CHEM Santa Ana College Unit Molecular Weight of Various Polymers Questions

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Please answer 11 Questions I have attached below and you can find the answers from the pdf file or the Internet (CHEGG)

Please answer 11 Questions I have attached below and you can find the answers from the pdf file or the Internet (CHEGG)

Please answer 11 Questions I have attached below and you can find the answers from the pdf file or the Internet (CHEGG)

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1 Introduction © blickwinkel/Alamy © iStockphoto/Mark Oleksiy Chapter © iStockphoto/Jill Chen A familiar item fabricated from three different material types is the beverage container. Beverages are marketed in aluminum (metal) cans (top), glass (ceramic) bottles (center), and plastic (polymer) bottles © blickwinkel/Alamy © iStockphoto/Mark Oleksiy (bottom). • 1 Learning Objectives After studying this chapter, you should be able to do the following: 4. (a) List the three primary classifications of solid 1. List six different property classifications of materials, and then cite the distinctive materials that determine their applicability. chemical feature of each. 2. Cite the four components that are involved in the (b) Note the four types of advanced materials design, production, and utilization of materials, and, for each, its distinctive feature(s). and briefly describe the interrelationships 5. (a) Briefly define smart material/system. between these components. (b) Briefly explain the concept of nanotechnol3. Cite three criteria that are important in the ogy as it applies to materials. materials selection process. 1.1 HISTORICAL PERSPECTIVE Materials are probably more deep seated in our culture than most of us realize. Transportation, housing, clothing, communication, recreation, and food production— virtually every segment of our everyday lives is influenced to one degree or another by materials. Historically, the development and advancement of societies have been intimately tied to the members’ ability to produce and manipulate materials to fill their needs. In fact, early civilizations have been designated by the level of their materials development (Stone Age, Bronze Age, Iron Age).1 The earliest humans had access to only a very limited number of materials, those that occur naturally: stone, wood, clay, skins, and so on. With time, they discovered techniques for producing materials that had properties superior to those of the natural ones; these new materials included pottery and various metals. Furthermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances. At this point, materials utilization was totally a selection process that involved deciding from a given, rather limited set of materials the one best suited for an application by virtue of its characteristics. It was not until relatively recent times that scientists came to understand the relationships between the structural elements of materials and their properties. This knowledge, acquired over approximately the past 100 years, has empowered them to fashion, to a large degree, the characteristics of materials. Thus, tens of thousands of different materials have evolved with rather specialized characteristics that meet the needs of our modern and complex society, including metals, plastics, glasses, and fibers. The development of many technologies that make our existence so comfortable has been intimately associated with the accessibility of suitable materials. An advancement in the understanding of a material type is often the forerunner to the stepwise progression of a technology. For example, automobiles would not have been possible without the availability of inexpensive steel or some other comparable substitute. In the contemporary era, sophisticated electronic devices rely on components that are made from what are called semiconducting materials. 1.2 MATERIALS SCIENCE AND ENGINEERING Sometimes it is useful to subdivide the discipline of materials science and engineering into materials science and materials engineering subdisciplines. Strictly speaking, materials science involves investigating the relationships that exist between the structures and 1 The approximate dates for the beginnings of the Stone, Bronze, and Iron Ages are 2.5 million bc, 3500 bc, and 1000 bc, respectively. 2 • 1.2 Materials Science and Engineering • 3 properties of materials. In contrast, materials engineering involves, on the basis of these structure–property correlations, designing or engineering the structure of a material to produce a predetermined set of properties.2 From a functional perspective, the role of a materials scientist is to develop or synthesize new materials, whereas a materials engineer is called upon to create new products or systems using existing materials and/or to develop techniques for processing materials. Most graduates in materials programs are trained to be both materials scientists and materials engineers. Structure is, at this point, a nebulous term that deserves some explanation. In brief, the structure of a material usually relates to the arrangement of its internal components. Subatomic structure involves electrons within the individual atoms and interactions with their nuclei. On an atomic level, structure encompasses the organization of atoms or molecules relative to one another. The next larger structural realm, which contains large groups of atoms that are normally agglomerated together, is termed microscopic, meaning that which is subject to direct observation using some type of microscope. Finally, structural elements that can be viewed with the naked eye are termed macroscopic. The notion of property deserves elaboration. While in service use, all materials are exposed to external stimuli that evoke some type of response. For example, a specimen subjected to forces experiences deformation, or a polished metal surface reflects light. A property is a material trait in terms of the kind and magnitude of response to a specific imposed stimulus. Generally, definitions of properties are made independent of material shape and size. Virtually all important properties of solid materials may be grouped into six different categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative. For each, there is a characteristic type of stimulus capable of provoking different responses. Mechanical properties relate deformation to an applied load or force; examples include elastic modulus (stiffness), strength, and toughness. For electrical properties, such as electrical conductivity and dielectric constant, the stimulus is an electric field. The thermal behavior of solids can be represented in terms of heat capacity and thermal conductivity. Magnetic properties demonstrate the response of a material to the application of a magnetic field. For optical properties, the stimulus is electromagnetic or light radiation; index of refraction and reflectivity are representative optical properties. Finally, deteriorative characteristics relate to the chemical reactivity of materials. The chapters that follow discuss properties that fall within each of these six classifications. In addition to structure and properties, two other important components are involved in the science and engineering of materials—namely, processing and performance. With regard to the relationships of these four components, the structure of a material depends on how it is processed. Furthermore, a material’s performance is a function of its properties. Thus, the interrelationship between processing, structure, properties, and performance is as depicted in the schematic illustration shown in Figure 1.1. Throughout this text, we draw attention to the relationships among these four components in terms of the design, production, and utilization of materials. We present an example of these processing-structure-properties-performance principles in Figure 1.2, a photograph showing three thin-disk specimens placed over some printed matter. It is obvious that the optical properties (i.e., the light transmittance) of each of the three materials are different; the one on the left is transparent (i.e., virtually Processing Structure Properties Performance Figure 1.1 The four components of the discipline of materials science and engineering and their interrelationship. 2 Throughout this text we draw attention to the relationships between material properties and structural elements. 4 • Chapter 1 / Introduction aluminum oxide that have been placed over a printed page in order to demonstrate their differences in light-transmittance characteristics. The disk on the left is transparent (i.e., virtually all light that is reflected from the page passes through it), whereas the one in the center is translucent (meaning that some of this reflected light is transmitted through the disk). The disk on the right is opaque—that is, none of the light passes through it. These differences in optical properties are a consequence of differences in structure of these materials, which have resulted from the way the materials were processed. all of the reflected light passes through it), whereas the disks in the center and on the right are, respectively, translucent and opaque. All of these specimens are of the same material, aluminum oxide, but the leftmost one is what we call a single crystal—that is, has a high degree of perfection—which gives rise to its transparency. The center one is composed of numerous and very small single crystals that are all connected; the boundaries between these small crystals scatter a portion of the light reflected from the printed page, which makes this material optically translucent. Finally, the specimen on the right is composed not only of many small, interconnected crystals, but also of a large number of very small pores or void spaces. These pores also effectively scatter the reflected light and render this material opaque. Thus, the structures of these three specimens are different in terms of crystal boundaries and pores, which affect the optical transmittance properties. Furthermore, each material was produced using a different processing technique. If optical transmittance is an important parameter relative to the ultimate in-service application, the performance of each material will be different. 1.3 WHY STUDY MATERIALS SCIENCE AND ENGINEERING? Why do we study materials? Many an applied scientist or engineer, whether mechanical, civil, chemical, or electrical, is at one time or another exposed to a design problem involving materials, such as a transmission gear, the superstructure for a building, an oil refinery component, or an integrated circuit chip. Of course, materials scientists and engineers are specialists who are totally involved in the investigation and design of materials. Many times, a materials problem is one of selecting the right material from the thousands available. The final decision is normally based on several criteria. First of all, the in-service conditions must be characterized, for these dictate the properties required of the material. On only rare occasions does a material possess the maximum or ideal combination of properties. Thus, it may be necessary to trade one characteristic for another. The classic example involves strength and ductility; normally, a material having a high strength has only a limited ductility. In such cases, a reasonable compromise between two or more properties may be necessary. A second selection consideration is any deterioration of material properties that may occur during service operation. For example, significant reductions in mechanical strength may result from exposure to elevated temperatures or corrosive environments. Specimen preparation, P. A. Lessing. Figure 1.2 Three thin-disk specimens of 1.3 Why Study Materials Science and Engineering? • 5 Finally, probably the overriding consideration is that of economics: What will the finished product cost? A material may be found that has the ideal set of properties but is prohibitively expensive. Here again, some compromise is inevitable. The cost of a finished piece also includes any expense