942 CHAPTER 25 Modern Materials I -p- I o I CH 2 0 IBa sel 1/ " 1 C C / \ H HI \ H \1 I H C C I \ o H I HO-P-O I o I CH 2 0 IBa s el 1/ " 1 C C / \ H HI \ H \1 I H C C I \ o H I HO-P-O I o I Repeating unit along DNA chain I CH 2 0 IB asel 1/" 1 C C / \ H HI \ H \1 I H C C I \ o H I HO-P-O I o I CH 2 0 IB asel 1/ " 1 C C / \ H HI \ H \1 I H C C I \ o H I HO-P-O I o I Figure 25.8 Structure of DNA. amino group of one amino acid and the carboxy group of another. (See Figure 10.13.) One water molecule is produced as each new C-N bond is formed. The C- N bond formed between an amino group and a carboxy group is called an amide bond because RCONR' R" is the general form of an amide [ ~~ Section 10.2, Table 10.2]. Poly- mers in which the monomers are connected by amide linkages are called polyamides. The amide bond between amino acids is more specifically called a peptide bond, and proteins are routinely referred to as polypeptides. Although polypeptides can form from a single amino acid (e.g., poly- valine), most proteins in organisms consist of chains of different amino acids. As a result, most proteins are random copolymers (Table 25.2). Nylon 6,6 is a synthetic condensation polymer formed from hexamethylenediamine, a mol- ecule with two amino group s, and adipic acid, which has two carboxy groups. Water is the small molecule eliminated in this condensation reaction. o 0 0 0 nH2N1 CH2 t- NH2 + nOH- ~-f CH 2t~ -OH _. NH-fCH2~NH-~1CH2t~-+- n Both hexamethylenediamine and adipic acid contain six carbon atoms, which is why the resulting polymer is called nylon 6, 6. Like proteins, nylon 6,6 contains amide linkages between monomers, so nylon 6,6 is a polyamide, too. Also like proteins, nylon 6,6 is a copolymer, although nylon 6,6 is an alternating copolymer (Table 25.2), not a random copolymer. Dacron is the trade name for the condensation copolymer formed from ethylene glycol (a diol a molecule with two alcohol group s) and para-terephthalic acid (a diacid-a molecule with two carboxy groups). Th e condensation reaction removes the hydrogen atom from the alcohol on one monomer and the OH group from the carboxy group on a second monomer, producing water. 0 \\ n '\: C < / OH The bond that form s between each ethylene glycol monomer and each para-terephthalic acid monomer is called an ester linkage becau se RCOOR' is the general form of an ester [ ~1 Section 10.2, Table 10.2]. Polymers in which the monomers are connected by ester linkages are called polyesters. Deoxyribonucleic acid (DNA; see Figure 25.8), which stores the genetic information for every known organism and has a molecular weight as large as billions of grams per mole, is a biological condensation copolymer. If your body contained anything but a tiny fraction of a mole of DNA, you would be extremely heavy! DNA is a copolymer of a five-carbon sugar called deoxy- ribose, four different molecular ba ses (adenine, thymine, guanine, and cytosine), and phosphoric acid (H 3 P0 4 ) . Ribonucleic acid (RNA) is a biological condensation copolymer analogous to DNA, except that RNA is made from a fi ve-carbon sugar called ribose and the four different molecular ba ses are adenine, uracil, guanine, and cytosine. RNA molecules vary greatly in size. They are large molecule s, but still much smaller than DNA molecules. Sample Problem 25.2 shows how to determine the monomers in a copolymer. Sample Problem 25.2 Kevlar, a condensation polymer used in bulletproof vest s, has the following general structure: o NH~ (I II (I f NH-C < - Write the structure of the two monomers that form Kevlar. o II > C n Strategy Identify the condensation linkage s, split them apart, and then add H to one of the resulting • exposed bonds and OH to the other. o I Setup Kevlar contains an amide (linkage between C and N). We split the C-N bond, adding an H to the exposed N and an OH to the exposed C to produce an amine and a carboxylic acid , respectively. Solution To determine what the monomers are, first remove the C - N bond: o o II I -NH < > NH C- ) C Next, add H 2 0 as H to the Nand OH to the C: o o I I NH- HO-C- > C Finally, add the components of another water molecule to the open end s of the molecule s- H to N and OH to C, as before: o o II I HO-C <. > C-OH Practice Problem Nylon 6, like nylon 6,6, is a condensation polymer (a polyamide ). Unlike nylon 6,6, however, nylon 6 is not a copolymer because it is synthesiz ed from a single monomer called 6-aminohexanoic acid: o i Draw the structure of at least three repeating units of ny Ion 6. ~, ~ Bringing Chemistry to life Electrically Conducting Polymers You have seen so far that it is possible to polymerize organic compounds with carbon-carbon double bonds (alkenes). The resulting addition polymers (e.g., polyethylene) have carbon-carbon single bonds. It is also possible, though, to polymerize organic compounds with carbon-carbon tri- . . . . . . . . . . . . . . . . . p Ie bonds. The simplest of these