References
Chapter 10: Advanced Polymerization Methods
10.1 Introduction
10.2 Cationic Polymerization [1–4]
10.3 Anionic Polymerization [5–7]
10.4 Kinetics of Anionic Polymerization [5, 6, 9]
10.5 Group-Transfer Polymerization
10.6 Atom Transfer Radical Polymerization
10.7 Heterogeneous Stereospecific Polymerization [13--15]
10.8 Grafted Polymer Surfaces
10.9 Summary
References
Chapter 11: Copolymerization
11.1 Introduction
11.2 Mechanism
11.3 Significance of Reactivity Ratios
11.4 Variation of Composition with Conversion
11.5 Copolymerization Kinetics
11.6 Penultimate Effects and Charge-Transfer Complexes
11.7 Summary
References
Chapter 12: Polymerization Practice
12.1 Introduction
12.2 Bulk Polymerization
12.3 Gas-Phase Olefin Polymerization
12.4 Solution Polymerization
12.5 Interfacial Polycondensation
12.6 Suspension Polymerization
12.7 Emulsion Polymerization
12.8 Summary
References
Part III: Polymer Properties
Chapter 13: Rubber Elasticity
13.1 Introduction
13.2 Thermodynamics of Elasticity
13.3 Statistics of Ideal Rubber Elasticity [1,2,5]
13.4 Summary
References
Chapter 14: Introduction to Viscous Flow and the Rheological Behavior of Polymers
14.1 Introduction
14.2 Basic Definitions
14.3 Relations Between Shear Force and Shear Rate: Flow Curves
14.4 Time-Dependent Flow Behavior
14.5 Polymer Melts and Solutions
14.6 Quantitative Representation of Flow Behavior
14.7 Temperature Dependence of Flow Properties
14.8 Influence of Molecular Weight on Flow Properties
14.9 The Effects of Pressure on Viscosity
14.10 Viscous Energy Dissipation
14.11 Poiseuille Flow
14.12 Turbulent Flow
14.13 Drag Reduction
14.14 Summary
Problems
References
Chapter 15: Linear Viscoelasticity
15.1 Introduction
15.2 Mechanical Models for Linear Viscoelastic Response
15.3 The Four-Parameter Model and Molecular Response
15.4 Viscous or Elastic Response? The Deborah Number [1]
15.5 Quantitative Approaches to Model Viscoelasticity [2–5]
15.6 The Boltzmann Superposition Principle
15.7 Dynamic Mechanical Testing
15.8 Summary
Problems
References
Chapter 16: Polymer Mechanical Properties
16.1 Introduction
16.2 Mechanical Properties of Polymers
16.3 Axial Tensiometers
16.4 Viscosity Measurement
16.5 Dynamic Mechanical Analysis: Techniques
16.6 Time–Temperature Superposition
16.7 Summary
Problems
References
Part IV: Polymer Processing and Performance
Chapter 17: Processing
17.1 Introduction
17.2 Molding
17.3 Extrusion [14–17]
17.4 Blow Molding [18]
17.5 Rotational, Fluidized-Bed, and Slush Molding
17.6 Calendering
17.7 Sheet Forming (Thermoforming) [19, 20]
17.8 Stamping
17.9 Solution Casting
17.10 Casting
17.11 Reinforced Thermoset Molding
17.12 Fiber Spinning
17.13 Compounding
17.14 Lithography
17.15 Three-Dimensional (Rapid) Prototyping
17.16 Summary
Problems
References
Chapter 18: Polymer Applications: Plastics and Plastic Additives
18.1 Introduction
18.2 Plastics
18.3 Mechanical Properties of Plastics
18.4 Contents of Plastic Compounds
18.5 Sheet Molding Compound for Plastics
18.6 Plastics Recycling
References
Chapter 19: Polymer Applications: Rubbers and Thermoplastic Elastomers
19.1 Introduction
19.2 Thermoplastic Elastomers [1, 2]
19.3 Contents of Rubber Compounds
19.4 Rubber Compounding [7, 8]
References
Chapter 20: Polymer Applications: Synthetic Fibers
20.1 Synthetic Fibers
20.2 Fiber Processing
20.3 Fiber Dyeing
20.4 Other Fiber Additives and Treatments
20.5 Effects of Heat and Moisture on Polymer Fibers
Chapter 21: Polymer Applications: Surface Finishes and Coatings
21.1 Surface Finishes
21.2 Solventless Coatings
21.3 Electrodeposition [5, 6]
21.4 Microencapsulation
References
Chapter 22: Polymer Applications: Adhesives
22.1 Adhesives
References
Index
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Library of Congress Cataloging-in-Publication Data:
Brazel, Christopher S., 1970-
Fundamental principles of polymeric materials / Christopher S. Brazel, Stephen L. Rosen. – 3rd ed.
pages cm
Revised edition of: Fundamental principles of polymeric materials / Stephen L. Rosen. 2nd ed. c1993.
Includes bibliographical references and index.
ISBN 978-0-470-50542-7
1. Polymers. I. Rosen, Stephen L., 1937- II. Rosen, Stephen L., 1937- Fundamental
principles of polymeric materials. III. Title.