incurred during fabrication to produce the desired shape. The more familiar an engineer or scientist is with the various characteristics and structure–property relationships, as well as the processing techniques of materials, the more proficient and confident he or she will be in making judicious materials choices based on these criteria. C A S E S T U D Y Liberty Ship Failures T he following case study illustrates one role that materials scientists and engineers are called upon to assume in the area of materials performance: analyze mechanical failures, determine their causes, and then propose appropriate measures to guard against future incidents. The failure of many of the World War II Liberty ships3 is a well-known and dramatic example of the brittle fracture of steel that was thought to be ductile.4 Some of the early ships experienced structural damage when cracks developed in their decks and hulls. Three of them catastrophically split in half when cracks formed, grew to critical lengths, and then rapidly propagated completely around the ships’ girths. Figure 1.3 shows one of the ships that fractured the day after it was launched. Subsequent investigations concluded one or more of the following factors contributed to each failure5: • When some normally ductile metal alloys are cooled to relatively low temperatures, they become susceptible to brittle fracture—that is, they experience a ductile-to-brittle transition upon cooling through a critical range of temperatures. These Liberty ships were constructed of steel that experienced a ductile-to-brittle transition. Some of them were deployed to the frigid North Atlantic, where the once ductile metal experienced brittle fracture when temperatures dropped to below the transition temperature.6 • The corner of each hatch (i.e., door) was square; these corners acted as points of stress concentration where cracks can form. • German U-boats were sinking cargo ships faster than they could be replaced using existing construction techniques. Consequently, it became necessary to revolutionize construction methods to build cargo ships faster and in greater numbers. This was accomplished using prefabricated steel sheets that were assembled by welding rather than by the traditional time-consuming riveting. Unfortunately, cracks in welded structures may propagate unimpeded for large distances, which can lead to catastrophic failure. However, when structures are riveted, a crack ceases to propagate once it reaches the edge of a steel sheet. • Weld defects and discontinuities (i.e., sites where cracks can form) were introduced by inexperienced operators. 3 During World War II, 2,710 Liberty cargo ships were mass-produced by the United States to supply food and materials to the combatants in Europe. 4 Ductile metals fail after relatively large degrees of permanent deformation; however, very little if any permanent deformation accompanies the fracture of brittle materials. Brittle fractures can occur very suddenly as cracks spread rapidly; crack propagation is normally much slower in ductile materials, and the eventual fracture takes longer. For these reasons, the ductile mode of fracture is usually preferred. Ductile and brittle fractures are discussed in Sections 9.3 and 9.4. 5 Sections 9.2 through 9.5 discuss various aspects of failure. 6 This ductile-to-brittle transition phenomenon, as well as techniques that are used to measure and raise the critical temperature range, are discussed in Section 9.8. (continued) 6 • Chapter 1 / Introduction Figure 1.3 The Liberty ship S.S. Schenectady, which, in 1943, failed before leaving the shipyard. (Reprinted with permission of Earl R. Parker, Brittle Behavior of Engineering Structures, National Academy of Sciences, National Research Council, John Wiley & Sons, New York, 1957.) Remedial measures taken to correct these problems included the following: • Improving welding practices and establishing welding codes. • Lowering the ductile-to-brittle temperature of the steel to an acceptable level by improving steel quality (e.g., reducing sulfur and phosphorus impurity contents). In spite of these failures, the Liberty ship program was considered a success for several reasons, the primary reason being that ships that survived failure were able to supply Allied Forces in the theater of operations and in all likelihood shortened the war. In addition, structural steels were developed with vastly improved resistances to catastrophic brittle fractures. Detailed analyses of these failures advanced the understanding of crack formation and growth, which ultimately evolved into the discipline of fracture mechanics. • Rounding off hatch corners by welding a curved reinforcement strip on each corner.7 • Installing crack-arresting devices such as riveted straps and strong weld seams to stop propagating cracks. 7 The reader may note that corners of windows and doors for all of today’s marine and aircraft structures are rounded. 1.4 CLASSIFICATION OF MATERIALS Tutorial Video: What are the Different Classes of Materials? Solid materials have been conveniently grouped into three basic categories: metals, ceramics, and polymers, a scheme based primarily on chemical makeup and atomic structure. Most materials fall into one distinct grouping or another. In addition, there are the composites, which are engineered combinations of two or more different materials. A brief explanation of these material classifications and representative characteristics is offered next. Another category is advanced materials—those used in high-technology applications, such as semiconductors, biomaterials, smart materials, and nanoengineered materials; these are discussed in Section 1.5. 1.4 Classification of Materials • 7 Density (g/cm3) (logarithmic scale) Figure 1.4 Bar chart of roomtemperature density values for various metals, ceramics, polymers, and composite materials. 40 Metals 20 Platinum Silver 10 8 6 Copper Iron/Steel Titanium 4 Aluminum 2 Magnesium Ceramics ZrO2 Al2O3 SiC,Si3N4 Glass Concrete Polymers PTFE GFRC CFRC PVC PS PE Rubber 1.0 0.8 0.6 Composites Woods 0.4 0.2 0.1 Metals Tutorial Video: Metals Metals are composed of one or more metallic elements (e.g., iron, aluminum, copper, titanium, gold, nickel), and often also nonmetallic elements (e.g., carbon, nitrogen, oxygen) in relatively small amounts.8 Atoms in metals and their alloys are arranged in a very orderly manner (as discussed in Chapter 3) and are relatively dense in comparison to the ceramics and polymers (Figure 1.4). With regard to mechanical characteristics, these materials are relatively stiff (Figure 1.5) and strong (Figure 1.6), yet are ductile (i.e., capable of large amounts of deformation without fracture) and are resistant to fracture (Figure 1.7), which accounts for their widespread use in structural applications. Metallic materials have large numbers of nonlocalized electrons; that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. For example, metals are extremely good conductors of electricity Stiffness [Elastic (or Young’s) modulus (in units of gigapascals)] (logarithmic scale) Figure 1.5 Bar chart of roomtemperature stiffness (i.e., elastic modulus) values for various metals, ceramics, polymers, and composite materials. 1000 100 10 1.0 Metals Tungsten Iron/Steel Titanium Aluminum Magnesium Ceramics SiC AI2O3 Si3N4 ZrO2 Glass Concrete Composites CFRC GFRC Polymers PVC PS, Nylon PTFE PE 0.1 0.01 Rubbers 0.001 8 The term metal alloy refers to a metallic substance that is composed of two or more elements. Woods 8 • Chapter 1 / Introduction Figure 1.6 Metals Composites Ceramics Strength (tensile strength, in units of megapascals) (logarithmic scale) Bar chart of roomtemperature strength (i.e., tensile strength) values for various metals, ceramics, polymers, and composite materials. 1000 Steel alloys Cu,Ti alloys 100 Aluminum alloys Gold CFRC Si3N4 Al2O3 GFRC SiC Polymers Glass Nylon PVC PS PE Woods PTFE 10 (Figure 1.8) and heat and are not transparent to visible light; a polished metal surface has a lustrous appearance. In addition, some of the metals (i.e., Fe, Co, and Ni) have desirable magnetic properties. Figure 1.9 shows several common and familiar objects that are made of metallic materials. Furthermore, the types and applications of metals and their alloys are discussed in Chapter 13. Ceramics Tutorial Video: Ceramics Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. For example, common ceramic materials include aluminum oxide (or alumina, Al2O3), silicon dioxide (or silica, SiO2), silicon carbide (SiC), silicon nitride (Si3N4), and, in addition, what some refer to as the traditional ceramics—those composed of clay minerals (e.g., porcelain), as well as cement and glass. With regard to mechanical behavior, ceramic materials are relatively stiff and strong—stiffnesses and strengths are comparable to those of the metals (Figures 1.5 and 1.6). In addition, they are typically very hard. Historically, ceramics have exhibited extreme brittleness (lack of ductility) and are highly susceptible to fracture (Figure 1.7). However, newer ceramics are being engineered to have improved resistance to fracture; these materials are used for Figure 1.7 (Reprinted from Engineering Materials 1: An Introduction to Properties, Applications and Design, third edition, M. F. Ashby and D. R. H. Jones, pages 177 and 178. Copyright 2005, with permission from Elsevier.) Metals Resistance to fracture (fracture toughness, in units of MPa m) (logarithmic scale) Bar chart of room-temperature resistance to fracture (i.e., fracture toughness) for various metals, ceramics, polymers, and composite materials. 100 Steel alloys Composites Titanium alloys Aluminum alloys 10 CFRC Ceramics Si3N4 Al2O3 SiC 1.0 Glass Concrete 0.1 Polymers Nylon Polystyrene Polyethylene Polyester Wood GFRC 1.4 Classification of Materials • 9 Figure 1.8 Metals 108 Electrical conductivity (in units of reciprocal ohm-meters) (logarithmic scale) Bar chart of roomtemperature electrical conductivity ranges for metals, ceramics, polymers, and semiconducting materials. Semiconductors 104 1 10–4 10–8 Ceramics Polymers 10–12 10–16 10–20 cookware, cutlery, and even automobile engine parts. Furthermore, ceramic materials are typically insulative to the passage of heat and electricity (i.e., have low electrical conductivities; Figure 1.8) and are more resistant to high temperatures and harsh environments than are metals and polymers. With regard to optical characteristics, ceramics may be transparent, translucent, or opaque (Figure 1.2), and some of the oxide ceramics (e.g., Fe3O4) exhibit magnetic behavior. Several common ceramic objects are shown in Figure 1.10. The characteristics, types, and applications of this class of materials are also discussed in Chapter 13. Polymers Tutorial Video: Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (i.e., O, N, and Si). Furthermore, they have very large molecular structures, often chainlike in nature, that often have a backbone of carbon atoms. Some common and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials typically have low densities (Figure 1.4), whereas their mechanical characteristics are generally dissimilar Figure 1.9 Familiar objects made of © William D. Callister, Jr. metals and metal alloys (from left to right): silverware (fork and knife), scissors, coins, a gear, a wedding ring, and a nut and bolt. 10 • Chapter 1 / Introduction Figure 1.10 Common objects made of © William D. Callister, Jr. ceramic materials: scissors, a china teacup, a building brick, a floor tile, and a glass vase. to those of the metallic and ceramic materials—they are not as stiff or strong as these other material types (Figures 1.5 and 1.6). However, on the basis of their low densities, many times their stiffnesses and strengths on a per-mass basis are comparable to those of the metals and ceramics. In addition, many of the polymers are extremely ductile and pliable (i.e., plastic), which means they are easily formed into complex shapes. In general, they are relatively inert chemically and unreactive in a large number of environments. One major drawback to the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some instances, limits their use. Furthermore, they have low electrical conductivities (Figure 1.8) and are nonmagnetic. Figure 1.11 shows several articles made of polymers that are familiar to the reader. Chapters 4, 13, and 14 are devoted to discussions of the structures, properties, applications, and processing of polymeric materials. Figure 1.11 Several common objects © William D. Callister, Jr. made of polymeric materials: plastic tableware (spoon, fork, and knife), billiard balls, a bicycle helmet, two dice, a lawn mower wheel (plastic hub and rubber tire), and a plastic milk carton. 