Transcript Introduction to Materials Science and Engineering

Review of Polymers
Highlights from MY2100
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Polyethylene
“Monomer”
(Ethylene gas)
Polymer
(Polyethylene,
PE)
Milk jugs, structural plastics
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Linear polymer molecules
Polyethylene is typical of a large number of polymers. They have
a long “chain” backbone, with side groups attached to the
backbone. Each molecule is like a long fiber.
Teflon, (PTFE)
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Lubrication applications
Linear polymer molecules
PVC, polyvinyl
chloride
Plumbing, structural
plastics
PP, polypropylene
Fig 4.2
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Fabrics, ropes,
structural plastics
Polystyrene
Benzene ring, or phenol group:
Cheap, clear plastic
drink cups
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Other polymer backbones
Some linear polymers have more complex backbones:
Nylon
Fabrics, ropes,
structural plastics
Polycarbonate, PC
Shatter-resistant
clear plastic
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Degree of Polymerization/Molecular Weight
One of the most important features associated with polymer
structures is the size of the molecule. Most useful polymers
have huge molecule sizes.
Molecular weights of 25,000 g/mol are not uncommon.
This means that there are a large number of mers in the backbone.
Degree of Polymerization is average number of mers in a chain.
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Shape of molecules
C-C bond angle is 109o, but there is rotational freedom.
This means that the molecules are not straight, and will
form random 3-D messes, like a plate of spaghetti.
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Polyethylene Spaghetti
Note: 2-D representation.
Will actually wander in
3-D (into and out of
paper) as well.
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Molecule structures
Controlled by
chemistry
and
processing.
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Thermosets and thermoplasts
Thermoplastic polymers soften and melt when heated. They may
be recycled.
Thermosetting polymers stay hard, and eventually burn when
heated. They may not be recycled.
• Many thermosetting polymers are formed by a chemical
reaction called condensation polymerization, where two
chemicals are added to form the polymer. A common
example is epoxy, which is formed by combining a resin and
a hardener.
• Thermosets are often highly cross-linked.
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Copolymers
Copolymers are like polymer alloys. Different mers are joined to
form a mixture in the backbone.
Example: ABS
Acrylonitrile-butadienestyrene copolymer
• Football helmets
Copolymers may be tailored
to obtain specific properties.
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Crystalline polymers
All the polymers we have talked about so far are ordered at the
atomic scale (C-C bond angle, etc). But they are amorphous (no
long-range order) at the scale above atomic bonding.
By processing, we can impose
order on the polymer by regularly
arranging the chains. We call this
crystallization, even though it does
not look very much like the metal
and ceramic crystals from Ch 3.
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Crystallinity
Fold the polymer chains
over on each other in an
ordered way.
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Crystallinity
“Spherulite” crystal
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Glasses
A glass (amorphous material) is quite different than a crystalline
metal or ceramic. In a glass, there is no long-range crystalline
order.
• Therefore, the molecular structures of liquid and solid
glasses are not very different, and amorphous materials are
often called super-cooled liquids.
• Mechanical behavior changes gradually and continuously,
for example, the viscosity (related to the ability to blow a
glass) changes smoothly with temperature.
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Glass transition
The temperature above which the glass becomes soft and viscous
enough to work is related to the glass transition temperature, Tg.
Below the glass transition temperature, the material is relatively
hard and stiff; above it, it becomes more viscous.
This shows up in the volume/temperature curve.
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Mechanical Properties
Mechanical behavior of amorphous and semi-crystalline polymers
is strongly affected by the glass transition temperature.
In general (although there are exceptions):
• Polymers whose glass transition temperature is above the
service temperature are strong, stiff and sometimes brittle
 e.g. Polystyrene (cheap, clear plastic drink cups)
• Polymers whose glass transition temperature is below the
service temperature are weaker, less rigid, and more ductile
 Polyethylene (milk jugs)
If the service temperature changes, and Tg is crossed, the behavior
can change drastically.
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Typical examples
??
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Fibers
We see that both nylon and polyester have glass transition
temperatures that are above room temperature. So in bulk form
they are stiff and relatively brittle. Many plastic gears and
bushings are made of nylon.
We also know that many clothing items, which are very flexible,
yet very resistant to tearing, are made of nylon and polyester.
This is accomplished by making the material in the form of a
fiber.
A fiber is a long, thin strand of material. Since the fiber is so thin,
it is flexible.
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Typical properties of selected materials
Material
Low density
polyethylene
UTS (ksi)
E (ksi)
Density (g/cc)
10
25
0.92
Polyethylene
fiber (Spectra 900)
350
17,000
1
7075 Aluminum
90
10,000
2.8
4340 Steel Q+T
250
30,000
7.8
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Specific Strength
We see that the fibers have an excellent combination of low
density, high stiffness, and high strength. We quantify these
combinations by using specific properties.
 Specific
strength = strength/density
 Specific stiffness = modulus/density
The higher these properties are, the better is the performance of the
material concerning light-weight design.
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Specific properties
Material
Low density
polyethylene
Specific Strength
Specific stiffness
11
27
Polyethylene
fiber (Spectra 900)
350
17,000
7075 Aluminum
32
3,570
4340 Steel Q+T
32
3,840
units are ksi/(g/cc) (should clean this up!)
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