TA455.P58R63 2012
668.9–dc23
2011052328
Preface
This work has been edited and organized to provide a solid understanding of the main concepts of polymeric materials at an introductory level, suitable for undergraduate and beginning graduate students in disciplines ranging from chemistry and chemical engineering to materials science, polymer engineering, and mechanical engineering. The second edition of the textbook was organized in a way that flowed naturally from molecular-level considerations to bulk properties, mechanical behavior, and processing methods. I have kept that organization intact with this third edition. I have used this book in teaching a polymer materials engineering course over the past several years, and find that enough information is presented without overwhelming students in detail (i.e., for more detailed courses beyond the introductory polymer class).
One of the big challenges in updating a textbook is to include some of the newer materials, methods, and issues surrounding polymer science while editing and refining the original material so that the end product remains fairly streamlined and provides a balance between describing theories and methodologies while treating each subject with an appropriate weighting. (Of course, instructors are certainly invited to pick and choose topics for their classes, and add material to that covered in the text, but I hope that this provides a good, solid read for students learning the material for the first time without a significant need to supplement the book on the instructor's behalf and without providing so much information that significant portions of the book must be passed over in a one-semester course.)
The text should be suitable for advanced undergraduates and beginning graduate students in disciplines ranging from chemical engineering and chemistry to materials science and mechanical engineering. I have taught mixed classes with just this background using the second edition, and usually found that some introductory information (such as organic chemical structures) was needed—both for students who had not been formally trained in organic chemistry and for those who needed a refresher. This edition now includes a short section in Chapters 1 and 02 on organic functional groups, with an emphasis on some of the structures found in condensation polymerizations. Several instances of natural polymers are included in structures and examples, including starches and polypeptides, to make the important connection that may building blocks of biology are also polymers.
Some reorganization and combination was done in the third edition, with Chapters 3 and 04 from the previous edition combined, and some of the detailed information on polymer rheology and transport was shortened so that students could be introduced to the material without being overwhelmed. Only small sections were removed, and at many instances, new materials were added, such as the addition of techniques for polymer analysis, processing techniques (including three-dimensional prototyping), and the inclusion of microencapsulation with the coatings section. Updates to advanced polymerization techniques includes some of the emerging techniques to make well-defined polymers, such as atom-transfer radical polymerization, although these methods are treated in a rather brief sense, so that students can understand the basics of the technique improvements and what advantages are achieved compared to other techniques. (In most cases, references are given for those seeking more detail.)
Some of the things that I liked best about this book for teaching an introductory polymers course have been retained. These areas include the description of processes to formulate different products, along with sketches of the processes, the arrangement of the book in going from molecular to macromolecular to physical structures, and the general tone of the book that attempts to connect with the reader through examples that may be familiar to them.
New homework problems have been introduced throughout, primarily those that I have found useful in teaching.
Christopher S. Brazel
Tuscaloosa, AL
August 2011
Preface to the Second Edition
This work was written to provide an appreciation of those fundamental principles of polymer science and engineering that are currently of practical relevance. I hope the reader will obtain both a broad, unified introduction to the subject matter that will be of immediate practical value and a foundation for more advanced study.
A decade has passed since the publication of the first Wiley edition of this book. New developments in the polymer area during that decade justify an update. Having used the book in class during the period, I've thought of better ways of explaining some of the material, and these have been incorporated in this edition.
But the biggest change with this edition is the addition of end-of-chapter problems at the suggestion of some academic colleagues. This should make the book more suitable as an academic text. Most of these problems are old homework problems or exam questions. I don't know what I'm going to do for new exam questions, but I'll think of something. Any suggestions for additional problems will be gratefully accepted.
The first Wiley edition of this book in 1982 was preceded by a little paperback intended primarily as a self-study guide for practicing engineers and scientists. I sincerely hope that by adding material aimed at an academic audience I have not made the book less useful to that original audience. To this end, I have retained the worked-out problems in the chapters and added some new ones. I have tried to emphasize a qualitative understanding of the underlying principles before tackling the mathematical details, so that the former may be appreciated independently of the latter (I don't recommend trying it the other way around, however), and I have tried to include practical illustrations of the material whenever possible.
In this edition, previous material has been generally updated. In view of commercial developments over the decade, the discussion of extended-chain crystals has been increased and a section on liquid-crystal polymers has been added. The discussion of phase behavior in polymer-solvent systems has been expanded and the Flory–Huggins theory is introduced. All kinetic expressions are now written in terms of conversion (rather than monomer concentration) for greater generality and ease of application. Also, in deference to the ready availability of numerical-solution software, kinetic expressions now incorporate the possibility of a variable-volume reaction mass, and the effects of variable volume are illustrated in several examples. A section on group-transfer polymerization has been added and a quantitative treatment of Ziegler–Natta polymerization has been attempted for the first time, including three new worked-out examples. Processes based on these catalysts are presented in greater detail. The “modified Cross” model, giving viscosity as a function of both shear rate and temperature, is introduced and its utility is illustrated. A section on scaleup calculations for the laminar flow of non-Newtonian fluids has been added, including two worked-out examples. The discussion of three-dimensional stress and strain has been expanded and includes two new worked-out examples. Tobolsky's “Procedure X” for extracting discrete relaxation times and moduli from data is introduced.
Obviously, the choice of material to be covered involves subjective judgment on the part of the author. This, together with space limitations and the rapid expansion of knowledge in the field, has resulted in the omission or shallow treatment of many interesting subjects. I apologize to friends and colleagues who have suggested incorporation of their work but don't find it here. Generally, it's fine work, but too specialized for a book of this nature. The end-of-chapter references are chosen to aid the reader who wishes to pursue a subject in greater detail.