1.4 Classification of Materials • 11 C A S E S T U D Y Carbonated Beverage Containers O ne common item that presents some interesting material property requirements is the container for carbonated beverages. The material used for this application must satisfy the following constraints: (1) provide a barrier to the passage of carbon dioxide, which is under pressure in the container; (2) be nontoxic, unreactive with the beverage, and, preferably, recyclable; (3) be relatively strong and capable of surviving a drop from a height of several feet when containing the beverage; (4) be inexpensive, including the cost to fabricate the final shape; (5) if optically transparent, retain its optical clarity; and (6) be capable of being produced in different colors and/or adorned with decorative labels. All three of the basic material types—metal (aluminum), ceramic (glass), and polymer (polyester plastic)—are used for carbonated beverage containers (per the chapter-opening photographs). All of these materials are nontoxic and unreactive with beverages. In addition, each material has its pros and cons. For example, the aluminum alloy is relatively strong (but easily dented), is a very good barrier to the diffusion of carbon dioxide, is easily recycled, cools beverages rapidly, and allows labels to be painted onto its surface. However, the cans are optically opaque and relatively expensive to produce. Glass is impervious to the passage of carbon dioxide, is a relatively inexpensive material, and may be recycled, but it cracks and fractures easily, and glass bottles are relatively heavy. Whereas plastic is relatively strong, may be made optically transparent, is inexpensive and lightweight, and is recyclable, it is not as impervious to the passage of carbon dioxide as aluminum and glass. For example, you may have noticed that beverages in aluminum and glass containers retain their carbonization (i.e., “fizz”) for several years, whereas those in two-liter plastic bottles “go flat” within a few months. Composites Tutorial Video: Composites 9 A composite is composed of two (or more) individual materials that come from the categories previously discussed—metals, ceramics, and polymers. The design goal of a composite is to achieve a combination of properties that is not displayed by any single material and also to incorporate the best characteristics of each of the component materials. A large number of composite types are represented by different combinations of metals, ceramics, and polymers. Furthermore, some naturally occurring materials are composites—for example, wood and bone. However, most of those we consider in our discussions are synthetic (or human-made) composites. One of the most common and familiar composites is fiberglass, in which small glass fibers are embedded within a polymeric material (normally an epoxy or polyester).9 The glass fibers are relatively strong and stiff (but also brittle), whereas the polymer is more flexible. Thus, fiberglass is relatively stiff, strong (Figures 1.5 and 1.6), and flexible. In addition, it has a low density (Figure 1.4). Another technologically important material is the carbon fiber–reinforced polymer (CFRP) composite—carbon fibers that are embedded within a polymer. These materials are stiffer and stronger than glass fiber–reinforced materials (Figures 1.5 and 1.6) but more expensive. CFRP composites are used in some aircraft and aerospace applications, as well as in high-tech sporting equipment (e.g., bicycles, golf clubs, tennis rackets, skis/ snowboards) and recently in automobile bumpers. The new Boeing 787 fuselage is primarily made from such CFRP composites. Chapter 15 is devoted to a discussion of these interesting composite materials. Fiberglass is sometimes also termed a glass fiber–reinforced polymer composite (GFRP). 12 • Chapter 1 1.5 / Introduction ADVANCED MATERIALS Materials utilized in high-technology (or high-tech) applications are sometimes termed advanced materials. By high technology, we mean a device or product that operates or functions using relatively intricate and sophisticated principles, including electronic equipment (camcorders, CD/DVD players, etc.), computers, fiber-optic systems, spacecraft, aircraft, and military rocketry. These advanced materials are typically traditional materials whose properties have been enhanced and also newly developed, highperformance materials. Furthermore, they may be of all material types (e.g., metals, ceramics, polymers) and are normally expensive. Advanced materials include semiconductors, biomaterials, and what we may term materials of the future (i.e., smart materials and nanoengineered materials), which we discuss next. The properties and applications of a number of these advanced materials—for example, materials that are used for lasers, integrated circuits, magnetic information storage, liquid crystal displays (LCDs), and fiber optics—are also discussed in subsequent chapters. Semiconductors Semiconductors have electrical properties that are intermediate between those of electrical conductors (i.e., metals and metal alloys) and insulators (i.e., ceramics and polymers)—see Figure 1.8. Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms, for which the concentrations may be controlled over very small spatial regions. Semiconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries (not to mention our lives) over the last four decades. Biomaterials Biomaterials are employed in components implanted into the human body to replace diseased or damaged body parts. These materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cause adverse biological reactions). All of the preceding materials—metals, ceramics, polymers, composites, and semiconductors—may be used as biomaterials. Smart Materials Smart (or intelligent) materials are a group of new and state-of-the-art materials now being developed that will have a significant influence on many of our technologies. The adjective smart implies that these materials are able to sense changes in their environment and then respond to these changes in predetermined manners—traits that are also found in living organisms. In addition, this smart concept is being extended to rather sophisticated systems that consist of both smart and traditional materials. Components of a smart material (or system) include some type of sensor (which detects an input signal) and an actuator (that performs a responsive and adaptive function). Actuators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and/or magnetic fields. Four types of materials are commonly used for actuators: shape-memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological/magnetorheological fluids. Shape-memory alloys are metals that, after having been deformed, revert to their original shape when temperature is changed (see the Materials of Importance box following Section 11.9). Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely, they also generate an electric field when their dimensions are altered (see Section 12.25). The behavior of magnetostrictive materials is analogous to that of the piezoelectrics, except that they are responsive to 1.5 Advanced Materials • 13 magnetic fields. Also, electrorheological and magnetorheological fluids are liquids that experience dramatic changes in viscosity upon the application of electric and magnetic fields, respectively. Materials/devices employed as sensors include optical fibers (Section 19.14), piezoelectric materials (including some polymers), and microelectromechanical systems (MEMS; Section 13.11). For example, one type of smart system is used in helicopters to reduce aerodynamic cockpit noise created by the rotating rotor blades. Piezoelectric sensors inserted into the blades monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer-controlled adaptive device, which generates noise-canceling antinoise. Nanomaterials One new material class that has fascinating properties and tremendous technological promise is the nanomaterials, which may be any one of the four basic types—metals, ceramics, polymers, or composites. However, unlike these other materials, they are not distinguished on the basis of their chemistry but rather their size; the nano prefix denotes that the dimensions of these structural entities are on the order of a nanometer (10−9 m)—as a rule, less than 100 nanometers (nm); (equivalent to approximately 500 atoms). Prior to the advent of nanomaterials, the general procedure scientists used to understand the chemistry and physics of materials was to begin by studying large and complex structures and then investigate the fundamental building blocks of these structures that are smaller and simpler. This approach is sometimes termed top-down science. However, with the development of scanning probe microscopes (Section 5.12), which permit observation of individual atoms and molecules, it has become possible to design and build new structures from their atomic-level constituents, one atom or molecule at a time (i.e., “materials by design”). This ability to arrange atoms carefully provides opportunities to develop mechanical, electrical, magnetic, and other properties that are not otherwise possible. We call this the bottom-up approach, and the study of the properties of these materials is termed nanotechnology.10 Some of the physical and chemical characteristics exhibited by matter may experience dramatic changes as particle size approaches atomic dimensions. For example, materials that are opaque in the macroscopic domain may become transparent on the nanoscale; some solids become liquids, chemically stable materials become combustible, and electrical insulators become conductors. Furthermore, properties may depend on size in this nanoscale domain. Some of these effects are quantum mechanical in origin, whereas others are related to surface phenomena—the proportion of atoms located on surface sites of a particle increases dramatically as its size decreases. Because of these unique and unusual properties, nanomaterials are finding niches in electronic, biomedical, sporting, energy production, and other industrial applications. Some are discussed in this text, including the following: • Catalytic converters for automobiles (Materials of Importance box, Chapter 5) • Nanocarbons (fullerenes, carbon nanotubes, and graphene) (Section 13.11) • Particles of carbon black as reinforcement for automobile tires (Section 15.2) • Nanocomposites (Section 15.16) • Magnetic nanosize grains that are used for hard disk drives (Section 18.11) • Magnetic particles that store data on magnetic tapes (Section 18.11) 10 One legendary and prophetic suggestion as to the possibility of nanoengineered materials was offered by Richard Feynman in his 1959 American Physical Society lecture titled “There’s Plenty of Room at the Bottom.” 14 • Chapter 1 / Introduction Whenever a new material is developed, its potential for harmful and toxicological interactions with humans and animals must be considered. Small nanoparticles have exceedingly large surface area–to–volume ratios, which can lead to high chemical reactivities. Although the safety of nanomaterials is relatively unexplored, there are concerns that they may be absorbed into the body through the skin, lungs, and digestive tract at relatively high rates, and that some, if present in sufficient concentrations, will pose health risks—such as damage to DNA or promotion of lung cancer. 