compounds (known collectively as alkynes) is acetylene (C 2 H 2 ), which is commonly used in welding torches. The polymerization of acetylene is very similar to the addition polymerization of ethylene. That is, a free-radical initiator molecule attaches to one of the carbon atoms and breaks one of the pi bonds between the two carbon atoms in acetylene. The resulting polymer, polyacetylene, thus retains a carbon-carbon double bond: H-C C-H H-C-C-H H-C-C-H ~~~~~~ H H H I I I -_. -C=C-C=C-C=C- I I I H H H The generalized form of the reaction can be represented as follows: n H-C=C-H +. H I -+C=C+ I H n The structure of polyacetylene contains alternating carbon-carbon single and carbon- carbon double bonds. Recall from Chapter 9 that the carbon atoms in a C=C double bond are - SECTION 25.1 Polymers 943 • Think About It To determine whether the monomer s in the final answer are reasonable, recombine them in a condensation reaction to see if they re-form Kev lar, the polymer in the problem. The syst e mati c name of a cety lene is eth yn e, bu t t he common na me a cetyle ne is far m ore co mmo nl y u sed , w hi ch is so mewhat unfortunate si nce the -ene e nd in g ma ke s it so und lik e it is an alkene with a double b on d inste ad of an al kyne with a t rip le bond I , 944 CHAPTER 25 Modern Materials • Think About It The repeating polymer unit in the general structure resembles the original monomer except that there is a carbon-carbon double bond in place of a carbon-carbon triple bond. If each of the carbon-carbon single bonds at the ends of the repeating unit in parentheses were split open, then one electron from each bond could be combined to re-form the second pi bond in the carbon- carbon triple bond of the monomer. Sp2 hybridized and that it is the leftover p orbital on each carbon atom that overlaps to form the pi bond. Each carbon atom in polyacetylene is identical (structurally and electronically), so there is extended overlap of the p orbitals throughout the polymer chain. The p electrons are delocalized!- that is, they can move throughout the network of overlapping orbitals that extend the length of the polymer chain. By being able to move electrons from one end of the polymer chain to the other, poly acetylene can conduct electricity much like a wire. That is, polyacetylene is a plastic that conducts electricity! For comparison, consider polyethylene and Teflon. Both are polyalkenes in which the poly- mer chain contains only carbon-carbon single bonds. Since all the electrons are localized in sigma bonds between the carbon atoms, there are no delocalized electrons available to carry charge throughout the polymer chain. The presence of electrons in delocalized orbitals, such as those in polyacetylene, is the key to electrical conductivity. Sample Problem 25.3 shows how to determine the structure of an electrically conducting polymer. Sample Problem 25.3 Propyne ( HC=CCH 3 ) can be used to form an electrically conducting polymer. Draw the structure of polypropyne, showing at least three repeating units, and write the general formula for the polymer. Strategy Addition polymer s, whether they are synthesized from alkenes or alkynes, form via a free-radical reaction in which one pair of pi electrons in the carbon-carbon multiple bond of a monomer molecule is used to form carbon-carbon single bonds to other monomer molecules. Draw the structural formula of propyne such that the triple bond can be "opened up" to form single bonds between consecutive monomer units. Setup The structural formula of propyne can be determined from the formula given and by analogy to acetylene. The only difference between propyne and acetylene is the CH 3 group in place of one of the hydrogen atoms: H-C-C-CH 3 By drawing three adjacent propyne molecules,'we can rearrange the bonds to show three repeating units: Solution The structure of polypropylene showing three monomer units is CH 3 CH 3 CH3 I I I -C=C-C=C-C=C- I I I H H H The repeating unit is -CH=C(CH 3 ) - , so the general structure is CH3 I -+-C=C-+- I H n Practice Problem Draw the structures of poly(l-butyne) and poly(l-butene), showing three or more repeating units. How are they different? How are they similar? Which of them, if any, do you think will conduct electricity? H-C C CH2CH3 I-butyne H2C CH CH2CH3 I-butene SECTION 25.2 Ceramics and Composite Materials 945 Checkpoint 25.1 Polymers 25.1 .1 25.1.2 Classify the following copolymer: - B-B-B-B-B-A-A-A-A-A-B-B-B-B - B-A-A-A-A-A- (Select all that apply.) a) Block b) Random c) Graft d) Alternating e) All the above What feature is common to molecules that can undergo polymerization? a) Fluorine b) Hydrogen bonds c) Sulfur d) Multiple bonds e) Lone pairs ······· 25.2 Ceramics and Composite Materials Ceramics The use of ceramics in the form of pottery dates back to antiquity. Along with common ceramic substances like brick and cement, modern advanced ceramics are found in electronic devices and on the exterior of spaceships. All these ceramics are polymeric inorganic compounds that share the properties of hardness, strength, and high melting points. 