I have used the previous edition to introduce the macromolecular gospel to a variety of audiences. Parts 1, 2 and most of 3 were covered in a one-semester course with chemistry and chemical engineering seniors and graduate students at Carnegie-Mellon. At Toledo, Parts 1 and 2 were covered in a one-quarter course with chemists and chemical engineers. A second quarter covered Part 3 with additional quantitative material on processing added. The audience for this included chemical and mechanical engineers (we didn't mention chemical reactions). Finally, I covered Parts 1 and 3 in one quarter with a diverse audience of graduate engineers at the NASA–Lewis Research Labs.
A word to the student: To derive maximum benefit from the worked-out examples, make an honest effort to answer them before looking at the solutions. If you can't do one, you've missed some important points in the preceding material, and you ought to go back over it.
Stephen L. Rosen
Rolla, Missouri
November 1992
Acknowledgments
The most important person to acknowledge is Dr. Stephen Rosen, who penned the first and second editions of this book with a great vision for organizing the wealth of information on polymers into a textbook covering the fundamentals that provided an excellent tool for classroom learning. The guinea pigs (or students) who helped do a trial run of this edition in my polymeric materials classes in 2011 provided corrections and suggestions throughout the semester.
I greatly appreciate my departmental colleagues and university for allowing me a sabbatical from my normal professor duties to expand my research and write several papers as well as updating this book. I am also grateful to the U.S.–U.K. Fulbright Commission, which partially funded my stay in the United Kingdom during which I began writing this third edition.
Chapter 1
Introduction
Although relatively new to the scene of materials science, polymers have become ubiquitous over the past century. In fact, since the Second World War, polymeric materials represent the fastest growing segment of the U.S.' chemical industry. It has been estimated that more than a third of the chemical research dollar is spent on polymers, with a correspondingly large proportion of technical personnel working in the area. From the beginning, the study of polymers was an interdisciplinary science, with chemists, chemical engineers, mechanical engineers, and materials scientists working to understand the chemical structure and synthesis of polymers, develop methods to scale up and process polymers, and evaluate the wide range of mechanical properties existing within the realm of polymeric materials. The molecular structure of polymers is far more complex than the molecules you may have studied in a general chemistry course: just compare the molecular weights, H2O is 18, NaCl is about 58, but polymers have molecular weights from 10,000 to tens of millions (or possibly much higher for cross-linked polymers). Many of the structures you might have seen in a general cell biology course are made of polymers––proteins, polysaccharides, and DNA are all notable biological polymers. In a material science course, you may have studied crystal structures in metals to understand the mechanical behavior of different alloys (polymers can form crystals, too, but imagine the difficulty of trying to line up a huge polymer molecule into a crystal structure). Polymers are a unique class of materials having wide ranging applications.
A modern automobile contains over 300 lb (150 kg) of plastics, and this does not include paints, the rubber in tires, or the fibers in tires and upholstery. Newer aircraft incorporate increasing amounts of polymers and polymer-based composites. With the need to save fuel and therefore weight, polymers will continue to replace traditional materials in the automotive and aircraft industries. Similarly, the applications of polymers in the building construction industry (piping, resilient flooring, siding, thermal and electrical insulation, paints, decorative laminates) are already impressive and will become even more so in the future. A trip through your local supercenter will quickly convince anyone of the importance of polymers in the packaging (bottles, films, trays), clothing (even cotton is a polymer), and electronics industries. Many other examples from pharmaceutical coatings to playground equipment could be cited, but to make a long story short, the use of polymers now outstrips that of metals not just on a volume basis but also on a mass basis.
People have objected to synthetic polymers because they are not “natural.” Well, botulism is natural, but it is not particularly desirable. Seriously, if all the polyester and nylon fibers in use today were to be replaced by cotton and wool, their closest natural counterparts, calculations show that there would not be enough arable land left to feed the populace, and we would be overrun by sheep. The fact is that there simply are no practical natural substitutes for many of the synthetic polymers used in modern society.
Since most modern polymers have their origins in petroleum, it has been argued that this increased reliance on polymers constitutes an unnecessary drain on energy resources. However, the raw materials for polymers account for less than 2% of total petroleum and natural gas consumption, so even the total elimination of synthetic polymers would not contribute significantly to the conservation of hydrocarbon resources. Furthermore, when total energy costs (raw materials plus energy to manufacture and ship) are compared, the polymeric item often comes out well ahead of its traditional counterpart, for example, glass versus plastic beverage bottles. In addition, the manufacturing processes used to produce polymers often generate considerably less environmental pollution than the processes used to produce the traditional counterparts, for example, polyethylene film versus brown kraft paper for packaging.
Ironically, one of the most valuable properties of polymers, their chemical inertness, causes problems because polymers do not normally degrade in the environment. As a result, they increasingly contribute to litter and the consumption of scarce landfill space. One of the challenges in using polymers in materials is developing suitable methods for recycle or effective methods to improve the degradation of disposable items.
Environmentally degradable polymers are being developed, although this is basically a wasteful approach and we are not yet sure of the impact of the degradation products. Burning polymer waste for its fuel value makes more sense, because the polymers retain essentially the same heating value as the raw hydrocarbons from which they were made. Still, the polymers must be collected and this approach wastes the value added in manufacturing the polymers.