1.6 MODERN MATERIALS’ NEEDS In spite of the tremendous progress that has been made in the discipline of materials science and engineering within the last few years, technological challenges remain, including the development of even more sophisticated and specialized materials, as well as consideration of the environmental impact of materials production. Some comment is appropriate relative to these issues so as to round out this perspective. Nuclear energy holds some promise, but the solutions to the many problems that remain necessarily involve materials, such as fuels, containment structures, and facilities for the disposal of radioactive waste. Significant quantities of energy are involved in transportation. Reducing the weight of transportation vehicles (automobiles, aircraft, trains, etc.), as well as increasing engine operating temperatures, will enhance fuel efficiency. New high-strength, low-density structural materials remain to be developed, as well as materials that have higher-temperature capabilities, for use in engine components. Furthermore, there is a recognized need to find new and economical sources of energy and to use present resources more efficiently. Materials will undoubtedly play a significant role in these developments. For example, the direct conversion of solar power into electrical energy has been demonstrated. Solar cells employ some rather complex and expensive materials. To ensure a viable technology, materials that are highly efficient in this conversion process yet less costly must be developed. The hydrogen fuel cell is another very attractive and feasible energy-conversion technology that has the advantage of being nonpolluting. It is just beginning to be implemented in batteries for electronic devices and holds promise as a power plant for automobiles. New materials still need to be developed for more efficient fuel cells and also for better catalysts to be used in the production of hydrogen. Furthermore, environmental quality depends on our ability to control air and water pollution. Pollution control techniques employ various materials. In addition, materials processing and refinement methods need to be improved so that they produce less environmental degradation—that is, less pollution and less despoilage of the landscape from the mining of raw materials. Also, in some materials manufacturing processes, toxic substances are produced, and the ecological impact of their disposal must be considered. Many materials that we use are derived from resources that are nonrenewable—that is, not capable of being regenerated, including most polymers, for which the prime raw material is oil, and some metals. These nonrenewable resources are gradually becoming depleted, which necessitates (1) the discovery of additional reserves, (2) the development of new materials having comparable properties with less adverse environmental impact, and/or (3) increased recycling efforts and the development of new recycling technologies. As a consequence of the economics of not only production but also environmental impact and ecological factors, it is becoming increasingly important to consider the “cradleto-grave” life cycle of materials relative to the overall manufacturing process. The roles that materials scientists and engineers play relative to these, as well as other environmental and societal issues, are discussed in more detail in Chapter 20. References • 15 SUMMARY Materials Science and Engineering • There are six different property classifications of materials that determine their applicability: mechanical, electrical, thermal, magnetic, optical, and deteriorative. • One aspect of materials science is the investigation of relationships that exist between the structures and properties of materials. By structure, we mean how some internal component(s) of the material is (are) arranged. In terms of (and with increasing) dimensionality, structural elements include subatomic, atomic, microscopic, and macroscopic. • With regard to the design, production, and utilization of materials, there are four elements to consider—processing, structure, properties, and performance. The performance of a material depends on its properties, which in turn are a function of its structure(s); furthermore, structure(s) is (are) determined by how the material was processed. • Three important criteria in materials selection are in-service conditions to which the material will be subjected, any deterioration of material properties during operation, and economics or cost of the fabricated piece. Classification of Materials • On the basis of chemistry and atomic structure, materials are classified into three general categories: metals (metallic elements), ceramics (compounds between metallic and nonmetallic elements), and polymers (compounds composed of carbon, hydrogen, and other nonmetallic elements). In addition, composites are composed of at least two different material types. Advanced Materials • Another materials category is the advanced materials that are used in high-tech applications, including semiconductors (having electrical conductivities intermediate between those of conductors and insulators), biomaterials (which must be compatible with body tissues), smart materials (those that sense and respond to changes in their environments in predetermined manners), and nanomaterials (those that have structural features on the order of a nanometer, some of which may be designed on the atomic/molecular level). REFERENCES Ashby, M. F., and D. R. H. Jones, Engineering Materials 1: An Introduction to Their Properties, Applications, and Design, 4th edition, Butterworth-Heinemann, Oxford, England, 2012. Ashby, M. F., and D. R. H. Jones, Engineering Materials 2: An Introduction to Microstructures and Processing, 4th edition, Butterworth-Heinemann, Oxford, England, 2012. Ashby, M. F., H. Shercliff, and D. Cebon, Materials: Engineering, Science, Processing, and Design, 3rd edition, ButterworthHeinemann, Oxford, England, 2014. Askeland, D. R., and W. J. Wright, Essentials of Materials Science and Engineering, 3rd edition, Cengage Learning, Stamford, CT, 2014. Askeland, D. R., and W. J. Wright, The Science and Engineering of Materials, 7th edition, Cengage Learning, Stamford, CT, 2016. Baillie, C., and L. Vanasupa, Navigating the Materials World, Academic Press, San Diego, CA, 2003. Douglas, E. P., Introduction to Materials Science and Engineering: A Guided Inquiry, Pearson Education, Upper Saddle River, NJ, 2014. Fischer, T., Materials Science for Engineering Students, Academic Press, San Diego, CA, 2009. Jacobs, J. A., and T. F. Kilduff, Engineering Materials Technology, 5th edition, Prentice Hall PTR, Paramus, NJ, 2005. McMahon, C. J., Jr., Structural Materials, Merion Books, Philadelphia, PA, 2006. Murray, G. T., C. V. White, and W. Weise, Introduction to Engineering Materials, 2nd edition, CRC Press, Boca Raton, FL, 2007. Schaffer, J. P., A. Saxena, S. D. Antolovich, T. H. Sanders, Jr., and S. B. Warner, The Science and Design of Engineering Materials, 2nd edition, McGraw-Hill, New York, NY, 1999. Shackelford, J. F., Introduction to Materials Science for Engineers, 8th edition, Prentice Hall PTR, Paramus, NJ, 2014. Smith, W. F., and J. Hashemi, Foundations of Materials Science and Engineering, 5th edition, McGraw-Hill, New York, NY, 2010. Van Vlack, L. H., Elements of Materials Science and Engineering, 6th edition, Addison-Wesley Longman, Boston, MA, 1989. White, M. A., Physical Properties of Materials, 2nd edition, CRC Press, Boca Raton, FL, 2012. 16 • Chapter 1 / Introduction QUESTIONS 1.1 Select one or more of the following modern items or devices and conduct an Internet search in order to determine what specific material(s) is (are) used and what specific properties this (these) material(s) possess(es) in order for the device/ item to function properly. Finally, write a short essay in which you report your findings. Cell phone/digital camera batteries Cell phone displays Solar cells Wind turbine blades Fuel cells Automobile engine blocks (other than cast iron) Automobile bodies (other than steel alloys) Space telescope mirrors Military body armor Sports equipment Soccer balls Basketballs Ski poles Ski boots Snowboards Surfboards Golf clubs Golf balls Kayaks Lightweight bicycle frames 1.2 List three items (in addition to those shown in Figure 1.9) made from metals or their alloys. For each item, note the specific metal or alloy used and at least one characteristic that makes it the material of choice. 1.3 List three items (in addition to those shown in Figure 1.10) made from ceramic materials. For each item, note the specific ceramic used and at least one characteristic that makes it the material of choice. 1.4 List three items (in addition to those shown in Figure 1.11) made from polymeric materials. For each item, note the specific polymer used and at least one characteristic that makes it the material of choice. 1.5 Classify each of the following materials as to whether it is a metal, ceramic, or polymer. Justify each choice: (a) brass; (b) magnesium oxide (MgO); (c) Plexiglas®; (d) polychloroprene; (e) boron carbide (B4C); and (f) cast iron. Chapter 4 Polymer Structures (a) Schematic representation of the arrangement of molecular chains for a crystalline region of polyethylene. Black and gray balls represent, respectively, carbon and hydrogen atoms. (a) (b) Schematic diagram of a polymer chain-folded crystallite—a plate-shaped crystalline region in which the molecular chains (red lines/curves) fold back and forth on themselves; these folds occur at the crystallite faces. (b) (c) Structure of a spherulite found in some semicrystalline polymers (schematic). Chain-folded crystallites radiate outward from a common center. Separating and connecting these crystallites are regions of amorphous material, wherein the molecular chains (red curves) assume misaligned and disordered configurations. (d) Transmission electron micrograph showing the spherulite structure. (c) Chain-folded lamellar crystallites (white lines) approximately 10 nm thick extend in radial directions from the center. 15,000×. (e) A polyethylene produce bag containing some fruit. [Photograph of Figure (d) supplied by P. J. Phillips. First published in R. Bartnikas and R. M. Eichhorn, Engineering Dielectrics, Vol. IIA, Electrical Properties of Solid Insulating Materials: Molecular Structure and Electrical Behavior, 1983. Copyright ASTM, 1916 Race Street, Philadelphia, PA 19103. Reprinted with permission.] Glow Images (d) (e) • 115 WHY STUDY Polymer Structures? A relatively large number of chemical and structural characteristics affect the properties and behaviors of polymeric materials. Some of these influences are as follows: 1. Degree of crystallinity of semicrystalline polymers—on density, stiffness, strength, and ductility (Sections 4.11 and 8.18). 2. Degree of crosslinking—on the stiffness of rubber-like materials (Section 8.19). 3. Polymer chemistry—on melting and glass-transition temperatures (Section 11.17). Learning Objectives After studying this chapter, you should be able to do the following: (b) the three types of stereoisomers, 1. Describe a typical polymer molecule in terms of (c) the two kinds of geometric isomers, and its chain structure and, in addition, how the (d) the four types of copolymers. molecule may be generated from repeat units. 5. Cite the differences in behavior and molecular 2. Draw repeat units for polyethylene, poly(vinyl structure for thermoplastic and thermosetting chloride), polytetrafluoroethylene, polypropylpolymers. ene, and polystyrene. 6. Briefly describe the crystalline state in polymeric 3. Calculate number-average and weight-average materials. molecular weights and degree of polymerization 7. Briefly describe/diagram the spherulitic structure for a specified polymer. for a semicrystalline polymer. 