'Cer<iiiiics ' are ' 'u'suaiiy' 'fai-mea' by melting and then solidifying inorganic substances (including clays). Most ceramic materials are electrical insulators, but some conduct electricity very well at very cold temperatures. Ceramics are also good heat insulators, which is why the outer layer of the space shuttle (shown in Figure 25.9) is made of ceramic tiles. These ceramic tiles can withstand the very hot temperatures of re-entry into the atmosphere (where temperatures can reach 1650°C), while keeping the underlying metal structural body of the orbiter (and the astronauts within) relatively cool. Ceramics can be prepared by heating a slurry of a powder of the inorganic substance in water to a very high temperature under high pressure. This process, called sintering, bonds the The larger the lattice energy of an ionic compound, the higher the melting point [ ~ Section 8.2, Table 8.1] . Ceramics all have large lattice energies. . • Multimedia Chemical Reactions- thermite reaction . Figure 25.9 Captain Wendy Lawrence inspects the ceramic tiles on the exterior of the space shuttle . • 946 CHAPTER 25 Modern Materials The sintering of bronze in the manufacture of bearings takes advantage of the inherent imperfections produced by the process. Porous bearings are sometimes preferred because the porosity allows a lubri cant to flow throughout the material. The "sol" in sol-gel refers to a colloidal suspension of individual particles. The "gel" refers to the suspension of the resulting polymer. • particles to each other, thus producing the finished ceramic. Sintering is a relatively easy process to perform, but the resulting particle size is irregular, so the solid may contain cracks, spaces, and . ' imperfections. Imperfections in the interior of a solid ceramic can be impossible to detect, but they make weak spots in the substance that can cause the ceramic piece to fail. To avoid these prob- lems, the sol-gel process is frequently employed for modem ceramics that are used in structural appilc · citlOns. · the' sol-gel process produces particles of nearly uniform size that are much more likely to produce a solid ceramic without gaps or cracks. The first step in the sol-gel process is the preparation of an alkoxide of the metal or metalloid that is going to be made into the ceramic. This can be illustrated with the yttrium(III) ion, which is used to prepare a yttrium-oxygen ceramic: Y + 3CH 3 CH 2 0H -_. Y(OCH 3 CH 2 )3 + 3H + yttrium alkoxide The resulting metal alkoxide is dissolved in alcohol. Water is then added to generate the metal hydroxide and regenerate the alcohol: Y(OCH 3 CH 2 )3 + 3H 2 0 +. Y(OH)3 + 3CH 3 CH 2 0H yttrium alkoride yttrium hydroxide ethanol The metal hydroxide, once fOlIned, undergoes condensation polymerization to form a chain with bridging oxygen atoms between the metal atoms: OH I HO-Y-O H OH I HO -Y-OH OH OH I I -_. HO-Y-O-Y-OH + H 2 0 (water) Just as in condensation polymerization between organic molecules, a small molecule (water, in this case) is produced by the combination of atoms from the two monomers. Repeated condensa- tion produces an insoluble chain of metal atoms connected by oxygen atoms. The polymer chain may contain branches (i.e., it may be three dimensional) because all three of the hydroxide groups attached to the central metal ion are identical (and are thus equally likely to react). The suspension of the metal oxide-hydroxide polymer is called a gel. The gel is carefully heated to remove the liquid, and what remains is a collection of tiny, remarkably uniformly sized particles. Sintering of material produced by the sol-gel process produces a ceramic with relatively few imperfections. Composite Materials A composite material is made from two or more substances with different properties that remain separate in the bulk material. Each contributes properties to the overall material, though, such that the composite exhibits the best properties of each of its components. One of the oldest examples of a human-made composite material is the formation of bricks from mud and straw, a process that dates back to biblical times. Modem composite materials commonly include reinforcing fibers, analogous to the straw in mud bricks, in a polymer matrix called a resin. Fiberglass, Kevlar, or carbon fibers can be used to give strength to the composite, whereas polyester, polyamide, or epoxy forms the matrix that holds the fibers together. These types of composites are called polymer matrix composites . . Composites made from a metal and a ceramic, organic polymer, or another metal are called metal matrix composites. Carbide drill tips are made with a combination of softer cobalt and tougher tungsten carbide. Toyota has used a metal matrix composite in the engine