This ultimate solution is recycling. If waste polymers are to be recycled, they must first be collected. Unfortunately, there are literally dozens (maybe hundreds) of different polymers in the waste mix, and mixed polymers have mechanical properties similar to Cheddar cheese. Thus, for anything but the least- demanding applications (e.g., parking bumpers, flower pots), the waste mix must be separated prior to recycling. To this end, several automobile manufacturers have standardized plastics used in cars that can be easily removed, remolded, and reused in newer models. Another identifier helpful in recycling plastics is obvious if you have ever looked at the bottom of a plastic soda bottle; there are molded-in numbers on most of the large volume commodity plastics, allowing hand sorting of different materials.
Processes have been developed to separate the mixed plastics in the waste. The simplest of these is a sink–float scheme that takes advantage of density differences among various plastics. Unfortunately, many plastic items are foamed, plated, or filled (mixed with nonpolymer components), which complicates density-based separations. Other separation processes are based on solubility differences between various polymers. An intermediate approach chemically degrades the waste polymer to the starting materials from which new polymer can be made. Other efforts related to polymeric waste have focused on reducing the seemingly infinite lifetime of many plastics in the environment by developing biodegradable commodity polymers.
There are five major areas of application for polymers: (1) plastics, (2) rubbers or elastomers, (3) fibers, (4) surface finishes and protective coatings, and (5) adhesives. Despite the fact that all five applications are based on polymers, and in many cases the same polymer is used in two or more, the industries pretty much grew up separately. It was only after Dr. Harmann Staudinger [1,2] proposed the “macromolecular hypothesis” in the 1920s explaining the common molecular makeup of these materials (for which he won the 1953 Nobel Prize in chemistry in belated recognition of the importance of his work) that polymer science began to evolve from the independent technologies. Thus, a sound fundamental basis was established for continued technological advances. The history of polymer science is treated in detail elsewhere [3,4].
Economic considerations alone would be sufficient to justify the impressive scientific and technological efforts expended on polymers in the past several decades. In addition, however, this class of materials possesses many interesting and useful properties completely different from those of the more traditional engineering materials and that cannot be explained or handled in design situations by the traditional approaches. A description of three simple experiments should make this obvious.
Although such behavior is unusual in terms of the more familiar materials, it is a perfectly logical consequence of the molecular structure of polymers. This molecular structure is the key to an understanding of the science and technology of polymers and will underlie the chapters to follow.
Figure 1.1 illustrates the followings questions to be considered:
The word polymer comes from the Greek word meaning “many- membered.” Strictly speaking, it could be applied to any large molecule formed from a relatively large number of smaller units or “mers,” for example, a sodium chloride crystal, but it is most commonly (and exclusively, here) restricted to materials in which the mers are held together by covalent bonding, that is, shared electrons. For our purposes, only a few bond valences need be remembered:
Figure 1.1 The key role of molecular structure in polymer science and technology.
It is always a good idea to “count the bonds” in any structure written to make sure they conform to the above.
The basic structure of a polymer consists of a backbone and pendant (or side) groups (Figure 1.2). Atoms that are covalently linked and stretch from one end of a polymer to the other make up the polymer backbone (which is often not only carbon, but can also contain other atoms such as N, O, or Si). All other atoms are part of the side groups (H is the simplest, methyl (–CH3) or alcohol (–OH) groups are among many possibilities, making for a wide variety of structures that make up polymers). Polymers are named based on the repeat unit since that can describe a rather lengthy molecule in a succinct way.
Figure 1.2 Structure of a typical polymer polypropylene is shown here, with an all-carbon backbone and side groups of –H or –CH3. Note that fixed bond angles make the three-dimensional structure more complex than shown here.
Because carbon is so versatile and can form the backbone of a polymer while being covalently attached to two other side groups, some basic organic chemistry will help to understand the molecular structure of polymers. While organic chemistry is not a prerequisite for learning about polymers, some basic terminology related to organic functional groups will help in understanding molecular structures and polymerization reactions.
Organic chemicals can be categorized based on common functional groups (types of bonds) that are found within a structure (Figure 1.3). These functionalities are helpful in determining if a molecule might be reactive, whether it is likely to be hydrophilic (or hydrophobic), and how (or if) a molecule can participate in a polymerization reaction. Alcohols (–OH) are perhaps one of the more familiar functional groups, and they can be reacted with carboxylic acids (–COOH) to form an ester bond. As long as each molecule has at least two reactive functional groups (e.g., a dialcohol (more commonly referred to as a diol) and a dicarboxylic acid (a diacid) can react to form a continuous polymer molecule with multiple ester linkages along the polymer backbone, yielding a polyester!). Alcohols can also be found in the side groups of polymers, as in poly(vinyl alcohol), where the –OH group stays intact after the reaction.
Figure 1.3 Common organic functional groups (note: R1 and R2 stand for general organic molecules that continue the molecule away from the functional group).
Note that the hydrogens written alongside carbons are not part of the backbone of a polymer molecule. They can bond only to one other atom (the carbon), so a more accurate drawing would show the hydrogens off to the side of the backbone, just as the –OH group is in the poly(vinyl alcohol).
Some of the organic functional groups that are reactive to form polymers include alcohols, amines, carboxylic acids, and alkenes. Although alkanes (C–C bonds) are common in polymers, both in the backbone and in the side groups, they are not reactive. Other bonds commonly found in polymers include amides (for nylons), esters, carbonates, imides, and urethanes. Of this last set, even those who have had organic chemistry probably have not run across the last three, as they are more common in polymers than in traditional organic chemistry. Further reviews of organic chemistry functional groups are also available, and many others can be found online 5, 6.
