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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)Lubrication applications

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Linear polymer molecules

PVC, polyvinyl

chloride

Plumbing, structural

plastics

PP, polypropylene

Fabrics, ropes,

structural plasticsFig 4.2

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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-butadiene-

styrene 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 UTS (ksi) E (ksi) Density (g/cc)

Low density

polyethylene 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 Specific Strength Specific stiffnessLow densitypolyethylene 11 27

Polyethylenefiber (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!)