block of some of its cars, and some bicycles are made with aluminum metal matrix composites. Composites made of a ceramic as the primary reinforcing material accompanied by organic polymers are called ceramic matrix composites. A common biological ceramic matrix composite is bone. Bones consist of reinforcing fibers made of collagen (a protein) surrounded by a matrix of hydroxyapatite, [Ca5(P04)30H]. Another type of composite material that does not fit neatly into one of these categories is a . reinforced carbon-carbon composite (RCC), which consists of carbon fibers in a graphite matrix. To prevent oxidation, a silicon carbide coating is applied. RCC is used on the nose cones of mis- siles, but it is most commonly known as the coating on the nose of the space shuttle. It is believed that the Space Shuttle Columbia disaster in 2003 occurred because the ceramic tiles and RCC panel material were both damaged on liftoff by the impact of falling foam insulation. The extreme heat of reentry into the atmosphere damaged the orbiter's structure through the breaches in the ceramic tiles and the RCC panel, and the orbiter and all seven astronauts on board were lost. Carbon fibers can be woven into fabrics and threads to be used as the structural components of vehicles and for sporting equipment such as bicycles, tennis rackets, and skateboards. SECTION 25.3 Liquid Crystals 947 Liquid Crystals Liquid crystals are substances that exhibit properties of both liquids, such as the ability to flow and to take on the shape of a container, and those of crystals, such as a regular arrangement of particles in a lattice. Some substances exhibit liquid crystal behavior when they are melted from a solid. As the temperature increases, the solid liquefies but retains some order in one or two dimensions. At a higher temperature, the ordered liquid becomes a more conventional liquid in which there is no consistent orientation of the molecules. Liquids are isotropic, because their properties are independent of the direction of testing. Because particles in a liquid are free to rotate, there are molecules present in every possible ori- entation, so the bulk sample gives the same measurements regardless of the direction in which the measurements are taken or observations are made. Liquid crystals are anisotropic because the properties they display depend on the direction (orientation) of the measurement. A box full of pencils, all neatly aligned, is an analogy for anisotropy, because the pencils are long and nar- row. Looking at the pencils end on is different than looking at them from the side. What you see depends on the direction from which you view the pencils. Similarly, what you see when you look at anisotropic molecules depends on the direction from which you view them. Frederick Reinitzer discovered the first compound to exhibit liquid crystal behavior in 1888. Cholesteryl benzoate was observed to form an ordered liquid crystalline phase when melted, and this liquid crystalline phase became an ordinary liquid at higher temperatures: Cholesteryl benzoate The structure of cholestery I benzoate is fairly rigid due to the presence of the fused rings and 2 ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . sp -hybridized carbon atoms. The molecule is relatively long compared to its width, too. Rigid- Recall from Chapter 9 that there is free rotation about carbon-carbon single bonds (i.e., bonds between Sp3 -hybridized carbon atoms), but not about carbon-carbon double bonds (i.e., bonds between Sp2 -hybridized carbon atoms). ity and this particular shape makes it possible for the cholesteryl benzoate molecules to arrange themselves in an orderly manner, in much the way that pencils, chopsticks, or tongue depressors can be arranged. There are three different types of ordering (called nematic, smectic, and cholesteric) that a liquid crystal can adopt (see Figure 25.10). A nematic liquid crystal contains molecules ordered in one dimension only. Nematic molecules are all aligned parallel to one another, but there is no organization into layers or rows. A smectic liquid crystal is ordered in two dimensions. Smectic molecules are aligned parallel to one other and are further arranged in layers that are parallel to one another. Cholesteric liquid crystal molecules are parallel to each other within each layer, but each layer is rotated with respect to the layers above and below it. The layers are rotated because there are repulsive intermolecular forces between layers. Liquid crystalline substances were curiosities for many years but have found applications in many fields. Liquid crystal displays (LCDs) in calculators, watches, and laptop computers are pos- sible, for example, because polarized light can be transmitted through liquid crystals in one phase but not transmitted through another (Figure 25.11). Polarized light is produced by passing ordi- nary light (which oscillates in all directions