While there is no simple rule about the structures found in a polymer, the most common polymers have only a few organic functional groups, that repeat over and over to make the macromolecules. Of the groups shown in Figure 1.3, alkanes, esters, urethanes, carbonates, aryls, silanes, amides, and imides are commonly found in the backbone of polymers. Alcohols, carboxylic acids, and amines are either reactive parts of monomers or can be found in polymer side groups.
Also keep in mind that when the covalently bound atoms differ in electronegativity, the electrons are not shared evenly between them. In gaseous HCl, for example, the electrons cluster around the electronegative chlorine atom, giving rise to a molecular dipole:
Electrostatic forces between such dipoles can play an important role in determining polymer properties, as can be imagined for polymers containing the electronegative oxygen, nitrogen, chlorine, or fluorine atoms (which are generally δ−). These atoms pull the electrons away from other atoms they are covalently attached to (often carbons), making the carbon atom somewhat positive (δ+).
The most important constituents of living organisms, cellulose and proteins, are naturally occurring polymers (as is DNA), but most commercially important polymers are synthetic or modified natural polymers.
Problems
References
1. Staudinger, H., Ber. Dtsch. Chem. Ges. 53, 1073 (1920).
2. Staudinger, H. and J. Fritsch, Helv. Chim. Acta 5, 778 (1922).
3. Morawetz, H., Polymers: The Origins and Growth of a Science, Wiley-Interscience, New York, 1985.
4. Stahl, G.A., CHEMTECH, August 1984, 492.
5. Richardson, P.N. and R.C. Kierstead, SPE J. 25(9), 54 (1969).
6. Leach, M.R., Organic Functional Groups, on website Chemistry Tutorials and Drills, www.chemistry-drills.com/functional-groups.php?q=simple, 2009.
Part I
Polymer Fundamentals
This part covers the fundamental building blocks, basic structures, and nomenclature for polymers. To start, we need to know a little about the molecular organization of polymers, how they are named, and some of the important techniques to characterize polymeric materials. Part I also covers some of the unique thermal, solution, and optical properties of polymers.
Some of the key points to learn from this part of the book are the following:
Chapter 2
Types of Polymers
The large number of natural and synthetic polymers has been classified in several ways: thermal behavior, route of chemical synthesis, and structural organization. These will be outlined below, and in the process many terms important in polymer science and technology will be introduced.
2.1 Reaction to Temperature
The earliest distinction between types of polymers was made long before any concrete knowledge of their molecular structure. It was a purely phenomenological distinction based on their reaction to heating and cooling.
2.1.1 Thermoplastics
It was noted that certain polymers would soften upon heating and could then be made to flow when a stress was applied. When cooled again, they would reversibly regain their solid or rubbery nature. These polymers are known as thermoplastics. By analogy, ice and solder, though not polymers, behave as thermoplastics.
Some of the most commercially important thermoplastics include polyethylene (PE), polypropylene (PP), poly(vinyl chloride) (PVC), and polystyrene (PS). PE is used in products ranging from plastic bags to detergent bottles and has the simplest possible repeat structure of any polymer, since all the pendant groups are hydrogens. PP is also found in a wide range of products, such as plastic storage containers, and competes with other polymers in making plastic bags and pipes. PVC is commonly found in materials as diverse as rigid drain pipes, shower curtains, and raincoats, while PS has been used in foam coffee cups and disposable cutlery. Because these materials are thermoplastics, they are typically made into pellets after polymerization, and then can be melted and extruded or shaped into final products.
2.1.2 Thermosets
Other polymers, although they might be heated to the point where they would soften and could be made to flow under stress once, would not do so reversibly; that is, heating caused them to undergo a “curing” reaction. Sometimes these materials emerge from the synthesis reaction in a cured state. Further heating of these thermosetting polymers ultimately leads only to degradation (as is sometimes attested to by the smell of a short-circuited electrical appliance) and not softening and flow. Again by analogy, eggs and concrete behave as thermosets. Continued heating of thermoplastics will also ultimately lead to degradation, but they will generally soften at temperatures below their degradation point.
Commercially important thermosets include epoxies, polyesters, and phenolic resins. Each of these materials starts out as (often viscous) liquids that set by curing into a final shape. Because these materials set the first time they are made, they cannot be reheated after the polymer is formed without degrading the structure.
Natural rubber is a classic example of the difference between a thermoplastic and a thermoset. Introduced to Europe by Columbus, natural rubber did not achieve commercial significance for centuries; because it was a thermoplastic, articles made of it would become soft and sticky on hot days. In 1839, Charles Goodyear discovered the curing reaction with sulfur (which he called vulcanization in honor of the Roman god of fire) that converted the polymer to a thermoset. This allowed the rubber to maintain its useful properties to much higher temperatures, which ultimately led to its great commercial importance.
2.2 Chemistry of Synthesis
Pioneering workers in the field of polymer chemistry soon observed that they could produce polymers by two familiar types of organic reactions: condensation and addition. As monomers react, new structures are created during the polymerization:
An oligomer is simply a small polymer, but it has unique properties since it has a small number of repeat units. Eventually, enough reactions take place to form a large polymer, where it is not uncommon to have thousands or tens of thousands of repeat units connected along a single polymer backbone. In the formation from monomer to polymer, the basic repeat unit remains the same, but the material changes from a monomer that is liquid (or even a gas) to oligomers that may be highly viscous liquids to solid polymers.
