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Materials 121 > Chapter 7 - Polymers > Flashcards

Chapter 7 - Polymers Flashcards

1
Q

True/False: Plastics are a kind of polymer.

A

False. In material engineering the word plastic refers exclusively to plastic deformation.

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2
Q

What does the microstructure of polymers look like?

A

Spaghetti. Long molecular structures, tangled together.

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3
Q

Define: Polymer.

A

The term polymer refers to a type of material with a structure made up of many repeating molecular units (called “mer”). These units are connected together into long chains.

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4
Q

True/False: Ethylene is also known as Ethene.

A

True.

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5
Q

Define: Polymerisation.

A

Polymerisation refers to the reaction that breaks down a molecule and then bonds them together into a chain.

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6
Q

How many “mers” are in a single chain of polyethylene?

A

3,500-25,000.

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7
Q

What is the “Degree of Polymerisation”?

A

An expression for the average length of the chain in mer.

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8
Q

How is “Degree of Polymerisation” calculated?

A

DP = Average Molecular Weight of the Chain / Molecular Weight of the Mer.

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9
Q

True/False: The “Degree of Polymerisation” always provides a steady number.

A

False. The degree of polymerisation is variable so we often take the average.

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10
Q

True/False: Polymer molecules perform straight chains.

A

False. The chains that are formed assume a zigzag formation. This is due to the 109.5* angle between the carbon atoms.

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11
Q

What kind of bond is the Carbon-Carbon bond in a polymer?

A

Covalent bonds.

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12
Q

True/False: Because the angle between the carbon atoms is fixed the polymer chains have to form pretty simple zigzag lines.

A

False. While the bonds always have the same angle they can rotate, allowing the chains to bend in three dimensions. This is why it looks so tangled.

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13
Q

What kind of bonds are there between the different chains?

A

The weaker secondary bonds are usually hydrogen, Van Der Waal’s etc.

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14
Q

How does having a short chain (low degree of polymerisation) affect the physical properties.

A

Because the chains are short, the chains are only attached by a few secondary bonds which means it’ll have a really low strength.

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15
Q

How does having a long chain (high degree of polymerisation) affect the physical properties.

A

Because the chains are long, the chains are attached by long array of secondary bonds which means it’ll have a really high strength.

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16
Q

What are co-polymers?

A

Co-polymers are polymers with two or more chemically different mers in different sequences.

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17
Q

What are the types of co-polymers?

A
  • Alternating Co-Polymers
  • Block Co-Polymers
  • Random Co-Polymers
  • Graft Polymer
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18
Q

What are linear polymers?

A

Repeating units joined end to end with no side branches. Can pack in close together (high density).

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19
Q

What are branched polymers?

A

Main polymer chain, to which side branches are connected. Branching reduces how close chains can pack together (low density).

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20
Q

What are cross-linked polymers?

A

Linear polymer chains that are joined by covalently bonded chain segments to form a 3D structure. Cross links prevent chains moving relative to each other (stiff, hard, strong).

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21
Q

What are thermoplastic polymers?

A

Linear/branched polymers with weak secondary bonds that soften with increasing temperature.

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22
Q

How does heat affect thermoplastic polymers?

A

The vibration reduces secondary bonding, allowing the chains to move relative to each other easily, then when it cools the secondary bonds reform.

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23
Q

What are thermosetting polymers?

A

Cross-linked polymer chains, with strong covalent bonds between chains. This causes them to be unable to melt or soften with heat.

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24
Q

How does heat affect thermosetting polymers?

A

Heat causes thermosetting polymers to burn, degrade and char.

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25
Q

What are Elastomers?

A

Polymers that can be stretched to large deformations and elastically spring back to their original dimensions.

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26
Q

Why are elastomers able to stretch?

A

The chains are highly coiled/kinked which allows them to straighten out as a load is applied, this caused the structure to elongate in the direction of the applied load.

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27
Q

Why do elastomers not plastically deform?

A

Elastomers have a few cross links, these cause the chains to be unable to permanently move relative to each other.

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28
Q

What is a completely amorphous polymer?

A

Completely amorphous polymers have no long-range order (completely random).

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29
Q

What are partially crystalline polymers?

A

Sections of the polymers are arranged in a regular repeating manner to produce a long-range ordered structure. Regions of ordered structure can be considered as “Crystallite’s”.

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30
Q

What effect does crystallinity have on polymers?

A

Higher crystallinity means higher strength (because there’s a higher concentration of secondary bonds in crystalline regions).

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31
Q

True/False: It is not possible to have a completely crystalline polymer.

A

True, the degrees of crystallinity can range from 5%-95%, but even if the chains are perfectly aligned the folds are still random.

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32
Q

What is a homopolymer?

A

A polymer made from one single repeated mer.

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33
Q

What factors affect the degree of crystallinity?