perpendicular to the beam) through a filter that allows (a) (b) (c) Figure 25.10 (a) Nematic, (b) cholesteric, and (c) smectic liquid • crystals. • 948 CHAPTER 25 Modern Materials Figure 25.11 Liquid crystal display. When the current is on, the liquid crystal molecules are aligned and polarized light cannot pass through. When the current is off, the molecules are not aligned and light can pass through. • Figure 25.12 LCD thermometer. A material that changes color as the temperature changes is called thermochromic . . . . . . - , Current off: molecules not aligned, light passes, bright region Polarizer Top plate LC layer Bottom plate __ -I-Polarizer ~-t-Mirror Current on: molecules aligned, light blocked, dark region only the light waves oscillating in one particular plane to pass through. This plane-polarized light can be rotated by a twisted liquid crystal such that it is allowed to pass through another filter when a voltage is applied to the liquid. If the light passes through the liquid when the voltage is off, though, the light will not pass through. Thjs principle can be used in calculator and watch displays because each digit is made up of no more than the seven segments shown in Figure 25.11, and the specific digit depends on which segments are bright and which are dark. LCDs in laptop comput- ers and televisions are more sophisticated, but still operate on the same basic principle. Liquid crystals can also be used in thermometers (Figure 25.12). The spacing between crys- tal layers depends on temperature, and the wavelength of light reflected by the crystal depends on tills spacing. Thus, the color reflected by the liquid crystal will change with temperature, and this color can be used to indicate the temperature to which the liquid crystal is exposed. Sample Problem 25.4 lets you practice determining whether or not a molecule might exhibit liquid crystal properties. Would the following molecule make a good liquid crystal? Why or why not? o o ~C < Strategy Cho le steryl benzoate exhibits liquid crystal properties because it has structurally rigid regions (fused rings and si-hybridized carbon atoms) and because itis relatively long compared to SECTION 25.4 Biomedical Materials 949 its width. Examine the structure in question to see if it has rigid regions and to see if it is longer in one dimension than in another. Setup Carbon atoms that are sp2-hybridized contribute to the rigidity of a molecule's shape. Solution The left-hand portion of the molecule contains an Sp2_ hybridized carbon atom, a benzene ring, another sp2-hybridized carbon atom, and another benzene ring. These features are relatively rigid and should allow the molecule to maintain its shape when heated. The chain of CH 2 groups ending in a CH 3 group is not rigid due to free rotation about the carbon-carbon single bonds (i.e., sp3-hybridized carbon atoms). The overall shape of the molecule is longer than it is wide, though, which combined with a large portion that is rigid should make the molecule a good candidate for liquid crystal behavior. Practice Problem Which compound would you expect to exhibit liquid crystal behavior, and why? o I OH-C < ~ o > d <(I o I > C-OH >-CH2-C - CH2 ' /CH 2 , / CH2 , CH2 CH2 CH 3 Checkpoint 25.3 Liquid Crystals 25.3.1 Which of the following is a good analogy for anisotropy? (Select all that apply.) a) Glass of water b) Bucket of ice cubes c) Box of spaghetti d) Box of rice e) Box of macaroni Biomedical Materials 25.3.2 What characteristics make a molecule likely to exhibit liquid crystal properties? (Select all that apply.) a) Long narrow shape b) Flexibility c) Rigidity d) Low molar mass e) High molar mass Many modern materials are finding uses in medical application s. Replacement joints, dental implants, and artificial organs all contain modern polymers, composites, and ceramics. To func- tion successfully in a biomedical application, though, a material mu st first be compatible with the living system. The human body very easily recognizes foreign substances and attacks them in an effort to rid them from the body. Thus, a biomaterial must be designed with enough similarity to the body's own systems that the body will accept the material as its own. Additionally, if the poly- mer, ceramic, or composite contains leftover chemicals from it s manufacture, these contaminants may lead to adverse reactions with the body over the lifetime of the medical implant. It is impor- tant, therefore, that the substance be as pure as possible. The physical properties of a biomedical material are important, too , because the longer the material lasts, the less often the medical device has to be replaced during the lifetime of the patient. · Most biomedical materials must possess great strength and flexibility to perform in the body. For example, the materials in artificial joints and heart valves must be able to flex many times without breaking. Materials used in dental