Several elementary organic functional groups are worth reviewing, as they play important roles in the synthesis of polymers (Figure 1.3). Because the chemical structures resulting from condensation and addition polymerizations are a bit different, the method of polymerization is one of the major distinctions used in describing polymers. Note that certain functional groups are reactive for forming polymers (common ones include alkenes, alcohols, carboxylic acids, and amines), while others are more commonly found in the resulting structures (alkanes, amides, esters, imides, and urethanes). Other functional groups may not participate in reactions, but be present as side groups or within the polymer backbone (e.g., aryl groups, and ethers).
2.2.1 Condensation Polymerization
Polymers formed from a typical organic condensation reaction, in which a small molecule (most often water) is split out, are known, logically enough, as condensation polymers. The common esterification reaction of an organic acid and an organic base (an alcohol in this case) illustrates the simple “lasso chemistry” involved:
The –OH group on the alcohol and the on the acid are known as functional groups, those parts of a molecule that participate in a reaction, while R and R′ are abbreviations that represent the remainder of the molecule––they are generic organic groups and will commonly be used in organic chemistry. Of course, the ester formed in the preceding reaction is not a polymer because we have hooked up only two small molecules, and the reaction is finished far short of anything that might be considered “many membered.”
At this point, it is useful to introduce the concept of monomer functionality. Functionality is the number of bonds a mer can form with the other mers in a reaction. In condensation polymerization, it is equal to the number of (organic) functional groups on the mer. These groups must be reactive (such as carboxylic acids, alcohols, aldehydes, or amines).
In the above example, the reactants are monofunctional (there is one alcohol group on one reactant and one carboxylic acid group on the other); thus, if there are no other functional groups present in the rest of the molecule (the R group), the reaction will form only one new bond (an ester). Consider, though, what happens if both reactants are difunctional and the reaction progresses at each end.
Here, the resulting product molecule is still difunctional (an alcohol on the left end and an acid on the right; the ester bonds, , are not reactive for polymerization). So, the left end of the molecule can react with another diacid molecule and its right end with another diol molecule. At each step, the growing molecule remains difunctional, so the reaction can continue until all alcohol and acid groups in the reaction mixture are added together (this includes the possibility that two larger molecules can react). Once enough of these molecules (hundreds to even millions of “mers”) have condensed together, a polymer is created.
2.2.1.1 Polyesters
In general, the polycondensation of x molecules of a diol with x molecules of a diacid to give a polyester molecule is written as in the reaction above.
In the polyester (many ester “mers”) molecule, the structure in brackets is the repeating unit, and this is what distinguishes one polymer from another. The linkage characterizes all polyesters. The generalized organic groups R and R′ may vary widely (with a consequent variation in the properties of the polymer), but as long as the repeating unit contains the ester linkage, the polymer is a polyester. Of course, there are other functional groups that can react, and thus there are many different types of condensation polymers (polyamides, polyimides, polyethers, etc.).
The quantity x is the degree of polymerization, the number of repeating units strung together as identical beads in the polymer chain. It is sometimes also called the chain length, but it is a pure number, not a measurable length.
The above nomenclature was introduced by Wallace Hume Carothers, who, with his group at Du Pont, invented two important polymers, neoprene (an addition polymer) and nylon, in the 1930s and was one of the founders of modern polymer science 1.
2.2.1.2 Polyamides (Nylons)
Another functional group that is capable of taking part in a condensation polymerization is the amine (–NH2) group, where one hydrogen from this group reacts with a carboxylic acid in a manner similar to the alcoholic hydrogen to form a polyamide (commonly known as nylon):
The linkage is characteristic of all nylons. This bond is also commonly referred to as a peptide bond when it connects two amino acids. Proteins are made from polypeptides, which are one type of natural polymers, and are discussed later.
2.2.1.3 Polyimides
Polyimides are actually a subset of polyamides, but forms when both carboxylic acid groups in diacid monomer react with an amine. As shown, a benzene ring with two acid groups on adjacent carbons (in the ortho position) condenses with an amine to form a single bond. The result is an imide linkage.
Compare the functionality of the diacid in this reaction versus the polyamides or polyesters. Here, the diacid acts as a monofunctional group, while when forming polyamides or polyesters, the diacid reacts separately on both ends. For the formation of a polyimide, the carboxylic acid groups must be close together. Of course, this forms only a single bond. To form a polyimide, there needs to be another diacid as part of the R1 functional group and another amine in the R2 group.
2.2.1.4 Polyurethanes
Polyurethanes are made by the reaction of a molecule with two isocyanate groups (a diisocyanate) and two alcohols (a diol). In this reaction, the hydrogen from the alcohol is transferred to the nitrogen atom of the isocyanate group, and a new bond is formed between the isocyanate carbon and the alcohol oxygen.
As observed for the polyesters, polyamides, and polyimides, polyurethanes also have a characteristic bond, –(O–C–N)–, in their backbone (with the carbon having a =O) that makes these polymers easily identifiable. Note that after the reaction to form a urethane bond, the end groups (an alcohol and an isocyanate) are still available to grow the polymer chain further.
2.2.1.5 Polycarbonates
One additional group of condensation polymers highly important in industry is polycarbonates. Polycarbonates are a special subcase of polyesters, in that one monomer has only a single carbon, resulting in the characteristic linkage:
A common reaction to produce polycarbonates involves a diol (bisphenol-A) and phosgene, which is a diacid chloride (with functionality similar to a dicarboxylic acid) (see part N in Example 2.4).
