Prentice Hall © 2003Chapter 12 Chapter 12 Modern Materials CHEMISTRY The Central Science 9th Edition David P. White

Prentice Hall © 2003Chapter 12 Solids are characterized by their order. Liquids are characterized by almost random ordering of molecules. There is an intermediate phase where liquids show a limited amount of ordering: –the liquid flows (liquid properties) but has some order (crystal properties). –Example: cholesterol benzoate above 179  C is clear. Between 145  C and 179  C cholesterol benzoate is milky and liquid crystalline. Liquid Crystals

Prentice Hall © 2003Chapter 12 Liquid Crystals Types of Liquid Crystalline Phases Liquid crystal molecules are usually long and rod-like. Three types of liquid crystalline phase depending on ordering: –nematic liquid crystals (least ordered): ordered along the long axis of the molecule only; –smectic liquid crystals: ordered along the long axis of the molecule and in one other dimension; –cholesteric liquid crystals (most ordered).

Prentice Hall © 2003Chapter 12 Liquid Crystals Types of Liquid Crystalline Phases Smectic liquid crystals: usually contain C=N or N=N bonds and benzene rings. –Recall: C=N and N=N structures are planar. –Recall: there is no rotation about C=N and N=N bonds. –Therefore, the molecules are rigid. –Also, the benzene rings (planar) add stiffness. –The molecules are long and rod-like.

Prentice Hall © 2003Chapter 12 Liquid Crystals Types of Liquid Crystalline Phases

Prentice Hall © 2003Chapter 12 Liquid Crystals Types of Liquid Crystalline Phases Cholesteric liquid crystals: based on the structure of cholesterol. –Molecules aligned along their long axis. –In addition the molecules are arranges in layers. –There is a twist between layers. –The molecules are long, flat, rod-like with a flexible tail. –The flexible tail causes the twist between the layers.

Prentice Hall © 2003Chapter 12 Liquid Crystals Types of Liquid Crystalline Phases

Prentice Hall © 2003Chapter 12 Polymers Polymers are giant molecules that are made up of many, many smaller molecules. Building blocks for polymers are called monomers. Examples: plastics, DNA, proteins, rubber etc. Carbon compounds have an unusual ability to form polymers.

Prentice Hall © 2003Chapter 12 Polymers Addition Polymerization Example: ethylene H 2 C=CH 2, can polymerize by opening the C-C  bond to form C-C  bonds with adjacent ethylene molecules. The result: polyethylene. This is called addition polymerization because ethylene molecules are added to each other.

Prentice Hall © 2003Chapter 12 Polymers Condensation Polymerization Condensation Polymerization: molecules are joined by the elimination of a small molecule (e.g. water): Example of condensation polymerization: formation of nylon. Physical properties of polymers can be designed by understanding the structure of polymers.

Polymers Types of Polymers Plastic: materials that can be formed into shapes. Thermoplastic: materials that can be shaped more than once. Thermosetting: materials that can only be shaped once. Elastomer: material that is elastic in some way. If a moderate amount of deforming force is added, the elastomer will return to its original shape. Useful for fibers.

Prentice Hall © 2003Chapter 12 Polymers Structure and Physical Properties of Polymers Polymer chains tend to be flexible and easily entangled or folded. Degree of crystallinity is the amount of ordering in a polymer. Stretching or extruding a polymer can increase crystallinity.

Prentice Hall © 2003Chapter 12 Polymers Structure and Physical Properties of Polymers Degree of crystallinity is also determined by average molecular mass: –low density polyethylene (LDPE) has an average molecular mass of 10 4 amu (used in plastic wrap); –high density polyethylene (HDPE) has an average molecular mass of 10 6 amu (used in milk cartons).

