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CHEMISTRY
The Central Science
9th Edition
Chapter 12
Modern Materials
David P. White
Prentice Hall © 2003
Chapter 12
Liquid Crystals
• 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.
Prentice Hall © 2003
Chapter 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 © 2003
Chapter 12
Liquid Crystals
Types of Liquid Crystalline Phases
• Smectic liquid crystals: usually contain C=N or N=N
bonds and benzene rings.
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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 © 2003
Chapter 12
Liquid Crystals
Types of Liquid Crystalline Phases
Prentice Hall © 2003
Chapter 12
Liquid Crystals
Types of Liquid Crystalline Phases
• Cholesteric liquid crystals: based on the structure of
cholesterol.
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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 © 2003
Chapter 12
Liquid Crystals
Types of Liquid Crystalline Phases
Prentice Hall © 2003
Chapter 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.
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Chapter 12
Polymers
Addition Polymerization
• Example: ethylene H2C=CH2, 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 © 2003
Chapter 12
Polymers
Condensation Polymerization
• Condensation Polymerization: molecules are joined by
the elimination of a small molecule (e.g. water):
O
H O
N H + H O C
N C
H
+ H O H
• Example of condensation polymerization: formation of
nylon.
• Physical properties of polymers can be designed by
understanding the structure of polymers.
Prentice Hall © 2003
Chapter 12
Polymers
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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.
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 © 2003
Chapter 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 104 amu (used in plastic wrap);
– high density polyethylene (HDPE) has an average molecular
mass of 106 amu (used in milk cartons).
Prentice Hall © 2003
Chapter 12
Polymers
Structure and Physical Properties of
Polymers
Prentice Hall © 2003
Chapter 12
Polymers
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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.
Prentice Hall © 2003
Chapter 12
Polymers
Cross-Linking Polymers
Polymers
Cross-Linking Polymers
Prentice Hall © 2003
Chapter 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 © 2003
Chapter 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 © 2003
Chapter 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.
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Chapter 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 © 2003
Chapter 12
Biomaterials
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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 © 2003
Chapter 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, -[-(CF2CF2)n-]-, is used for
smaller vascular grafts.
Prentice Hall © 2003
Chapter 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 © 2003
Chapter 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 © 2003
Chapter 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 (Al2O3), silicon carbide (SiC),
zirconia (ZrO2) and beryllia (BeO).
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Chapter 12
Ceramics
Prentice Hall © 2003
Chapter 12
Ceramics
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Processing of Ceramics
Small defects developed during processing make
ceramics weaker.
Sintering: heating of very pure uniform particles (about
10-6 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(OCH2CH3)4).
Sol formed by reacting alkoxide with water (to form
Ti(OH)4).
Prentice Hall © 2003
Chapter 12
Ceramics
Processing of Ceramics
Ti(s) + 4CH3CH2OH(l)  Ti(OCH2CH3)4 + H2(g)
Ti(OCH2CH3)4 + H2O(l)  Ti(OH)4 + 4CH3CH2OH(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 0.003 and 0.1
m in diameter.
Prentice Hall © 2003
Chapter 12
Ceramics
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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 © 2003
Chapter 12
Ceramics
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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 © 2003
Chapter 12
Superconductivity
• Superconductors show no resistance to flow of electricity.
• Superconducting behavior only starts below the
superconducting transition temperature, Tc.
• Meissner effect: permanent magnets levitate over
superconductors. The superconductor excludes all
magnetic field lines, so the magnet floats in space.
Prentice Hall © 2003
Chapter 12
Superconducting
Ceramic Oxides
Thin Films
• Thin films generally have a thickness between 0.1 m
and 300 m.
• Useful thin films must
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be chemically stable,
adhere well to the surface,
be uniform,
be pure,
have low density of imperfections.
Prentice Hall © 2003
Chapter 12
Thin Films
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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 © 2003
Chapter 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: MgF2, Al2O3, and SiO2.
Prentice Hall © 2003
Chapter 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 © 2003
Chapter 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:
TiBr4(g) + 2H2(g)  Ti(s) + 4HBr(g)
SiCl4(g) + 2H2(g)  Si(s) + 4HCl(g)
SiCl4(g) + 2H2(g) + 2CO2(g)  SiO2(s) + 4HCl(g) + 2CO(g)
3SiH4(g) + 4NH3(g)  Si3N4(s) + 12H2(g)
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Chapter 12
End of Chapter 12
Modern Materials
Prentice Hall © 2003
Chapter 12