4. Name and briefly describe: (a) the four general types of polymer molecular structures, 4.1 INTRODUCTION Naturally occurring polymers—those derived from plants and animals—have been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk. Other natural polymers, such as proteins, enzymes, starches, and cellulose, are important in biological and physiological processes in plants and animals. Modern scientific research tools have made possible the determination of the molecular structures of this group of materials and the development of numerous polymers that are synthesized from small organic molecules. Many of our useful plastics, rubbers, and fiber materials are synthetic polymers. In fact, since the conclusion of World War II, the field of materials has been virtually revolutionized by the advent of synthetic polymers. The synthetics can be produced inexpensively, and their properties may be managed to the degree that many are superior to their natural counterparts. In some applications, metal and wood parts have been replaced by plastics, which have satisfactory properties and can be produced at a lower cost. As with metals and ceramics, the properties of polymers are intricately related to the structural elements of the material. This chapter explores molecular and crystal structures of polymers; Chapter 8 discusses the relationships between structure and some of the mechanical properties. 4.2 HYDROCARBON MOLECULES Because most polymers are organic in origin, we briefly review some of the basic concepts relating to the structure of their molecules. First, many organic materials are hydrocarbons—that is, they are composed of hydrogen and carbon. Furthermore, the intramolecular bonds are covalent. Each carbon atom has four electrons that may 116 • 4.2 Hydrocarbon Molecules • 117 participate in covalent bonding, whereas every hydrogen atom has only one bonding electron. A single covalent bond exists when each of the two bonding atoms contributes one electron, as represented schematically in Figure 2.12 for a molecule of hydrogen (H2). Double and triple bonds between two carbon atoms involve the sharing of two and three pairs of electrons, respectively.1 For example, in ethylene, which has the chemical formula C2H4, the two carbon atoms are doubly bonded together, and each is also singly bonded to two hydrogen atoms, as represented by the structural formula H H C C H H where  and  denote single and double covalent bonds, respectively. An example of a triple bond is found in acetylene, C2H2: H unsaturated saturated Table 4.1 Compositions and Molecular Structures for Some Paraffin Compounds: CnH2n+2 C C H Molecules that have double and triple covalent bonds are termed unsaturated—that is, each carbon atom is not bonded to the maximum (four) other atoms. Therefore, it is possible for another atom or group of atoms to become attached to the original molecule. Furthermore, for a saturated hydrocarbon, all bonds are single ones, and no new atoms may be joined without the removal of others that are already bonded. Some of the simple hydrocarbons belong to the paraffin family; the chainlike paraffin molecules include methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). Compositions and molecular structures for paraffin molecules are contained in Table 4.1. The covalent bonds in each molecule are strong, but only weak van der Waals bonds exist between molecules, and thus these hydrocarbons have relatively low melting and boiling points. However, boiling temperatures rise with increasing molecular weight (Table 4.1). Name Composition Boiling Point (°C) Structure H Methane CH4 H −164 H C H Ethane Propane C3H8 H H H C C H H −88.6 H H H H C C C H H H H −42.1 C4H10 −0.5 Pentane C5H12 36.1 Hexane C6H14 69.0 Butane 1 C2H6 H In the hybrid bonding scheme for carbon (Section 2.6), a carbon atom forms sp3 hybrid orbitals when all its bonds are single ones; a carbon atom with a double bond has sp2 hybrid orbitals; and a carbon atom with a triple bond has sp hybridization. 118 • Chapter 4 isomerism / Polymer Structures Hydrocarbon compounds with the same composition may have different atomic arrangements, a phenomenon termed isomerism. For example, there are two isomers for butane; normal butane has the structure H H H H H C C C C H H H H H whereas a molecule of isobutane is represented as follows: H H H C H H C C C H H H H H Some of the physical properties of hydrocarbons depend on the isomeric state; for example, the boiling temperatures for normal butane and isobutane are −0.5°C and −12.3°C (31.1°F and 9.9°F), respectively. Table 4.2 Some Common Hydrocarbon Groups Family Characteristic Unit Representative Compound H R Alcohols OH H C OH Methyl alcohol H H Ethers R O R! H H O C C H OH Acids R C O C C Acetic acid O H C Aldehydes OH H R O C H O Formaldehyde H R OH Aromatic hydrocarbonsa Phenol C H a The simplified structure Dimethyl ether H H H H C denotes a phenyl group, C H H C C H C H 4.4 The Chemistry of Polymer Molecules • 119 There are numerous other organic groups, many of which are involved in polymer structures. Several of the more common groups are presented in Table 4.2, where R and R′ represent organic groups such as CH3, C2H5, and C6H5 (methyl, ethyl, and phenyl). Concept Check 4.1 Differentiate between polymorphism (see Chapter 3) and isomerism. (The answer is available in WileyPLUS.) 4.3 POLYMER MOLECULES macromolecule The molecules in polymers are gigantic in comparison to the hydrocarbon molecules already discussed; because of their size they are often referred to as macromolecules. Within each molecule, the atoms are bound together by covalent interatomic bonds. For carbon-chain polymers, the backbone of each chain is a string of carbon atoms. Many times each carbon atom singly bonds to two adjacent carbons atoms on either side, represented schematically in two dimensions as follows: C repeat unit monomer C C C C C C Each of the two remaining valence electrons for every carbon atom may be involved in side bonding with atoms or radicals that are positioned adjacent to the chain. Of course, both chain and side double bonds are also possible. These long molecules are composed of structural entities called repeat units, which are successively repeated along the chain.2 The term monomer refers to the small molecule from which a polymer is synthesized. Hence, monomer and repeat unit mean different things, but sometimes the term monomer or monomer unit is used instead of the more proper term repeat unit. 4.4 THE CHEMISTRY OF POLYMER MOLECULES Consider again the hydrocarbon ethylene (C2H4), which is a gas at ambient temperature and pressure and has the following molecular structure: H H C C H H If the ethylene gas is reacted under appropriate conditions, it will transform to polyethylene (PE), which is a solid polymeric material. This process begins when an active center is formed by the reaction between an initiator or catalyst species (R.) and the ethylene monomer, as follows: R· 2 polymer H H C C H H R H H C C· H H (4.1) A repeat unit is also sometimes called a mer. Mer originates from the Greek word meros, which means “part”; the term polymer was coined to mean “many mers.” 120 • Chapter 4 / Polymer Structures The polymer chain then forms by the sequential addition of monomer units to this actively growing chain molecule. The active site, or unpaired electron (denoted by ⋅ ), is transferred to each successive end monomer as it is linked to the chain. This may be represented schematically as follows: R H H H H C C· " C C H H H H R H H H H C C C C· H H H H (4.2) The final result, after the addition of many ethylene monomer units, is the polyethylene molecule.3 A portion of one such molecule and the polyethylene repeat unit are shown in Figure 4.1a. This polyethylene chain structure can also be represented as : VMSE H Repeat Unit Structures H C )n (C H or alternatively as H —( CH2 — CH2 — )n Here, the repeat units are enclosed in parentheses, and the subscript n indicates the number of times it repeats.4 The representation in Figure 4.1a is not strictly correct, in that the angle between the singly bonded carbon atoms is not 180° as shown, but rather is close to 109°. A more accurate three-dimensional model is one in which the carbon atoms form a zigzag pattern (Figure 4.1b), the CC bond length being 0.154 nm. In this discussion, depiction of polymer molecules is frequently simplified using the linear chain model shown in Figure 4.1a. Figure 4.1 For polyeth- ylene, (a) a schematic representation of repeat unit and chain structures, and (b) a perspective of the molecule, indicating the zigzag backbone structure. H H H H H H H H C C C C C C C C H H H H H H H H Repeat unit (a) C H (b) 3 A more detailed discussion of polymerization reactions, including both addition and condensation mechanisms, is given in Section 14.11. 4 Chain ends/end groups (i.e., the Rs in Equation 4.2) are not normally represented in chain structures. 4.4 The Chemistry of Polymer Molecules • 121 Of course polymer structures having other chemistries are possible. For example, the tetrafluoroethylene monomer, CF2 CF2, can polymerize to form polytetrafluoroethylene (PTFE) as follows: : VMSE Repeat Unit Structures n F F F C C (C F F F F C )n (4.3) F Polytetrafluoroethylene (trade name Teflon) belongs to a family of polymers called the fluorocarbons. The vinyl chloride monomer (CH2 CHCl) is a slight variant of that for ethylene, in which one of the four H atoms is replaced with a Cl atom. Its polymerization is represented as : VMSE Repeat Unit Structures n H H H C C (C H Cl H H C )n (4.4) Cl and leads to poly(vinyl chloride) (PVC), another common polymer. Some polymers may be represented using the following generalized form: H (C H : VMSE Repeat Unit Structures homopolymer copolymer bifunctional functionality trifunctional H C )n R where the R depicts either an atom [i.e., H or Cl, for polyethylene or poly(vinyl chloride), respectively] or an organic group such as CH3, C2H5, and C6H5 (methyl, ethyl, and phenyl). For example, when R represents a CH3 group, the polymer is polypropylene (PP). Poly(vinyl chloride) and polypropylene chain structures are also represented in Figure 4.2. Table 4.3 lists repeat units for some of the more common polymers; as may be noted, some of them—for example, nylon, polyester, and polycarbonate—are relatively complex. Repeat units for a large number of relatively common polymers are given in Appendix D. When all of the repeating units along a chain are of the same type, the resulting polymer is called a homopolymer. Chains may be composed of two or more different repeat units, in what are termed copolymers (see Section 4.10). The monomers discussed thus far have an active bond that may react to form two covalent bonds with other monomers forming a two-dimensional chainlike molecular structure, as indicated earlier for ethylene. Such a monomer is termed bifunctional. In general, the functionality is the number of bonds that a given monomer can form. For example, monomers such as phenol–formaldehyde (Table 4.3) are trifunctional: they have three active bonds, from which a three-dimensional molecular network structure results. Concept Check 4.2 On the basis of the structures presented in the previous section, sketch the repeat unit structure for poly(vinyl fluoride). (The answer is available in WileyPLUS.) 