A
  1. The rate of cooling.
  2. The presence of large/bulky side groups.
  3. The arrangement of the side groups on the chain.
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34
Q

How does the rate of cooling affect the crystallinity of a polymer?

A

The quicker a polymer is cooled the less time the chains have to reorder themselves into crystallites.

35
Q

How does the presence of large/bulky side groups affect the crystallinity of a polymer?

A

Large or bulky side groups stop the chains from being able to form a regular ordered arrangement.

36
Q

How does the arrangement of side groups affect the crystallinity of a polymer?

A

A random arrangement of side groups stops the chains from packing together into an ordered structure.
Atactic form - low crystallinity
Isotactic form - high crystallinity

37
Q

What is an atactic polymer?

A

An atactic formed polymer is a polymer where the side groups are all randomly arranged around the central chain. This causes low crystallinity and therefore low strength, stiffness, hardness.

38
Q

What is an isotactic polymer?

A

An isotactic formed polymer is a polymer where all the side groups are arranged on a single side. This allows for high crystallinity and therefore high strength, stiffness, hardness.

39
Q

What features does a completely amorphous thermoplastic have below the Tg (Glass Transition Temperature)?

A

Below the Tg the polymers form a glassy amorphous solid. They tend to be brittle, and the chains are frozen in place.

40
Q

What features does a completely amorphous thermoplastic have at around the Tg (Glass Temperature)?

A

At around the Tg the polymer chains are able to slide against each other, allowing the material to be ductile and soft, with a rubbery or leathery texture.

41
Q

What features does a completely amorphous thermoplastic have above the Tg (Glass Temperature)?

A

Well above the Tg the polymer grows in volume, and becomes a flowable viscous liquid. Increasing temperature further reduces the viscosity.

42
Q

With regards to thermoplastics what is Tg?

A

Tg is the glass transition temperature, which describes the temperature at which the thermoplastic will change from a brittle/glassy behaviour to a rubbery/viscous behaviour.

43
Q

How does temperature affect the crystalline portions of a partailly crystalline polymer?

A

There isn’t much effect except the standard thermal expansion until it reaches the melting temperature of the material, at which point the crystallites break down and the material experiences a sudden surge in volume.

44
Q

Why does HDPE (High Density Polyethylene) have high strength?

A

It has a high degree of crystallinity, so lots of secondary bonding interactions between chains.

45
Q

Why does LDPE (Low Density Polyethylene) have low strength?

A

LDPE has branches of PE grafted onto the main PE chain. This prevents crystallites from forming, causing the structure to be amorphous. The lower number of secondary bonds reduces the strength.

46
Q

What are the two forms of Polypropylene (PP)?

A
  • Atactic form.

- Isotactic form.

47
Q

What are the features of Atactic formed Polypropylene (PP)?

A

Atactic formed polypropylene (PP) has irregular positioning of its CH3 side groups which prevent crystallites from forming. Low crystallinity = low strength = low density.

48
Q

What are the features of Isotactic formed Polypropylene (PP)?

A

Isotactic formed polypropylene (PP) has the CH3 side groups all ordered on one side which allows crystallites to form. Higher crystallinity = Higher strength = Higher density.

49
Q

True/False: Polymethylmethacrylate (PMMA) is completely amorphous.

A

True. Polymethylmethacrylate (PMMA) has large bulky side groups which prevent crystallites from being able to form, ergo fully amorphous.

50
Q

Why is Polymethylmethacrylate (PMMA) transparent?

A

Because amorphous polymers do not have crystallites they are more likely to be transparent.

51
Q

Why are some semi-crystalline polymers transparent?

A

If the crystallite dimensions are greater than the wavelength of light the polymer will be opaque.

52
Q

True/False: If a polymer is transparent it MUST be amorphous.

A

True.

53
Q

Apart from changing the crystallinity of a polymer how can you change it’s optical properties?

A

By adding dyes, pigments, etc…

54
Q

How does Polytetrafluoroethylene (PTFE, “Teflon”) form?

A

Polytetrafluoroethylene (PTFE, “Teflon”) forms highly symmetrical chains which enables it to be highly crystalline.

55
Q

True/False: Elastomers can exhibit very large non-linear tensile elastic strain.

A

True. It’s in the name.

56
Q

What are the four requirements for large strain elastomeric behaviour?

A
  1. Long, Linear chains.
  2. Random arrangement of polymer chains.
  3. Sufficient but not too great, density of cross-links between chains.
  4. Chain segments must be in a state of constant thermal vibration.
57
Q

Why must a polymer have long linear chains for it to have large strain elastomeric behaviour?

A

Long chains allow for a high amount of strain when stretched. If they’re short then they quickly stretch out fully and are liable to snap.

58
Q

Why must a polymer have random arrangement of polymer chains for it to have large strain elastomeric behaviour?