implants must show great hardness, moreover , so as not to crack during biting and chewing. It takes years of research to develop successful biomedical materials that meet these needs. Think About It Double bonds in a molecular structure indicate the presence of pi bonds. Recall that it is the pi bonds that restrict rotation about bonds in a molecule [ ~ Sect ion 9.5] and lend rigidity to the structure. • 950 CHAPTER 25 Modern Materials • Dental Implants Dental implant materials have been used for many years. The oldest dental fillings were made of various materials including lead (which fell out of favor due to its softness the danger of lead poisoning was as yet unknown), tin, platinum, gold, and aluminum. The remains of Confeder- ate soldiers have been shown to include fillings made of a lead-tungsten mixture, probably from shotgun pellets; tin-iron; a mercury amalgam; gold; and even radioactive thorium. (The dentist probably thought he was using tin, which is similar in appearance.) Many modern fillings are made of dental amalgam [ ~~ Chapter 19], a solution of several metals in mercury. Modern dental amalgam consists of 50 percent mercury and 50 percent of an alloy powder that usually contains (in order of abundance) silver, tin, copper, and zinc. These metals tend to expand slightly with age, causing fissures and cracks in the tooth that may require further intervention (e.g., crowns, root canals, or tooth replacement). Some people consider amalgam fillings to be unsightly, too. Dental composite materials are now used that have several advantages over traditional amal- gams. The composites can be made in a wide range of colors, for example, to match the color of the other teeth. The existing healthy portion of the tooth can be etched with acid, moreover, to cre- ate pores into which the composite material can bond. With traditional dental amalgam, the dentist must create indentations in the healthy tooth to hold the amalgam in place. Destroying a portion of the healthy tooth is undesirable and can be avoided with the use of the composite. The dental composite consists of a matrix (made from a methacrylate resin) and a silica filler. The composite material is applied to the cavity in a putty-like consistency and then is dried and polymerized by a photochemical reaction initiated by light of a particular wavelength. Because the light does not penetrate very far into the composite, the thickness of the applied composite is critical. If the layer of composite is too thick, some of the composite will remain soft. A properly constructed dental composite filling will last more than 10 years. Over time, though, composite fillings tend to shrink, leaving breaches that can cause leakage, a situation that must be addressed to prevent further tooth decay. Porcelain ceramic fillings and crowns are very common, but the ceramic has two disadvan- tages: it is very hard and can wear on the opposing teeth, and it is brittle and may crack if subjected to great force. Most dentists, therefore, do not recommend porcelain ceramic crowns and fillings for the molar teeth, which do the bulk of the grinding work during chewing. A material used in dentistry must have properties that maximize both patient comfort and the lifetime of the implant. Dental fillings and tooth replacements must be resistant to acids, for example, because many foods (such as citrus fruits and soft drinks) contain acids directly. Any food containing carbohydrates can produce acids, though, if traces are left in the mouth because the bacteria that reside there consume carbohydrates, producing acids in the process. These acids eat away at the natural hydroxyapatite in teeth, creating caries (cavities). Materials like dental amalgam, gold, and dental composite resist attack by acids in the mouth. A dental material should also have low thermal conductivity; that is, it should conduct heat poorly. This is especially important in applications where the implant material is in contact with the nerve inside the tooth. If the implant material conducts heat well, then whenever hot or cold foods come in contact with the implant, the hot or cold will be transmitted through the material to the nerve, causing pain. Metals are good thermal conductors, so dental amalgams must not touch nerves. Finally, dental materials should resist wear (so they last a long time) and they should resist expansion and contraction as the temperature fluctuates. The close spacing and precision fits involved in dental fillings and implants would result in discomfort and possibly failure of the implant if the material expanded or contracted appreciably with changes in temperature. Soft Tissue Materials Burn victims who have lost large amounts of skin do not have enough cells to grow new skin, so a synthetic substitute must be used. The most promising artificial skin material is based on a polymer of lactic acid and glycolic acid. Both of these