122 • Chapter 4 / Polymer Structures Figure 4.2 Repeat unit and chain structures for (a) polytetrafluoroethylene, (b) poly(vinyl chloride), and (c) polypropylene. F F F F F F F F C C C C C C C C F F F F F F F F Repeat unit (a) H H H H H H H H C C C C C C C C H Cl H Cl H Cl H Cl H H H Repeat unit (b) H H H H H C C C C C C C C H CH3 H CH3 H CH3 H CH3 Repeat unit (c) Table 4.3 Repeat Units for 10 of the More Common Polymeric Materials Polymer Polyethylene (PE) : VMSE Repeat Unit Structures Poly(vinyl chloride) (PVC) Polytetrafluoroethylene (PTFE) Polypropylene (PP) Polystyrene (PS) Repeat Unit H H C C H H H H C C H Cl F F C C F F H H C C H CH3 H H C C H (continued) 4.5 Molecular Weight • 123 Table 4.3 (Continued) Polymer Repeat Unit Poly(methyl methacrylate) (PMMA) H CH3 C C H C O O CH3 OH CH2 CH2 Phenol-formaldehyde (Bakelite) CH2 H Poly(hexamethylene adipamide) (nylon 6,6) Poly(ethylene terephthalate) (PET, a polyester) Polycarbonate (PC) N C H H O a C 6 H O C C C H H O C a O N O O 4 H H C C H H O O CH3 C O C CH3 H H a The symbol in the backbone chain denotes an aromatic ring as C C C C C H 4.5 C H MOLECULAR WEIGHT Extremely large molecular weights5 are observed in polymers with very long chains. During the polymerization process not all polymer chains will grow to the same length; this results in a distribution of chain lengths or molecular weights. Ordinarily, an average molecular weight is specified, which may be determined by the measurement of various physical properties such as viscosity and osmotic pressure. There are several ways of defining average molecular weight. The number-average molecular weight Mn is obtained by dividing the chains into a series of size ranges and 5 Molecular mass, molar mass, and relative molecular mass are sometimes used and are really more appropriate terms than molecular weight in the context of the present discussion—in fact, we are dealing with masses and not weights. However, molecular weight is most commonly found in the polymer literature and thus is used throughout this book. / Polymer Structures Hypothetical polymer molecule size distributions on the basis of (a) number and (b) weight fractions of molecules. Number fraction Figure 4.3 0.3 0.3 0.2 0.2 Weight fraction 124 • Chapter 4 0.1 0 0 5 10 15 20 25 30 35 0.1 0 40 0 5 10 15 20 25 30 Molecular weight (103 g/mol) Molecular weight (103 g/mol) (a) (b) 35 40 then determining the number fraction of chains within each size range (Figure 4.3a). The number-average molecular weight is expressed as Number-average molecular weight Mn = (4.5a) ∑ xiMi where Mi represents the mean (middle) molecular weight of size range i, and xi is the fraction of the total number of chains within the corresponding size range. A weight-average molecular weight Mw is based on the weight fraction of molecules within the various size ranges (Figure 4.3b). It is calculated according to Weight-average molecular weight Mw = (4.5b) ∑ wiMi where, again, Mi is the mean molecular weight within a size range, whereas wi denotes the weight fraction of molecules within the same size interval. Computations for both number-average and weight-average molecular weights are carried out in Example Problem 4.1. A typical molecular weight distribution along with these molecular weight averages is shown in Figure 4.4. Figure 4.4 Distribution of molecular weights for a typical polymer. Number-average, Mn Amount of polymer Weight-average, Mw Molecular weight 4.5 Molecular Weight • 125 degree of polymerization Degree of polymerization— dependence on number-average and repeat unit molecular weights An alternative way of expressing average chain size of a polymer is as the degree of polymerization, DP, which represents the average number of repeat units in a chain. DP is related to the number-average molecular weight Mn by the equation DP = Mn m (4.6) where m is the repeat unit molecular weight. EXAMPLE PROBLEM 4.1 Computations of Average Molecular Weights and Degree of Polymerization Assume that the molecular weight distributions shown in Figure 4.3 are for poly(vinyl chloride). For this material, compute (a) the number-average molecular weight, (b) the degree of polymerization, and (c) the weight-average molecular weight. Solution (a) The data necessary for this computation, as taken from Figure 4.3a, are presented in Table 4.4a. According to Equation 4.5a, summation of all the xiMi products (from the right-hand column) yields the number-average molecular weight, which in this case is 21,150 g/mol. (b) To determine the degree of polymerization (Equation 4.6), it is first necessary to compute the repeat unit molecular weight. For PVC, each repeat unit consists of two carbon atoms, three hydrogen atoms, and a single chlorine atom (Table 4.3). Furthermore, the atomic weights of C, H, and Cl are, respectively, 12.01, 1.01, and 35.45 g/mol. Thus, for PVC, m = 2(12.01 g/mol) + 3(1.01 g/mol) + 35.45 g/mol = 62.50 g/mol and DP = 21,150 g/mol Mn = 338 = m 62.50 g/mol Table 4.4a Data Used for Number-Average Molecular Weight Computations in Example Problem 4.1 Molecular Weight Range (g/mol) Mean Mi (g/mol) xi xiMi 5,000–10,000 7,500 0.05 375 10,000–15,000 12,500 0.16 2000 15,000–20,000 17,500 0.22 3850 20,000–25,000 22,500 0.27 6075 25,000–30,000 27,500 0.20 5500 30,000–35,000 32,500 0.08 2600 35,000–40,000 37,500 0.02 750 Mn = 21,150 126 • Chapter 4 / Polymer Structures (c) Table 4.4b shows the data for the weight-average molecular weight, as taken from Figure 4.3b. The wiMi products for the size intervals are tabulated in the right-hand column. The sum of these products (Equation 4.5b) yields a value of 23,200 g/mol for Mw. Table 4.4b Data Used for Weight-Average Molecular Weight Computations in Example Problem 4.1 Molecular Weight Range (g/mol) Mean Mi (g/mol) wi wiMi 5,000–10,000 7,500 0.02 150 10,000–15,000 12,500 0.10 1250 15,000–20,000 17,500 0.18 3150 20,000–25,000 22,500 0.29 6525 25,000–30,000 27,500 0.26 7150 30,000–35,000 32,500 0.13 4225 35,000–40,000 37,500 0.02 750 Mw = 23,200 Many polymer properties are affected by the length of the polymer chains. For example, the melting or softening temperature increases with increasing molecular weight (for M up to about 100,000 g/mol). At room temperature, polymers with very short chains (having molecular weights on the order of 100 g/mol) will generally exist as liquids. Those with molecular weights of approximately 1000 g/mol are waxy solids (such as paraffin wax) and soft resins. Solid polymers (sometimes termed high polymers), which are of prime interest here, commonly have molecular weights ranging between 10,000 and several million g/mol. Thus, the same polymer material can have quite different properties if it is produced with a different molecular weight. Other properties that depend on molecular weight include elastic modulus and strength (see Chapter 8). 4.6 MOLECULAR SHAPE Previously, polymer molecules have been shown as linear chains, neglecting the zigzag arrangement of the backbone atoms (Figure 4.1b). Single-chain bonds are capable of rotating and bending in three dimensions. Consider the chain atoms in Figure 4.5a; a third carbon atom may lie at any point on the cone of revolution and still subtend about a 109° angle with the bond between the other two atoms. A straight chain segment results when successive chain atoms are positioned as in Figure 4.5b. However, chain bending and twisting are possible when there is a rotation of the chain atoms into other positions, as illustrated in Figure 4.5c.6 Thus, a single chain molecule composed of many chain atoms might assume a shape similar to that represented schematically in Figure 4.6, having a multitude of bends, twists, and kinks.7 Also indicated in this figure 6 For some polymers, rotation of carbon backbone atoms within the cone may be hindered by bulky side group elements on neighboring chain atoms. 7 The term conformation is often used in reference to the physical outline of a molecule, or molecular shape, that can be altered only by rotation of chain atoms about single bonds. 4.6 Molecular Shape • 127 109° (b) (a) (c) Figure 4.5 Schematic representations of how polymer chain shape is influenced by the positioning of backbone carbon atoms (gray circles). For (a), the rightmost atom may lie anywhere on the dashed circle and still subtend a 109° angle with the bond between the other two atoms. Straight and twisted chain segments are generated when the backbone atoms are situated as in (b) and (c), respectively. is the end-to-end distance of the polymer chain r; this distance is much smaller than the total chain length. Polymers consist of large numbers of molecular chains, each of which may bend, coil, and kink in the manner of Figure 4.6. This leads to extensive intertwining and entanglement of neighboring chain molecules, a situation similar to what is seen in a heavily tangled fishing line. These random coils and molecular entanglements are responsible for a number of important characteristics of polymers, to include the large elastic extensions displayed by the rubber materials. Some of the mechanical and thermal characteristics of polymers are a function of the ability of chain segments to experience rotation in response to applied stresses or thermal vibrations. Rotational flexibility is dependent on repeat unit structure and chemistry. For example, the region of a chain segment that has a double bond (CC) is rotationally rigid. Also, introduction of a bulky or large side group of atoms restricts rotational movement. For example, polystyrene molecules, which have a phenyl side group (Table 4.3), are more resistant to rotational motion than are polyethylene chains. r Figure 4.6 Schematic representation of a single polymer chain molecule that has numerous random kinks and coils produced by chain bond rotations. 128 • Chapter 4 4.7 / Polymer Structures MOLECULAR STRUCTURE The physical characteristics of a polymer depend not only on its molecular weight and shape, but also on differences in the structure of the molecular chains. Modern polymer synthesis techniques permit considerable control over various structural possibilities. This section discusses several molecular structures including linear, branched, crosslinked, and network, in addition to various isomeric configurations. Linear Polymers linear polymer Linear polymers are those in which the repeat units are joined together end to end in single chains. These long chains are flexible and may be thought of as a mass of “spaghetti,” as represented schematically in Figure 4.7a, where each circle represents a repeat unit. For linear polymers, there may be extensive van der Waals and hydrogen bonding between the chains. Some of the common polymers that form with linear structures are polyethylene, poly(vinyl chloride), polystyrene, poly(methyl methacrylate), nylon, and the fluorocarbons. Branched Polymers branched polymer Polymers may be synthesized in which side-branch chains are connected to the main ones, as indicated schematically in Figure 4.7b; these are fittingly called branched polymers. The branches, considered to be part of the main-chain molecule, may result from side reactions that occur during the synthesis of the polymer. The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density. Polymers that form linear structures may also be branched. For example, high-density polyethylene (HDPE) is primarily a linear polymer, whereas lowdensity polyethylene (LDPE) contains short-chain branches. (a) (b) (c) (d) Figure 4.7 Schematic representations of (a) linear, (b) branched, (c) crosslinked, and (d) network (three-dimensional) molecular structures. Circles designate individual repeat units. 4.8 Molecular Configurations • 129 Crosslinked Polymers crosslinked polymer In crosslinked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds, as represented in Figure 4.7c. The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction. Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains. Many of the rubber elastic materials are crosslinked; in rubbers, this is called vulcanization, a process described in Section 8.19. Network Polymers network polymer Multifunctional monomers forming three or more active covalent bonds make threedimensional networks (Figure 4.7d) and are termed network polymers. Actually, a polymer that is highly crosslinked may also be classified as a network polymer. These materials have distinctive mechanical and thermal properties; the epoxies, polyurethanes, and phenol-formaldehyde belong to this group. Polymers are not usually of only one distinctive structural type. For example, a predominantly linear polymer may have limited branching and crosslinking. 4.8 MOLECULAR CONFIGURATIONS For polymers having more than one side atom or group of atoms bonded to the main chain, the regularity and symmetry of the side group arrangement can significantly influence the properties. Consider the repeat unit H H C C H R in which R represents an atom or side group other than hydrogen (e.g., Cl, CH3). One arrangement is possible when the R side groups of successive repeat units are bound to alternate carbon atoms as follows: H H H H C C C C H R H R This is designated as a head-to-tail configuration.8 Its complement, the head-to-head configuration, occurs when R groups are bound to adjacent chain atoms: H H H H C C C C H R R H In most polymers, the head-to-tail configuration predominates; often a polar repulsion occurs between R groups for the head-to-head configuration. Isomerism (Section 4.2) is also found in polymer molecules, wherein different atomic configurations are possible for the same composition. Two isomeric subclasses— stereoisomerism and geometric isomerism—are topics of discussion in the succeeding sections. 