Prentice Hall © 2003Chapter 12 Polymers Structure and Physical Properties of Polymers

Prentice Hall © 2003Chapter 12 Polymers Cross-Linking Polymers Bonds formed between polymer chains make the polymer stiffer. Natural rubber is too soft and chemically reactive to make a useful material. By vulcanizing the rubber (crosslinking the polymer chains) useful materials are made. Rubber is usually cross-linked with sulfur. Cross-linked rubber is stiffer, more elastic and less susceptible to chemical reaction.

Polymers Cross-Linking Polymers

Prentice Hall © 2003Chapter 12 Polymers Cross-Linking Polymers

Prentice Hall © 2003Chapter 12 Biomaterials Characteristics of Biomaterials Biomaterials are any materials that have biomedical applications. For example, the materials that are used to fill teeth are biomaterials. The biomaterials must be biocompatible: The body’s immune system must not attack the biomaterial.

Prentice Hall © 2003Chapter 12 Biomaterials Characteristics of Biomaterials Physical requirements: Biomaterials must be created for a specific environment. Artificial heart valves must open and close 70 to 80 times per minute. Chemical requirements: Biomaterials must be of medical grade. Polymers are very important biomaterials.

Prentice Hall © 2003Chapter 12 Biomaterials Polymeric Biomaterials The degree to which the body tolerates foreign materials depends on the nature of the atomic groups in the material. Naturally occurring biomaterials are polymers of sugars and nucleotides. These polymers are polyamino acids.

Prentice Hall © 2003Chapter 12 Biomaterials Examples of Biomaterial Applications Heart Replacement and Repairs: A heart that fails completely must be replaced by a donor organ. About 60,000 people in the US suffer heart failure and only 2,500 donor hearts are available. About 250,000 heart valve replacements are made each year. About 45 % of these valve replacements occur with a mechanical valve.

Prentice Hall © 2003Chapter 12 Biomaterials Examples of Biomaterial Applications The replacement valve must be smooth to prevent the destruction of blood vessels. The valve must also be anchored to the inside of the heart. Polyethylene terephthalate, called Dacron™, is often used in the manufacture of artificial heart valves. Dacron™ is used because tissue will grow through a polyurethane mesh.

Prentice Hall © 2003Chapter 12 Biomaterials Examples of Biomaterial Applications Vascular grafts: A vascular graft is the replacement for a piece of blood vessel. Dacron™ is used for large arteries. Polytetrafluoroethylene, -[-(CF 2 CF 2 ) n -]-, is used for smaller vascular grafts.

Prentice Hall © 2003Chapter 12 Biomaterials Examples of Biomaterial Applications Artificial Tissue: Artificial skin, which is grown in the laboratory, is used to treat patients with extensive skin loss. The challenge with growing artificial skin is getting the cells to align properly. Therefore a scaffold must be used for the cells. The most successful scaffold is lactic acid-glycolic acid copolymer.

Prentice Hall © 2003Chapter 12 Biomaterials Examples of Biomaterial Applications Hip Replacements: About 200,000 total hip replacements are performed each year. A metal ball, a cobalt chromium alloy, is often used in a hip replacement. This alloy is attached to a titanium alloy and cemented using a tough thermoset polymer. The acetabulum, which accommodates the femur, is lined with a polyethylene layer.

Prentice Hall © 2003Chapter 12 Ceramics Ceramics are –inorganic, nonmetallic, solids, crystalline, amorphous (e.g. glass), hard, brittle, stable to high temperatures, less dense than metals, more elastic than metals, and very high melting. Ceramics can be covalent network and/or ionic bonded. Typical examples: alumina (Al 2 O 3 ), silicon carbide (SiC), zirconia (ZrO 2 ) and beryllia (BeO).

Prentice Hall © 2003Chapter 12 Ceramics

Prentice Hall © 2003Chapter 12 Ceramics Processing of Ceramics Small defects developed during processing make ceramics weaker. Sintering: heating of very pure uniform particles (about m in diameter) under pressure to force particles to bond. Sol-gel process: formation of pure uniform particles. Metal alkoxide is formed (e.g. Ti(OCH 2 CH 3 ) 4 ). Sol formed by reacting alkoxide with water (to form Ti(OH) 4 ).

