ENGI1002 - Topic 4

What are polymers?

Polymers are a branch of materials comprised of many repeat units.

What are some natural polymers?

• Latex

• Proteins

• DNA

What are saturated hydrocarbons?

In saturated hydrocarbons, each carbon is bonded to four other atoms. Polyethylene is a saturated polymer.

What are unsaturated hydrocarbons?

Unsaturated hydrocarbons contain double or triple bonds.

What is isomerism?

Two compounds with the same chemical composition can have quite different structures.

What is free radical polymerisation?

Free radical polymerisation is used in the synthesis of polyethylene, polypropylene, poly (vinyl chloride), and polystyrene.

What are the polymer molecules?

• Homopolymer

• Co-polymer

• Bi-functional

• Tri-functional

What is a homopolymer?

Composed of the same repeat unit.

What is a copolymer?

Composed of two or more different repeat units. Two or more monomers polymerized together.

What is functionality?

Functionality is the number of bonds that a given monomer can have.

What properties are affected by a polymers molecular weight?

• Strength

• Stiffness

• Toughness

How to change molecular configuration?

To change molecular configuration, bonds must be broken.

What does configuration describe?

1. The direction in which monomers are linked together

2. The order in which monomers are joined together.

What is tacticity?

Tacticity is the stereoregularity of chains (arrangement of R groups).

What are the three types of tacticity?

1. Isotactic

2. Syndiotactic

3. Atactic

What does isotactic mean?

All R groups are on the same side of the chain.

What does syndiotactic mean?

R groups alternate sides.

What does atactic mean?

R groups are random.

What changes a polymer’s conformation?

Conformation is only changed by bond rotation.

What is cis-isoprene structure?

Bulky groups are on the same side of the chain.

What is trans-isoprene structure?

Bulky groups are on opposite sides of the chain.

Can cis and trans configurations change by bond rotation?

Conversion of cis to trans is not possible by a simple chain bond rotation. Bonds must be broken.

What are linear polymers?

Repeat units are joined together end-to-end in single chains.

What forces act in linear polymers?

Van der Waals forces act between chains in linear polymers.

What are some examples of linear polymers?

• Polyethylene

• Poly (vinyl chloride)

• Polystyrene

• Nylon

What are branched polymers?

Contain side-branch chains which are connected to the main ones.

What are cross-linked polymers?

Adjacent linear chains are joined at various positions by covalent bonds.

What are network polymers?

Multifunctional monomers forming three or more active covalent bonds.

What are some network polymer examples?

• Epoxies

• Polyurethanes

What is the strongest polymer chain?

Network polymers are the strongest.

What polymers are thermoplastics?

• Linear polymers

• Branched polymers

What polymers are thermosets?

• Cross-linked polymers

• Network polymers

Temperature’s affect on thermoplastics?

Thermoplastics soften when heated and harden when cooled.

What’s stronger, thermoplastics or thermosets?

Thermosets are harder and stronger than thermoplastics.

What’s denser, crystalline or amorphous polymers?

Crystalline polymers are denser than amorphous polymers.

What does crystallinity depend on?

1. Rate of cooling during solidification

2. Nature of repeat unit

How do the different types of tacticity crystallise?

Isotactic and syndiotactic polymers crystallise more readily than atactic polymers.

What factors influence the mechanical properties of semicrystalline polymers?

• Temperature

• Molecular weight

• Degree of crystallinity

• Heat treating (annealing)

What is glass transition at Tg?

Rubbery to rigid solid as T decreases.

What is melting at Tm?

Solid to liquid as T increases.

What is melting?

Melting is the rearrangement of molecules into a loosely bound amorphous state.

Factors that influence melting temperature?

1. Presence of double bonds

2. Bulky side groups

3. Polar groups

4. Molecular weight

5. Degree of branching

What is glass transition temperature?

When a solid polymer transitions from a rigid to a rubbery state.

Tg increases with?

• Branching

• Cross-linking

What are the seven polymer categories?

1. Plastics

2. Elastomers (rubber)

3. Fibres

4. Coatings

5. Adhesives

6. Foams

7. Films

What are plastics?

Plastics are solid molecules with some structural rigidity under load. Plastics can be thermoplastics or thermosets.

What are elastomers?

Elastomers experience large and reversible elastic deformation.

What are additives?

Additives are used to improve mechanical properties, processing, and durability.

What are fillers?

Fillers are added to improve tensile strength and abrasion resistance.

What are plasticizers?

Plasticizers are added to reduce glass transition temperature.