2.2.1.6 Monomers with Tri- (or Higher) Functionality
The above examples are for polymers formed from difunctional reactants. If either reactant has only one functional group, no polymer can result. However, monomers with functionalities higher than two are also used to make polymers. For example, glycerin is a trifunctional molecule (three alcohol groups) that can be used in polyesterification. Molecules with three or more functional groups tend to make network (or cross-linked) polymers, as the reaction can proceed to form a three-dimensional network instead of just a linear polymer as difunctional monomers do.
2.2.1.7 Monomers with Two Different Functional Groups
It is possible that the starting molecule for a condensation polymerization has two (or more) different functional groups and does not require the presence of two different monomers. Two examples of such monomers are amino acids and hydroxy acids:
Of course, most of us have heard of amino acids in biology classes, as these are the building blocks for polypeptides, which are given the “poly” prefix because they are polymers of amino acids. Multiple polypeptides make up the structure of proteins. An amino acid can react with another amino acid to form an amide bond (or peptide bond if it is an α amino acid) because they have both an amine functional group and a carboxylic acid functional group. Similarly, hydroxy acids condense to form polyesters. However, these reactions may not always proceed in a straightforward fashion. If the R group is large enough, it is possible that the molecule becomes configured so that it can “bite its own tail,” condensing to form a cyclic structure rather than a polymer.
This cyclic compound can then undergo a ring-scission polymerization, in which the polymer is formed without splitting out a small molecule (e.g., water), because the small molecule had been eliminated previously in the cyclization step.
Despite the lack of elimination of a small molecule in the actual polymerization step, the products can be thought of as having been formed by a direct condensation from the monomer and are usually considered condensation polymers. Also of note is that not all condensation polymerizations yield water as the by-product, sometimes carboxylic acids are formed into acid chlorides (with the functional –OH group replaced by a chlorine atom), so instead of an –OH leaving group, a –Cl group leaves, resulting in HCl as the “condensation” by-product.
Note that in this case, the characteristic nylon linkage, , is split up in the above reaction, and not immediately obvious in the repeating unit as written. If you place a second repeating unit next to the one shown, it becomes evident. This illustrates the somewhat arbitrary location of the brackets, which should not obscure the fact that the polymer is a nylon (though the structure inside the brackets is the repeating “mer”). One important difference between typical synthetic polymers and biological polypeptides is that in most synthetic polymers, the R group is the same throughout the polymer, whereas polypeptides are made up of a sequence of 20 different amino acids (as coded by DNA), each with a unique R group.
Though not formed from a monomer with two different functional groups, another important class of natural polymers is polysaccharides (literally poly sugars). The most common monomer here is glucose (C6H12O6), and when bonded together by glycosidic linkages (a condensation reaction), it forms starch, cellulose, and other polymers that are important for structural rigidity (particularly in plants) and energy storage in living organisms, as most starches can be broken down (depolymerized) easily.
2.2.2 Addition Polymerization
The second polymer formation reaction is known as addition polymerization and its products as addition (or chain or free radical) polymers. Addition polymerizations have two distinct characteristics:
Monomers of the general type undergo addition polymerization as
The double bond “opens up,” forming bonds to other monomers at each end, so a carbon–carbon double bond is difunctional according to our general definition. (Note that a monomer with two double bonds has a functionality of four; this type of monomer is often referred to as a cross-linking agent as it results in the formation of network structures.) The question of what happens at the ends of addition polymers will be deferred until we reach Chapter 9 on polymerization mechanisms. Also, note that carbon–oxygen double bonds, while common in monomers and polymers, are stable and unreactive for addition polymerization.
An important subclass of the double bond containing monomers is the vinyl monomer:
Addition polymerization is occasionally referred to as vinyl polymerization. Table 2.1 lists some commercially important vinyl monomers, with different substituents replacing X.
Table 2.1 Some Commercially Important Vinyl Monomers.
Monomer−XEthylene−HStyreneVinyl chloride−ClPropylene−CH3AcrylonitrileAcrylic acid is a vinyl monomer with a double bond (since it has only one carboxylic acid, it would not be useful in a condensation polymerization). It undergoes addition polymerization to form
Interestingly, the ester linkage in poly(lactic acid), PLA, is easily hydrolyzed, breaking down the polymer. Thus, this polymer has found applications in biodegradable plastics for products ranging from food packaging to tissue engineering scaffolds for medical applications. Poly(acrylic acid), or PAA, is not degradable, but the pendant carboxylic acid group makes this polymer extremely hydrophilic and thus it is used as a superabsorbent (particularly important in diapers).
Although most molecules with two C=C bonds (dialkenes or simply dienes) have a functionality of 4 for addition polymerization, in conjugated dienes, the two double bonds are separated by only one single C–C bond, having a functionality of 2, so they form linear polymers. A conjugated diene is a unique monomer for polymerization because (unlike molecules where the double bonds that act as cross-linking agents are further separated) the double bonds here act in the same way as a single double bond in vinyl polymerizations.