compounds contain an alcohol group (-OH) and a carboxy group (-COOH), so they can form a polyester copolymer in a condensation reaction that mirrors the formation of Dacron polyester (Section 25.1): o 0 0 0 II II II II HOCH2C~, O _ H _ H ,t OCHC-OH +. +-OCH2C-OCHC-+- I I CH3 CH3 n glycolic acid lactic acid SECTION 25.4 Biomedical Materials 95 1 This copolymer forms the structural mesh that supports the growth of skin tissue cells from a source other than the patient. Once the skin cells grow on the structural mesh, the artificial skin is applied to the patient, and the copolymer mesh eventually disappears as the ester linkages are . . . . . . . . . hydrolyzed. During this treatment, the patients must take drugs that will suppress their immune systems, which ordinarily would attack these substances as foreign to prevent their bodies from rejecting the new skin. Sutures, commonly known as stitches, are now made of the same lactic acid-glycolic acid copolymer as the structural mesh of artificial skin. Before, sutures were made of materials that did not degrade so they eventually had to be removed again. This type of suture is still used in cases where the degrading suture is deemed a risk or there might be a need to reopen the incision at a later time. Biodegradable sutures are routinely used during operations on animals, too, which decreases the number of trips to the veterinarian, thereby reducing the stress on the pet and its owner. When portions of blood vessels need to be replaced or circumvented in the body, it may or may not be possible to obtain a suitable vascular graft from another part of the patient's body. Dacron polyester (Section 25.1) can be woven into a tubular shape for this purpose, and pores can . . be worked into the material so that the graft can integrate its elf into the tissue around it , as well as to allow capillaries to connect. Unfortunately, the body 's immune system recognizes this as a foreign substance and blood platelets stick to the walls of the Dacron graft, causing clotting. Artificial hearts and heart valves are potential solutions to the shortage of donor organs. Artificial hearts have not yet been perfected to the point that they can work on a permanent basis, but valve replacement is a common procedure that can successfully prolong a patient's life. The most common replacement heart valve is fixed in position by surrounding the valve with lactic acid-glycolic acid copolymer, the same polyester that is used in skin grafts and biodegradable sutures. The polymer is woven into a mesh ring that allows the body tissue to grow into the ring and hold it in place. Researchers today are using electrospinning to place nanofibers directly onto wounds. (Nanofibers are products of nanotechnology; see Section 25.5.) Electrospinning uses a high- voltage electric field to create a jet of polymer solution that transforms into a spiraling dry fiber only nanometers in diameter. The wound dressings applied by direct electrospinning have high rates of moisture transmission and bacterial resistance. Enzymes can be added to the solution con- taining the nanofiber polymer, and these enzymes can be released into the skin after the dressing is applied. Proteins can also be cross-linked onto the nanofiber, making possible many applications of the nanofiber wound dressing. Artificial Joints Joint replacements, such as artificial knees, elbows, and hips, require major surgery but have become relatively commonplace because suitable materials have become available for not only the bone replacement but also the contact surfaces. In knee replacement, for example, the goal is usu- ally to replace worn-out cartilage surfaces (which may have resulted from arthritis or injury). It is possible to replace shattered bones that cannot heal back to their original state as well. The materials originally used in joint replacements were ivory, metal s, and glass, but they are fast being replaced by polymeric plastics and ceramics. Polymethyl methacrylate (PMMA), which wa s the subject of Practice Problem 25.lB in Section 25.1 , and polyethylene have been used in many total joint replacements. However, these joints tend to fail over time when they are implanted in younger, more active patients, so efforts are under way to improve the wear quality of the PMMA and polyethylene parts. Although metallurgical research provided new substances that were much stronger and less likely to break, including titanium-based alloys, total joint replacements were predicted to last no more than 20 years in a patient before needing to be replaced again. Ultrahigh-molecular- weight polyethylene, treated with radiation to initiate cross-linking of the polymer chains and stored under inert nitrogen gas to stop free-radical damage to the chain from the radiation, is now commonly used in many replacement joint s, with the expectation of improved wear. It remains to be seen whether ultrahigh-molecular-weight polyethylene will prolong the life of these devices, since many of the joints have been in service in patients for less than a decade. Other surfaces have also been researched to reduce wear. Metal-on-metal bearing surfaces suffered early setbacks due to the manufacturing tolerances of the cobalt-based alloy used. As the bearing surfaces wore, they produced metal sludge, and high friction between components caus ed loosening of the joint. Modern metal-on-metal bearing surfaces have improv ed as the manufactur- ing has become more consistent and produced components lower in friction. H ydroly sis is e ssentially th e oppos ite of a con d ensation re a ct ion. , [...]