8 The term configuration is used in reference to arrangements of units along the axis of the chain, or atom positions that are not alterable except by the breaking and then re-forming of primary bonds. 130 • Chapter 4 / Polymer Structures Stereoisomerism stereoisomerism Stereoisomerism denotes the situation in which atoms are linked together in the same order (head to tail) but differ in their spatial arrangement. For one stereoisomer, all of the R groups are situated on the same side of the chain as follows: C : VMSE Stereo and Geometric Isomers isotactic configuration syndiotactic configuration C C H R H R C H H C C H H C H C H H This is called an isotactic configuration. This diagram shows the zigzag pattern of the carbon chain atoms. Furthermore, representation of the structural geometry in three dimensions is important, as indicated by the wedge-shaped bonds; solid wedges represent bonds that project out of the plane of the page, and dashed ones represent bonds that project into the page.9 In a syndiotactic configuration, the R groups alternate sides of the chain:10 H H C C : VMSE H H R R C C C C H R R C C H H H H H H H H H R H H R R H H C and for random positioning R C C : VMSE Stereo and Geometric Isomers atactic configuration C H H R Stereo and Geometric Isomers H R H R R H H C C H H C C H H C C R C H H the term atactic configuration is used.11 9 The isotactic configuration is sometimes represented using the following linear (i.e., nonzigzag) and two-dimensional schematic: H H H H H H H H H C C C C C C C C C R H R H R H R H R 10 The linear and two-dimensional schematic for the syndiotactic configuration is represented as H H R H H H R H H C C C C C C C C C R H H H R H H H R 11 For the atactic configuration the linear and two-dimensional schematic is H H H H R H H H R C C C C C C C C C R H R H H H R H H 4.8 Molecular Configurations • 131 Conversion from one stereoisomer to another (e.g., isotactic to syndiotactic) is not possible by a simple rotation about single-chain bonds. These bonds must first be severed; then, after the appropriate rotation, they are re-formed into the new configuration. In reality, a specific polymer does not exhibit just one of these configurations; the predominant form depends on the method of synthesis. Geometric Isomerism Other important chain configurations, or geometric isomers, are possible within repeat units having a double bond between chain carbon atoms. Bonded to each of the carbon atoms participating in the double bond is a side group, which may be situated on one side of the chain or its opposite. Consider the isoprene repeat unit having the structure CH3 C : VMSE Stereo and Geometric Isomers cis (structure) H C CH2 CH2 in which the CH3 group and the H atom are positioned on the same side of the double bond. This is termed a cis structure, and the resulting polymer, cis-polyisoprene, is natural rubber. For the alternative isomer CH3 C : VMSE Stereo and Geometric Isomers trans (structure) CH2 CH2 C H the trans structure, the CH3 and H reside on opposite sides of the double bond.12 Transpolyisoprene, sometimes called gutta percha, has properties that are distinctly different from those of natural rubber as a result of this configurational alteration. Conversion of trans to cis, or vice versa, is not possible by a simple chain bond rotation because the chain double bond is extremely rigid. To summarize the preceding sections: Polymer molecules may be characterized in terms of their size, shape, and structure. Molecular size is specified in terms 12 For cis-polyisoprene the linear chain representation is as follows: H CH3 H H C C C C H H whereas the linear schematic for the trans structure is H CH3 C H C H C C H H 132 • Chapter 4 / Polymer Structures Figure 4.8 Molecular characteristics Classification scheme for the characteristics of polymer molecules. Chemistry (repeat unit composition) Size (molecular weight) Shape (chain twisting, entanglement, etc.) Linear Structure Branched Crosslinked Network Isomeric states Stereoisomers Isotactic Syndiotactic Geometric isomers Atactic cis trans of molecular weight (or degree of polymerization). Molecular shape relates to the degree of chain twisting, coiling, and bending. Molecular structure depends on the manner in which structural units are joined together. Linear, branched, crosslinked, and network structures are all possible, in addition to several isomeric configurations (isotactic, syndiotactic, atactic, cis, and trans). These molecular characteristics are presented in the taxonomic chart shown in Figure 4.8. Note that some of the structural elements are not mutually exclusive, and it may be necessary to specify molecular structure in terms of more than one. For example, a linear polymer may also be isotactic. Concept Check 4.3 What is the difference between configuration and conformation in relation to polymer chains? (The answer is available in WileyPLUS.) 4.9 THERMOPLASTIC AND THERMOSETTING POLYMERS thermoplastic polymer thermosetting polymer The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure. In fact, one classification scheme for these materials is according to behavior with rising temperature. Thermoplastics (or thermoplastic polymers) and thermosets (or thermosetting polymers) are the two subdivisions. 4.10 Copolymers • 133 Thermoplastics soften when heated (and eventually liquefy) and harden when cooled— processes that are totally reversible and may be repeated. On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied. Irreversible degradation results when a molten thermoplastic polymer is raised to too high a temperature. In addition, thermoplastics are relatively soft. Most linear polymers and those having some branched structures with flexible chains are thermoplastic. These materials are normally fabricated by the simultaneous application of heat and pressure (see Section 14.13). Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride). Thermosetting polymers are network polymers. They become permanently hard during their formation and do not soften upon heating. Network polymers have covalent crosslinks between adjacent molecular chains. During heat treatments, these bonds anchor the chains together to resist the vibrational and rotational chain motions at high temperatures. Thus, the materials do not soften when heated. Crosslinking is usually extensive, in that 10% to 50% of the chain repeat units are crosslinked. Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability. Most of the crosslinked and network polymers, which include vulcanized rubbers, epoxies, phenolics, and some polyester resins, are thermosetting. Concept Check 4.4 Some polymers (such as the polyesters) may be either thermoplastic or thermosetting. Suggest one reason for this. (The answer is available in WileyPLUS.) 4.10 COPOLYMERS random copolymer alternating copolymer block copolymer graft copolymer Average repeat unit molecular weight for a copolymer Polymer chemists and scientists are continually searching for new materials that can be easily and economically synthesized and fabricated with improved properties or better property combinations than are offered by the homopolymers previously discussed. One group of these materials are the copolymers. Consider a copolymer that is composed of two repeat units as represented by and in Figure 4.9. Depending on the polymerization process and the relative fractions of these repeat unit types, different sequencing arrangements along the polymer chains are possible. For one, as depicted in Figure 4.9a, the two different units are randomly dispersed along the chain in what is termed a random copolymer. For an alternating copolymer, as the name suggests, the two repeat units alternate chain positions, as illustrated in Figure 4.9b. A block copolymer is one in which identical repeat units are clustered in blocks along the chain (Figure 4.9c). Finally, homopolymer side branches of one type may be grafted to homopolymer main chains that are composed of a different repeat unit; such a material is termed a graft copolymer (Figure 4.9d). When calculating the degree of polymerization for a copolymer, the value m in Equation 4.6 is replaced with the average value m determined from m= ∑ fj mj (4.7) In this expression, fj and mj are, respectively, the mole fraction and molecular weight of repeat unit j in the polymer chain. 134 • Chapter 4 / Polymer Structures Figure 4.9 Schematic representations of (a) random, (b) alternating, (c) block, and (d) graft copolymers. The two different repeat unit types are designated by blue and red circles. (a) (b) (c) (d) Synthetic rubbers, discussed in Section 13.13, are often copolymers; chemical repeat units that are employed in some of these rubbers are shown in Table 4.5. Styrene– butadiene rubber (SBR) is a common random copolymer from which automobile tires are made. Nitrile rubber (NBR) is another random copolymer composed of acrylonitrile and butadiene. It is also highly elastic and, in addition, resistant to swelling in organic solvents; gasoline hoses are made of NBR. Impact-modified polystyrene is a block copolymer that consists of alternating blocks of styrene and butadiene. The rubbery isoprene blocks act to slow cracks propagating through the material. 4.11 POLYMER CRYSTALLINITY polymer crystallinity The crystalline state may exist in polymeric materials. However, because it involves molecules instead of just atoms or ions, as with metals and ceramics, the atomic arrangements will be more complex for polymers. We think of polymer crystallinity as the packing of molecular chains to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex. For example, Figure 4.10 shows the unit cell for polyethylene and its relationship to the molecular chain structure; this unit cell has orthorhombic geometry (Table 3.6). Of course, the chain molecules also extend beyond the unit cell shown in the figure. 4.11 Polymer Crystallinity • 135 Table 4.5 Chemical Repeat Units That Are Employed in Copolymer Rubbers Repeat Unit Name Repeat Unit Structure H Acrylonitrile VMSE Repeat Units for Rubbers Styrene Repeat Unit Name H C C H C H H C C Isoprene N H Butadiene H H H C C C C H Chloroprene Cl H H C C C H C H C C C C H H CH3 C C H CH3 CH3 Dimethylsiloxane Si O CH3 H H H CH3 H H Isobutylene H Repeat Unit Structure H Figure 4.10 Arrangement of molecular chains in a unit cell for polyethylene. 