A

An amorphous structure is necessary to reduce the number of secondary bonds between chains. This reduces the stress required to strain the material.

59
Q

Why must a polymer have cross links for it to have large strain elastomeric behaviour?

A

The cross links prevent irreversible chain sliding (plastic deformation) but still allow stretching. Too many cross links will make the material brittle however.

60
Q

Why must a polymer have constant thermal vibration for it to have large strain elastomeric behaviour?

A

Thermal vibration helps the chains to unkink and slide past one another to generate elastic strain. If a material is below Tg it is brittle.

61
Q

True/False: If a polymer is below Tg you will only get elastic deformation from an applied stress.

A

True. If a polymer is below Tg it will be brittle, therefore there will only be elastic deformation till the material fails.

62
Q

What is a travelling neck?

A

Above Tg, a thermoplastic stressed to above its yield stress will begin to neck. The neck will then continue to get longer without getting thinner.

63
Q

True/False: while a thermoplastic is being stressed during its travelling neck, the strength of the material gets stronger.

A

True. As the material is stressed the polymer chains in the neck straighten and become aligned. This causes the neck to increase in strength.

64
Q

Where is the plastic deformation occurring during travelling necking?

A

At the ends of the neck. The neck becomes crystalline as it is stretched/straightened out, this makes it stronger whereas the amorphous regions of the original material are relatively weak. So the original material is what plastically deforms.

65
Q

True/False: The aligned chains in the travelling neck are stronger in the direction parallel to the strain than perpendicular to the strain.

A

True.

66
Q

In what ways can polymers deform?

A
  • Elastically
  • Viscously
  • Viscoelastically
67
Q

What determines how polymers will deform?

A
  1. Structure of the polymer.
  2. Temperature of the polymer.
  3. Rate of loading of the polymer.
68
Q

How is elastic deformation of polymers simulated?

A

Using a spring element. Stress = Youngs Modulus x Strain

69
Q

How is viscous deformation of polymers simulated?

A

Using a dashpot element.

70
Q

How does temperature affect how an amorphous thermoplastic polymer will deform?

A

The higher the temperature is the more it will use a dashpot model for deformation, the lower the temperature is the more it will use a spring model for deformation.

71
Q

True/False: Increasing rate of deformation also increases the stiffness of the polymer.

A

True. Increasing rate of deformation is the same as decreasing temperature.

72
Q

How does temperature affect how a highly crystalline thermoplastic polymer will deform?

A

Below the melting temperature the high stiffness means the material will deform using the spring model. As the material reaches the melting temperature it will rapidly switch as it melts and becomes a viscous fluid.

73
Q

What influences the mechanical properties of polymers?

A
  1. Degree of polymerisation.
  2. Structure (Crystalline, Amorphous, Crosslinked, etc…)
  3. Temperature.
  4. Rate of loading.
74
Q

What is stress relaxation?

A

If a polymer is above its Tg and is deformed to a given strain, the stress on the polymer required to maintain that strain will gradually reduce as a consequence of chain sliding.

75
Q

How does stress relaxation occur in terms of spring and dashpot deformation?

A

The spring initially elastically deforms, then over time the elastic portion is recovered, and replaced by plastic dashpot deformation.

76
Q

What is the Maxwell model?

A

The Maxwell model is a method of modelling polymer deformation using both a spring and a dashpot elements connected in series.

77
Q

What is the Kelvin/Voight model?

A

The Kelvin/Voight model is a method of modelling polymer deformation using any combination of spring and dashpot elements connected in parallel.

78
Q

True/False: Once a polymer under constant strain has been allowed to completely relax through stress relaxation, the stress is 0.

A

True. Stress relaxation will eventually remove all stress on the material.

79
Q

How can the stress of a material undergoing stress relaxation at a specific time be calculated?

A

Stress (t) = Initial Stress x e ^ ( - time / tau)

Tau = Viscosity of Dashpot / Modulus of Spring

80
Q

True/False: Tau is a material property.

A

True. It is strongly dependent on the temperature.

Tau decreases as temperature increases.

81
Q

How is Tau related to temperature?

A

1 / Tau = C x e ^ ( - Q / [R x T])
C = Constant
Q = Activation energy for polymer chain segments to overcome energy barrier to jump to new positions.
R = Universal gas constant, 8.314 J/mol.K
T = Absolute temperature in Kelvin

82
Q

How do you convert from celsius to kelvin?

A

K = C + 273.15

83
Q

Can the Kelvin/Voight model be used to describe stress relaxation?

A

No. The Kelvin/Voight model does not allow for stress to be completely removed.

84
Q

True/False: Stress relaxation can only occur above Tg.

A

True. Stress relaxation can only occur above the glass transition temperature.

Materials 121 (8 decks)

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