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Materials Science > Polymers > Flashcards

Polymers Flashcards

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1
Q

What are the features of plastics?

A
  • Good thermal/electrical insulation
  • Low density
  • Easy to manufacture
  • Low cost
  • Useful as adhesives
  • Good transparency
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2
Q

What are polymers?

A
  • Polymers are materials with large macro-molecules, of which plastics are just one group.
  • The extent of cross-linking between chains influences the final material.
  • Stronger attractive forces between chains leads to a stronger, less flexible polymer (e.g. nylon).
  • If chains are less able to slide past each other, the polymer is likely to be flexible.
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3
Q

What are the different types of polymer?

A
  • Thermoplastics
  • Thermosets
  • Elastomers
  • Polysiloxanes
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4
Q

What are thermoplastics?

A
  • Linear/branches chains with minimal cross-linking.
  • Weak intermolecular bonds allow melting when heated.
  • Greater ability to form crystalline regions.
  • Ductile.
  • e.g. polyethylene, polypropylene, polystyrene
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5
Q

What are thermosets?

A
  • Heavily cross-linked.
  • React and harden causing the material to burn when heated.
  • Brittle.
  • e.g. vulcanised rubber, epoxies, phenolic resin
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6
Q

What are elastomers?

A
  • Linear with some cross-linking, allowing them to carry much higher strains.
  • Shape memory.
  • Useful for heat shrink, insulating electrical cables, and structural bearing pads.
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7
Q

What are the different types of chains in polymers?

A

Linear polymers:

  • Single chains
  • Flexible

Branched polymers:

  • Side branches
  • Less efficient packing
  • Lower density
  • Less crystalline

Cross-linked polymers:

  • Adjacent chains are joined at various positions by covalent bonds
  • Sometimes referred to as a network
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8
Q

What are copolymers?

A

Copolymers consist of two ‘met’ units, resulting in an irregular chain structure which can improve or tailor specific properties by modifying the intermolecular bonds.

Random copolymers:

  • ABBABA

Alternating copolymers:

  • ABABAB

Block copolymers:

  • AAABBB

Graft copolymers:

  • AAAAAA

BB

(active site)

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9
Q

What is polymerisation?

A

Polymerisation is the chemical process of reacting monomer molecules to form 3D networks or polymer chains.

Condensation polymerisation:

  • Elimination of small molecules

Free radical polymerisation:

  • Produces an addition reaction
  1. Initiation - free bonds produced
  2. Propagation - chains get longer
  3. Termination - longer molecules join together
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10
Q

What is the molecular weight?

A
  • The mass of a mol of chains.
  • It affects the tensile strength, stiffness and viscosity of the material.
  • Longer chains are more likely to entangle and therefore can ‘anchor’ better.
  • The critical length before strength increases is 100 ‘mers’ (40 for nylon).
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11
Q

How do you solidify a polymer?

A
  1. Annealing causes crystalline regions to grow.
  2. These crystalline regions are resistant to solvents.
  3. Secondary bonding such as hydrogen bonds and dipole - dipole reactions are very important.
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12
Q

How are crystalline regions discouraged?

A
  • Random copolymerisation
  • Bulky side groups
  • Chain branching
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13
Q

What is the difference between semi-crystalline and amorphous structures?

A

Semi-crystalline:

  • Stiff
  • Less ductile
  • High tensile strength

Amorphous:

  • Less stiff
  • Ductile
  • Low tensile strength
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14
Q

What is the crystallinity and molecular weight of plastics/waxes/grease?

A
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15
Q

Which factors affect the rate of crystallisation?

A
  • For crystallisation, considerable molecular mobility is necessary.
  • The formation rate of these crystals is the product of the nucleation rate and the growth rate.
  • Growth rate diffusion is controlled and at a maximum just below the melting point.
  • Nucleation rate is at a maximum just above the glass transition temperature.
  • Overall formation rate is maximum half way between these two temperatures.
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16
Q

What is the affect of crazing?

A
  • Crazing in linear polymers produces voids which scatter light.
  • Molecules are drawn out on a very small scale producing strong strands which bridge microcracks.
17
Q

Which factors affect the mechanical properties of polymers?

A
  • Molecular structure
  • Operating temperature
18
Q

What is the affect of cold drawing?

A
  1. Cold drawing causes molecules to be drawn out and aligned.
  2. This makes the material much stronger in a particular direction that it once was.
  3. Therefore, elasticity and tensile strength are increased but the ductility is reduced.
19
Q

What does the stress-strain graph for different polymers look like?

A
20
Q

What effect does temperature have on polymers?

A
  1. When heated, polymers deform plastically.
  2. Therefore ductility increases, but stiffness and strength decrease.
  • Chain stiffness and the bulkiness affects the ease of rotation of side groups.
  • Bulkier side groups are harder to rotate, and so melting temperature is higher.
  • The presence of polar side groups can also lead to significant intermolecular bonding forces.
21
Q

What is the glass transition temperature equation?

A
22
Q

How are polymers formed?

A
  • Injection moulding
  • Extrusion
  • Thermoforming
  • Blow moulding
  • Compression moulding
23
Q

What is injection moulding?

A

Thermoplastic - two hoppers feed resin and colourant onto a tapered screw root which forces them into a cooled mould.

Thermoset - a hopper feeds resin onto a screw root which forces it into a heated mould.

  • Produces complex shapes.
  • Rapidly manufactured.
  • Moulds are expensive, but production is very cheap once installed.
24
Q

What is extrusion?

A
  1. The polymer is heated causing it to melt into a liquid.
  2. This is then forced through a die of required cross-section.
  3. The polymer is then extruded out of the die and cooled.
  • This produces complex shapes.
  • Rapidly manufactured.
  • Moulds are expensive, but production is very cheap.
25
Q

What is thermoforming?

A
  • Different types such as vacuum, pressure and mechanical forming.
  • A sheet of material is heated before being pressed onto a one-sided mould in order to shape.
26
Q

What is blow moulding?

A
  • High temperature and stretching in the desired direction causes molecules to polarise, line up and essentially crystallise.
  • This can be used to produce bottles with a greater strength.
27
Q

What is compression moulding?

A
  1. A preheated platform is placed in the open mould, which then closes increasing compression.
  2. This causes the resin to become a hot fluid.
  3. The mould is then held shut until the resin cures, before opening and ejecting the finished part.
  • Used in thermoset materials.
28
Q

What are some common additives used in polymers?

A
29
Q

What are polysiloxanes?

A

The backbone of polysiloxanes consist of Si-O-Si units (commonly known as silicones).

30
Q

What are the benefits of using polysiloxanes?

A
  • Good electrical insulator
  • Good flexibility
  • Thermal stability over wide range of temperatures
  • Resistant to oxygen, ozone and UV light
  • Non-stick
  • Low chemical reactivity and toxicity
  • Doesn’t support microbial growth
31
Q

What are the applications of polysiloxanes?

A
  • Silicone sealants
  • Coatings
  • Fire protection
  • Glazing seals
  • Plumbing

Materials Science (8 decks)

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