... semiconductor because the semiconductivity has been enhanced by the addition of negative particles, the extra electrons It is also possible to dope a semiconductor with an element that has fewer valence electrons than the semiconductor itself For example, a silicon semiconductor could be doped with small amounts (again, parts per million) of gallium (Group 3A, three valence electrons) Now, instead of an... element and one Group 5A element, or one Group 2B element and one Group 6A element Ceramics are sintered to bond the particles to each other and to close any gaps that may exist in the material The sol-gel process is used to prepare ceramics for structural applications because it produces particles of nearly uniform size that are much more likely to produce a solid ceramic without gaps or cracks Nanotechnology... an electrical insulator based on the behavior of the electrons in the bands (Figure 25.15) The valence band is filled with the valence (bonding) electrons, whereas the conduction band is empty or only partially filled with electrons It is the conduction band that allows electrons to move between atoms The gap between the valence band and the conduction band can vary from nothing (for an electrical conductor)... semiconductor because gallium contributes three valence electrons and phosphorus contributes five, giving a total of eight valence electrons Semiconductors have also been formed between Group 2B elements (particularly Zn and Cd) and Group 6A elements Sample Problem 25.5 lets you practice identifying combinations of elements that can exhibit semiconductor properties ~ Sample State whether each of the following... valence electron than the natural semiconductor For the purpose of simplicity, we will consider a pure silicon semiconductor Silicon is a Group 4A element, so it has four valence electrons per atom A small (parts per million) amount of phosphorus (Group SA, five valence electrons) can be added, thus doping the silicon with phosphorus Since each phosphorus atom has an "extra" electron relative to the pure... This, in turn, makes it possible to align groups of tubes, potentially forming long strands and fibers Lightweight fibers and strands with tremendous strength have many applications in such things as auto parts, medical implants, and sports equipment Besides carbon, other elements have been fashioned into nanotubes, too Molybdenum sulfide nanotubes have been synthesized, for example, with openings between... Prize has been won by scientists who research and develop modern materials! You might be skeptical that two negatively charged electrons would attract each other, but there is evidence that all subatomic particles do exert some attractive forces on each other • , 958 CHAPTER 25 \ Modern Materials I Applying What You've Learned Now that you know more about polymers, we can apply some of the concepts you've... anisotropic, meaning that their apparent properties depend on the direction from which we view them • Thermoplastic polymers can be melted and reshaped, whereas thermosetting polymers assume their final shape as part of the chemical reaction that forms them in the first place • Addition polymers are formed via a radical reaction in which a pi bond in an alkene or alkyne is opened up to form new bonds to adjacent... not others This may have potential for the nanoscale storage of gases for fuel cells Many universities have developed large-scale facilities for nanotechnology research in collaboration with industrial partners Since the lower limits of size have been reached in the production of silicon chip-based electronic circuits, nanotechnology may provide the pathway to circuits that are much smaller than have . refers to a colloidal suspension of individual particles. The "gel" refers to the suspension of the resulting polymer. • particles to each other, thus producing the finished. bond the particles to each other and to close any gaps that may exist in the material. • The sol-gel process is used to prepare ceramics for structural applications because it produces particles. that form Kevlar. o II > C n Strategy Identify the condensation linkage s, split them apart, and then add H to one of the resulting • exposed bonds and OH to the other. o I Setup