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Materials Science > Topic 7- Polymers > Flashcards

Topic 7- Polymers Flashcards

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1
Q

What is the Eden Project made from?

A

Ethylene tetrafluoro ethylene (ETFE)

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2
Q

Does ETFE have high or low density?

A

Low density

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3
Q

Important feature of ETFE?

A

Optically transparent.

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4
Q

Why are polymers used a lot for buildings?

A

Cheap and easy to produce.

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5
Q

What are polymers and plastics?

A

Polymers are materials with large macro-molecules, of which plastics is just on group.

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6
Q

What 6 useful properties do plastics have that mean they are widely used?

A
  1. Good thermal/electrical insulation.
  2. Low density/
  3. Easy to manufacture and low cost.
  4. Useful as adhesives (including composites).
  5. Good transparency.
  6. Durable
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7
Q

When do plastics have poor durability?

A

Under UV light.

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8
Q

What is the main characteristic of thermoplastic polymers?

A

Melt when heated so can form different shapes.

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9
Q

What is the main characteristic of elastomers?

A

Very high strain to failure (carbon chains are curled up so will stretch out).

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10
Q

What is the main characteristic of thermosets?

A

Crosslinks mean they will not melt when reheated, so will burn.

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11
Q

What is the main characteristic of Polysiloxanes?

A

No carbon backbone - alternating string of oxygen and silicon atoms.

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12
Q

What is the plastic most commonly used in building and construction?

A

PVC

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13
Q

What is the main component in polymers?

A

Carbon

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14
Q

What are the three organic elements in polymers? What is the fourth (non-organic?) element?

A

Carbon (C), Hydrogen (H), Oxygen (O).

Silicon (Si)

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15
Q

How big are polymer molecules in comparison to hydrocarbons?

A

Gigantic (macro-molecules).

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16
Q

What is the bonding in a polymer?

A

Atoms bound by covalent bonds to form long and flexible chains, secondary bonds between chains.

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17
Q

Are polymers crystalline or amorphous?

A

Amorphous or partially crystalline.

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18
Q

What is the name of the repeated unit in a polymer chain?

A

‘mer’ unit.

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19
Q

What does polymer mean?

A

Many parts

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20
Q

What is polymerisation?

A

Process of reacting polymer monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains.

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21
Q

What is one mer of polyethylene (PE)?

A

H-C-H H-C-H

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22
Q

What is one mer of polyvinyl chloride (PVC)?

A

H-C-H H-C-Cl

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23
Q

What is one mer of polypropylene (PP)?

A

H-C-H H-C-CH3

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24
Q

What are the two determining characteristics of polymers?

A

Low stiffness and low melting points.

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25
Q

What does a large molecular ‘weight’ mean?

A

Very long chains.

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26
Q

What are linear polymers? Give three examples.

A

Single chains which are flexible. ‘Mass of spaghetti’. Van der Waals between molecules.
Polyethylene (PE), PVC, Polystyrene

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27
Q

What are branched polymers?

A

Side branches off chains. Packing is less efficient so lower density. These will be less crystalline than linear polymers.

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28
Q

Which two types of polymer molecular structure are thermoplastics (melt when reheated?

A

Linear and branched.

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29
Q

Which two types of polymer molecular structure are thermosets (burn when reheated)?

A

Cross-linked and network.

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30
Q

What are cross-linked polymers? Give an example.

A

Adjacent chains that are joined at various positions by covalent bonds.
Rubbers.

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31
Q

Why are cross-links often non-reversible?

A

Due to chemical reactions.

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32
Q

What is a network polymer?

A

A heavily cross-linked polymer.

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33
Q

What is the order of increasing strength for polymer molecular structure?

A

Linear, branched, cross-linked, network.

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34
Q

What are copolymers? Give two examples/

A

Composed of two ‘mer’ units. E.g. ABABAB.

Polyamides and polyesters.

35
Q

Why are the properties of copolymers often tailored and improved?

A

When more than one monomer is used, and irregular chain structure will result. This can be helpful to modify the intermolecular bonds.

36
Q

What are the two main types of polymerisation?

A

Condensation polymerisation.

Free radical polymerisation.

37
Q

What does condensation polymerisation do?

A

Eliminates a small molecule (H2O, HCl) by reacting a hyrdroxyl (-OH) and carboxylic acid (-C=O-OH) group to give an ester.

38
Q

What does free radical polymerisation do?

A

Produce an addition reaction. Something with a dangling (incomplete) bond.

39
Q

What are the four categories of copolymer?

A

Random, Alternating, Block, Graft.

40
Q

What is the structure of a random copolymer? What random copolymers are used in tyres?

A

ABBABABBAAB.

Synthetic rubbers such as styrene butadiene (SBR).

41
Q

What is the structure of an alternating copolymer?

A

ABABABABAB.

42
Q

What is the structure of a block copolymer?

A

BBBBBAAAAA.

43
Q

What might block copolymers be useful for?

A

Creating self-constructing fabrics with potential utility in semiconductor arrays.