The addition polymerization of a conjugated diene results in an unsaturated polymer, or a polymer that contains double bonds, either in the backbone of the polymer or in the pendant group. Furthermore, conjugated diene polymerizations can result in several structural isomers in the polymer. If the monomer is symmetrical with regard to substituent groups on carbons one through four, it can undergo 1,2-addition and 1,4-addition. If the monomer is asymmetrical, the 3,4 addition will result in yet another structural isomer. (For symmetrical dienes, the 1,2 and 3,4 reactions result in the same structure.) This is illustrated below for the addition polymerization of isoprene (2-methyl-1,3-butadiene), an asymmetrical conjugated diene:
Polyisoprene is an important flexible rubbery material, one of the first rubbers to be used as it is found in nature (thus the alternate name “natural rubber”). It is used in belts and hoses in automobiles, in laboratory gloves, and in rubber bands. The 1,2 and 3,4 reactions are sometimes known as vinyl addition, because part of the diene monomer simply acts as an X group in a vinyl monomer.
2.3 Structure
As an appreciation for the molecular structure of polymers was gained, three major structural categories emerged. These are illustrated schematically in Figure 2.1.
Figure 2.1 Schematic representation of polymer structures.
2.3.1 Linear
If a polymer is built from strictly difunctional monomers, the result is a linear polymer chain. A scale model of a typical linear polymer molecule made from 0.5 cm diameter clothesline rope would be about 3 m long. This is not a bad analogy: the chains are long, flexible, and essentially one-dimensional structures. The term linear can be somewhat misleading, however, because the molecules do not necessarily assume a geometrically linear conformation as shown in the figure. Some of the better analogies for what these macromolecules look like is a bowl of spaghetti or tangled strands of yarn. The nomenclature for homopolymers usually simply puts “poly” before the name of the repeat unit, which is particularly simple for most addition polymers, such as in polystyrene. If the repeat unit has more than one word, parentheses are used to indicate what is repeated, such as in poly(vinyl chloride). Many condensation polymers would have fairly complex names using this rule, so most are simply referred to by their class (e.g., a polyester) or by their trade names (Dacron®, Nylon®, Lexan®, etc.).
2.3.1.1 Random Copolymers
Polymers consisting of chains that contain a single repeating unit are known as homopolymers (this includes many polymers made by addition and condensation polymerization). If, however, the chains contain a random arrangement of two separate and distinct repeating units, the polymer is known as a random or statistical copolymer, or simply copolymer. A random copolymer might be formed by the addition polymerization of a mixture of two different vinyl monomers A and B (the degree of “randomness” depends on the relative amounts and reactivities of A and B, as we shall see later) and can be represented as
and called poly(A-co-B), where the first repeating unit listed is the one present in the greater amount. For example, a random synthetic rubber copolymer of 75% butadiene and 25% styrene would be termed poly(butadiene-co-styrene). Of course, ter- (3-) and higher multipolymers are possible: nature makes a regular tetrapolymer (RNA and DNA, which are complex polymers with four bases arranged according to genetic codes) and polypeptides that are copolymers with 20 different repeat units (amino acids).
It must be emphasized that the products of condensation polymerizations that require two different monomers to provide the necessary functional groups, for example, a diacid and a diamine, are not copolymers because they contain a single repeating unit. If, however, two different diamines were used in the polymerization, there would be two distinct repeating units and this could be correctly termed a copolymer.
2.3.1.2 Block Copolymers
Under certain conditions, linear chains can be formed that contain long contiguous blocks of two (or more) repeating units combined in the chains termed a block copolymer.
These structures are called a diblock copolymer, poly(A-b-B), and a triblock copolymer, poly(A-b-B-b-A), respectively. Here, the b (meaning “block”) replaces “co” to indicate the organized structure. These structures are important for nanotechnology as several block copolymers can self-assemble into nanospherical micelles, with the A and B blocks arranged to form the core and shell (Figure 2.2). This works especially well if the A and B groups have opposite polarities, so they prefer to segregate rather than intermingle.
Figure 2.2 Block copolymers self-assembling into a micelle with core of B groups and corona of A groups.
2.3.2 Branched
If a few molecules of tri (or higher) functionality are introduced (intentionally or through side reactions) to the reaction, the resulting polymer will have a branched structure. One such example is the grafting of branches made from repeating unit “B” to a linear backbone of A repeating units. Here, B is said to be grafted onto A:
This graft copolymer would be termed poly(A-g-B), as the backbone repeat unit is listed first. Note that “few molecules” is key here, since if even as little as 0.1% of the reaction mixture contains monomers with functionalities of 3 or higher, a network or cross-linked structure is likely to form (particularly at higher reaction conversions).
2.3.3 Cross Linked or Network
As the length and frequency of the branches on polymer chains increase, the probability that the branches will ultimately reach from one backbone chain to another increases. When the backbone chains are connected in this way, the molecular structure becomes a network, with all the chains linked through covalent bonds. This creates a three-dimensional cross-linked polymer and the entire mass of the polymer becomes one single tremendously large molecule! One impressive example is a bowling ball, which is a cross-linked polymer, so the entire mass is one molecule; it has a molecular weight on the order of 1027 g/mol. Remember this the next time someone suggests that individual molecules are too small to be seen with the naked eye.
Cross-linked or network polymers may be formed in two ways: (1) by starting with reaction masses containing sufficient amounts of tri- or higher functional monomer, or (2) by chemically creating cross-links between previously formed linear or branched molecules (“curing”). The latter is precisely what vulcanization does to natural rubber, and this fact serves to introduce the connection between the phenomenological “reaction to temperature” classification and the more fundamental concept of molecular structure. This important connection will be clarified through a discussion of bonding in polymers.