0.255 nm 0.494 nm 0.741 nm H C 136 • Chapter 4 Percent crystallinity (semicrystalline polymer)— dependence on specimen density, and densities of totally crystalline and totally amorphous materials / Polymer Structures Molecular substances having small molecules (e.g., water and methane) are normally either totally crystalline (as solids) or totally amorphous (as liquids). As a consequence of their size and often complexity, polymer molecules are often only partially crystalline (or semicrystalline), having crystalline regions dispersed within the remaining amorphous material. Any chain disorder or misalignment will result in an amorphous region, a condition that is fairly common, because twisting, kinking, and coiling of the chains prevent the strict ordering of every segment of every chain. Other structural effects are also influential in determining the extent of crystallinity, as discussed shortly. The degree of crystallinity may range from completely amorphous to almost entirely (up to about 95%) crystalline; in contrast, metal specimens are almost always entirely crystalline, whereas many ceramics are either totally crystalline or totally noncrystalline. Semicrystalline polymers are, in a sense, analogous to two-phase metal alloys, discussed in subsequent chapters. The density of a crystalline polymer will be greater than an amorphous one of the same material and molecular weight because the chains are more closely packed together for the crystalline structure. The degree of crystallinity by weight may be determined from accurate density measurements, according to % crystallinity = ρc (ρs − ρa ) ρs (ρc − ρa ) × 100 (4.8) where 𝜌s is the density of a specimen for which the percent crystallinity is to be determined, 𝜌a is the density of the totally amorphous polymer, and 𝜌c is the density of the perfectly crystalline polymer. The values of 𝜌a and 𝜌c must be measured by other experimental means. The degree of crystallinity of a polymer depends on the rate of cooling during solidification as well as on the chain configuration. During crystallization upon cooling through the melting temperature, the chains, which are highly random and entangled in the viscous liquid, must assume an ordered configuration. For this to occur, sufficient time must be allowed for the chains to move and align themselves. The molecular chemistry as well as chain configuration also influence the ability of a polymer to crystallize. Crystallization is not favored in polymers that are composed of chemically complex repeat units (e.g., polyisoprene). However, crystallization is not easily prevented in chemically simple polymers such as polyethylene and polytetrafluoroethylene, even for very rapid cooling rates. For linear polymers, crystallization is easily accomplished because there are few restrictions to prevent chain alignment. Any side branches interfere with crystallization, such that branched polymers never are highly crystalline; in fact, excessive branching may prevent any crystallization whatsoever. Most network and crosslinked polymers are almost totally amorphous because the crosslinks prevent the polymer chains from rearranging and aligning into a crystalline structure. A few crosslinked polymers are partially crystalline. With regard to the stereoisomers, atactic polymers are difficult to crystallize; however, isotactic and syndiotactic polymers crystallize much more easily because the regularity of the geometry of the side groups facilitates the process of fitting together adjacent chains. Also, the bulkier or larger the side-bonded groups of atoms, the less is the tendency for crystallization. For copolymers, as a general rule, the more irregular and random the repeat unit arrangements, the greater is the tendency for the development of noncrystallinity. For alternating and block copolymers there is some likelihood of crystallization. However, random and graft copolymers are normally amorphous. To some extent, the physical properties of polymeric materials are influenced by the degree of crystallinity. Crystalline polymers are usually stronger and more resistant to dissolution and softening by heat. Some of these properties are discussed in subsequent chapters. 4.11 Polymer Crystallinity • 137 Concept Check 4.5 (a) Compare the crystalline state in metals and polymers. (b) Compare the noncrystalline state as it applies to polymers and ceramic glasses. (The answer is available in WileyPLUS.) EXAMPLE PROBLEM 4.2 Computations of the Density and Percent Crystallinity of Polyethylene (a) Compute the density of totally crystalline polyethylene. The orthorhombic unit cell for polyethylene is shown in Figure 4.10; also, the equivalent of two ethylene repeat units is contained within each unit cell. (b) Using the answer to part (a), calculate the percent crystallinity of a branched polyethylene that has a density of 0.925 g/cm3. The density for the totally amorphous material is 0.870 g/cm3. Solution (a) Equation 3.8, used in Chapter 3 to determine densities for metals, also applies to polymeric materials and is used to solve this problem. It takes the same form, namely ρ= nA VC NA where n represents the number of repeat units within the unit cell (for polyethylene n = 2) and A is the repeat unit molecular weight, which for polyethylene is A = 2(AC ) + 4(AH ) = (2) (12.01 g/mol) + (4) (1.008 g/mol) = 28.05 g/mol Also, VC is the unit cell volume, which is just the product of the three unit cell edge lengths in Figure 4.10; or VC = (0.741 nm) (0.494 nm) (0.255 nm) = (7.41 × 10 −8 cm) (4.94 × 10 −8 cm) (2.55 × 10 −8 cm) = 9.33 × 10 −23 cm3/unit cell Now, substitution into Equation 3.8 of this value, values for n and A cited previously, and the value of NA leads to ρ= = nA VC NA (2 repeat units/unit cell) (28.05 g/mol) (9.33 × 10 −23 cm3/unit cell) (6.022 × 1023 repeat units/mol) = 0.998 g/cm3 (b) We now use Equation 4.8 to calculate the percent crystallinity of the branched polyethylene with 𝜌c = 0.998 g/cm3, 𝜌a = 0.870 g/cm3, and 𝜌s = 0.925 g/cm3. Thus, % crystallinity = = ρc (ρs − ρa ) ρs (ρc − ρa ) × 100 0.998 g/cm3 (0.925 g/cm3 − 0.870 g/cm3 ) 0.925 g/cm3 (0.998 g/cm3 − 0.870 g/cm3 ) = 46.4% × 100 138 • Chapter 4 / Polymer Structures Figure 4.11 Electron micrograph of a polyethylene single crystal. 20,000×. [From A. Keller, R. H. Doremus, B. W. Roberts, and D. Turnbull (Editors), Growth and Perfection of Crystals. General Electric Company and John Wiley & Sons, Inc., 1958, p. 498.] 1 μm 4.12 POLYMER CRYSTALS crystallite chain-folded model spherulite It has been proposed that a semicrystalline polymer consists of small crystalline regions (crystallites), each having a precise alignment, which are interspersed with amorphous regions composed of randomly oriented molecules. The structure of the crystalline regions may be deduced by examination of polymer single crystals, which may be grown from dilute solutions. These crystals are regularly shaped, thin platelets (or lamellae) approximately 10 to 20 nm thick and on the order of 10 μm long. Frequently, these platelets form a multilayered structure like that shown in the electron micrograph of a single crystal of polyethylene in Figure 4.11. The molecular chains within each platelet fold back and forth on themselves, with folds occurring at the faces; this structure, aptly termed the chain-folded model, is illustrated schematically in Figure 4.12. Each platelet consists of a number of molecules; however, the average chain length is much greater than the thickness of the platelet. Many bulk polymers that are crystallized from a melt are semicrystalline and form a spherulite structure. As implied by the name, each spherulite may grow to be roughly spherical in shape; one of them, as found in natural rubber, is shown in the transmission ~10 nm Figure 4.12 The chain-folded structure for a plate-shaped polymer crystallite. 4.12 Polymer Crystals • 139 Transmission electron micrograph showing the spherulite structure in a natural rubber specimen. electron micrograph in chapter-opening photograph (d) for this chapter and in the photograph that appears in the adjacent left margin. The spherulite consists of an aggregate of ribbon-like chain-folded crystallites (lamellae) approximately 10 nm thick that radiate outward from a single nucleation site in the center. In this electron micrograph, these lamellae appear as thin white lines. The detailed structure of a spherulite is illustrated schematically in Figure 4.13. Shown here are the individual chain-folded lamellar crystals that are separated by amorphous material. Tie-chain molecules that act as connecting links between adjacent lamellae pass through these amorphous regions. As the crystallization of a spherulitic structure nears completion, the extremities of adjacent spherulites begin to impinge on one another, forming more-or-less planar boundaries; prior to this time, they maintain their spherical shape. These boundaries are evident in Figure 4.14, which is a photomicrograph of polyethylene using cross-polarized light. A characteristic Maltese cross pattern appears within each spherulite. The bands or rings in the spherulite image result from twisting of the lamellar crystals as they extend like ribbons from the center. Spherulites are considered to be the polymer analogue of grains in polycrystalline metals and ceramics. However, as discussed earlier, each spherulite is really composed of many different lamellar crystals and, in addition, some amorphous material. Polyethylene, polypropylene, poly(vinyl chloride), polytetrafluoroethylene, and nylon form a spherulitic structure when they crystallize from a melt. Figure 4.13 Schematic Direction of spherulite growth representation of the detailed structure of a spherulite. Lamellar chain-folded crystallite Amorphous material Tie molecule Nucleation site Interspherulitic boundary 140 • Chapter 4 / Polymer Structures photomicrograph (using cross-polarized light) showing the spherulite structure of polyethylene. Linear boundaries form between adjacent spherulites, and within each spherulite appears a Maltese cross. 525×. μm SUMMARY Polymer Molecules • Most polymeric materials are composed of very large molecular chains with side groups of various atoms (O, Cl, etc.) or organic groups such as methyl, ethyl, or phenyl groups. • These macromolecules are composed of repeat units—smaller structural entities— which are repeated along the chain. The Chemistry of Polymer Molecules • Repeat units for some of the chemically simple polymers [polyethylene, polytetrafluoroethylene, poly(vinyl chloride), polypropylene, etc.] are presented in Table 4.3. • A homopolymer is one for which all of the repeat units are the same type. The chains for copolymers are composed of two or more kinds of repeat units. • Repeat units are classified according to the number of active bonds (i.e., functionality): For bifunctional monomers, a two-dimensional chainlike structure results from a monomer that has two active bonds. Trifunctional monomers have three active bonds, from which three-dimensional network structures form. Molecular Weight • Molecular weights for high polymers may be in excess of a million. Because all molecules are not of the same size, there is a distribution of molecular weights. • Molecular weight is often expressed in terms of number and weight averages; values for these parameters may be determined using Equations 4.5a and 4.5b, respectively. • Chain length may also be specified by degree of polymerization—the number of repeat units per average molecule (Equation 4.6). Molecular Shape • Molecular entanglements occur when the chains assume twisted, coiled, and kinked shapes or contours as a consequence of chain bond rotations. • Rotational flexibility is diminished when double chain bonds are present and also when bulky side groups are part of the repeat unit. Molecular Structure • Four different polymer molecular chain structures are possible: linear (Figure 4.7a), branched (Figure 4.7b), crosslinked (Figure 4.7c), and network (Figure 4.7d). Courtesy F. P. Price, General Electric Company Figure 4.14 A transmission Summary • 141 Molecular Configurations • For repeat units that have more than one side atom or groups of atoms bonded to the main chain: Head-to-head and head-to-tail configurations are possible. Differences in spatial arrangements of these side atoms or groups of atoms lead to isotactic, syndiotactic, and atactic stereoisomers. • When a repeat unit contains a double chain bond, both cis and trans geometric isomers are possible. Thermoplastic and Thermosetting Polymers • With regard to behavior at elevated temperatures, polymers are classified as either thermoplastic or thermosetting. Thermoplastic polymers have linear and branched structures; they soften when heated and harden when cooled. In contrast, thermosetting polymers, once they have hardened, will not soften upon heating; their structures are crosslinked and network. Copolymers • Th...
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High School Chemistry



High School Chemistry

Assignment No.2- EGME 459 Answer all the questions and solve the given problems. Questions
1 to 8 carry one (1) point each. Problems Numbered 9 and 10 each carry four (4) points. Problem
numbered 11 carry six (6) points. Total points= 20
1. Name four natural polymers:


2. Hydrocarbons consists of elements carbon (C) and hydrogen (H)
3. Ethylene (C2H4) is a gas at ambient temperature ...

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