44
Q

What is the structure of a graft copolymer?

A

Chains of different mers grafted onto side of a longer chain.

45
Q

How does the extent of cross-linking between chains influence the material?

A

Stronger attractive forces between chains leads to a stronger, less flexible polymer (e.g. nylon).

46
Q

What will make a polymer flexible?

A

If chains are able to slide past each other easily (VdW bonding).

47
Q

What do thermoplastics have a greater ability to form?

A

(Semi-) crystalline regions.

48
Q

Are thermoplastics ductile? Why?

A

Yes, long period of plastic deformation.

49
Q

Why are thermosets difficult to recycle?

A

Do not soften on reheating once formed.

50
Q

What percentage of mers are cross-links in thermosets?

A

10-50 %.

51
Q

Which type of polymer provides shape ‘memory’/retention?

A

Elastomers - linear with some cross-linking.

52
Q

What is ‘heat shrink’?

A

A property of elastomers to insulate electrical cables.

53
Q

What is molecular weight Mw?

A

Mass of a mole of chains.

54
Q

Which three properties are linked to molecular weight?

A

Stiffness, tensile strength and viscosity.

55
Q

How does tensile strength vary with number of repeating units?

A

Tensile strength increases with the number of repeating units.

56
Q

What is the approximate critical length needed before strength increases?

A

Approx. 100 but only around 40 for nylons.

57
Q

How are spherlites formed?

A

Folded chain lamellae grow and spread out to reach more crystallisable material. The ends of the lamellae twist producing spherelites.

58
Q

What is the optical feature of amorphous polymers?

A

Transparent in pure state.

59
Q

How does % crystallinity vary with molecular weight?

A

Greater the molecular weight, greater the % crystallinity.

60
Q

In what temperature range will crystallinity occur?

A

Between Tg and Tm.

61
Q

What is formation rate of crystals a product of?

A

Nucleation rate and growth rate.

62
Q

When in crystallisation growth rate a maximum?

A

Just below Tm.

63
Q

When is nucleation rate maximum?

A

Just above Tg.

64
Q

When is overall crystallisation formation rate maximum?

A

About halfway between Tg and Tm.

65
Q

What is the equation for total volume of a polymer?

A

V = Vc + Va

66
Q

What is the equation for mass of a polymer?

A

W = Wc + Wa

67
Q

What is the equation for density of a polymer?

A

pV = pcVc + paVa

68
Q

What is the equation for mass fraction of crystalline regions?

A

xc = Wc/W = pcVc/pV

69
Q

What two things can dramatically affect the mechanical properties of polymers?

A

The molecular structure.

Temperature at which it is operating.

70
Q

What will the structure of a brittle polymer consist of?

A

Heavily crosslinked and chains aligned.

71
Q

What happens when force is applied to a crystalline region of a polymer? (Semicrystalline)

A

Crystalline regions align themselves with direction of force.

72
Q

What is crazing? (Linear)

A

When molecules are drawn out on a very small scale producing strong strands which bridge microcracks. E.g. clear ruler turns white when you bend it.

73
Q

What happens when a polymer is cold drawn?

A

Molecules draw out and align so material is much stronger in draw direction than before. Annealing possible.

74
Q

How does cold drawing affect the properties of a polymer?

A

Increases elastic modulus and tensile strength.

Reduces ductility.

75
Q

How does the structure rearrange when an elastomer is stretched?

A

Chains are straight but still cross-linked.

76
Q

What does the mechanical state of a polymer depend on?

A

Glass transition temperature, Tg.

77
Q

What is the Tg for Polyethylene? What is its mechanical state at room temperature?

A

3 degrees C.

Ductile, low E.

78
Q

What is Tg for PMMA? What is its mechanical state at room temperature?

A

60 degrees C.

Brittle/ high E.

79
Q

How does the amount of side groups affect Tg and Tm?

A

More side groups mean less ease of rotation, therefore melting point and glass transition temperatures are higher.

80
Q

What is Tg and Tm for Polyvinyl Chloride and Polyethylene?

A

Polyvinyl chloride:
Tm = 220 degrees C Tg = 100 degrees C
Polyethylene:
Tm = 115 degrees C Tg = -18 degrees C

81
Q

What happens to stiffness, strength and ductility of PMMA as you reduce the temperature?

A

Strength and stiffness increase.

Ductility decreases.

82
Q

What are the three differences between injection moulding for thermoplastics and thermosets?

A
  1. Thermoplastic: screw root tapered. Thermoset: screw root straight.
  2. Thermoplastic: mold is cooled. Thermoset: mold is heated.
  3. Thermoplastic: two hoppers (resin and colorant). Thermoset: one hopper (just resin)
83
Q

What is important to consider when injection moulding thermosets?

A

To melt the resin but not let cross-links form until it is in the mold.

84
Q

What are four advantages of injection moulding polymers?

A
  1. Complex shapes.
  2. Rapid manufacturing.
  3. Thousands of samples can be manufactured (hence polymers are cheap).
  4. Very cheap once infrastructure in place.

Materials Science (8 decks)

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