Prentice Hall © 2003Chapter 12 Ceramics Processing of Ceramics Ti(s) + 4CH 3 CH 2 OH(l)  Ti(OCH 2 CH 3 ) 4 + H 2 (g) Ti(OCH 2 CH 3 ) 4 + H 2 O(l)  Ti(OH) 4 + 4CH 3 CH 2 OH(l) Gel is formed by condensing the sol and eliminating water. Gel is heated to remove water and is converted into finely divided oxide powder. Oxide powder has particle sizes between and 0.1  m in diameter.

Prentice Hall © 2003Chapter 12 Ceramics Ceramic Composites Composite: two or more materials making up a ceramic. Result: tougher ceramic. Most effective method: add fibers to ceramic material. Example: SiC fibers added to alluminosilicate glass. Fiber must have a length  100 times its diameter.

Prentice Hall © 2003Chapter 12 Ceramics Applications of Ceramics Used in cutting tool industry. Used in electronic industry (semiconductor integrated circuits usually made of alumina). Piezoelectric materials (generation of an electrical potential after mechanical stress) used in watches and ultrasonic generators. Used to make tiles on the space shuttle.

Prentice Hall © 2003Chapter 12 Superconductivity Superconductors show no resistance to flow of electricity. Superconducting behavior only starts below the superconducting transition temperature, T c. Meissner effect: permanent magnets levitate over superconductors. The superconductor excludes all magnetic field lines, so the magnet floats in space.

Superconducting Ceramic Oxides

Prentice Hall © 2003Chapter 12 Thin Films Thin films generally have a thickness between 0.1  m and 300  m. Useful thin films must –be chemically stable, –adhere well to the surface, –be uniform, –be pure, –have low density of imperfections.

Prentice Hall © 2003Chapter 12 Thin Films Uses of Thin Films Microelectronics (conductors, resistors and capacitors). Optical coatings (reduce reflected from a lens). Protective coatings for metals. Increase hardness on tools. Reduce scratching on glass.

Prentice Hall © 2003Chapter 12 Thin Films Formation of Thin Films Vacuum Deposition – thin film to be vaporized without breaking chemical bonds. –Material is placed in one chamber and the objects to be coated in another. –Pressure is reduced (low pressure means low sublimation point) while the material is heated. –The material vaporizes and condenses on the object to be coated. –To ensure an even coating, the objects are often rotated. –Examples: MgF 2, Al 2 O 3, and SiO 2.

Prentice Hall © 2003Chapter 12 Thin Films Formation of Thin Films Sputtering –The material used in the thin film is removed from the target using a high voltage. –The atoms move through and ionized gas in a chamber and they are deposited on the substrate. –The target is the negative electrode and the substrate the positive electrode. –Ar atoms (in the chamber) are ionized to Ar +. The Ar + ions strike the negative electrode and force an M atom to be ejected. M atoms have a high kinetic energy and travel in all directions. Some M atoms eventually hit the substrate and are coated.

Prentice Hall © 2003Chapter 12 Thin Films Formation of Thin Films Chemical Vapor Deposition –Surface is coated with a volatile compound at a high temperature (below the melting point of the substrate). –On the surface, the compound undergoes a chemical reaction to form a stable coating. –Examples: TiBr 4 (g) + 2H 2 (g)  Ti(s) + 4HBr(g) SiCl 4 (g) + 2H 2 (g)  Si(s) + 4HCl(g) SiCl 4 (g) + 2H 2 (g) + 2CO 2 (g)  SiO 2 (s) + 4HCl(g) + 2CO(g) 3SiH 4 (g) + 4NH 3 (g)  Si 3 N 4 (s) + 12H 2 (g)

Prentice Hall © 2003Chapter 12 End of Chapter 12 Modern Materials