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Materials Chemistry 1 > Introduction to polymer chemistry > Flashcards

Introduction to polymer chemistry Flashcards

1
Q

The Glass Transition Temperature (T g )

A

the lowest temperature at which molecular motion of the polymer chain is possible

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2
Q

Elastomers

A
  • crosslinked rubbery networks (above T g at room temp)
  • can be stretched to high extension but recover when stress is released
  • once crosslinked, it cannot be processed
  • lightly crosslinked polymers which can be deformed
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3
Q

Thermosets

A
  • rigid network polymers with a high crosslink density
  • once formed, they cannot be processed or stretched
  • rigid, inelastic structure so cannot be stretched/ deformed
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4
Q

Thermoplastics

A
  • linear or branched polymers and solid at room temp
  • when heated above a characteristic temperature, polymers can be processed as viscous liquids (they can be molded into shape)
  • upon cooling they solidify but can be reheated and reprocessed, therefore recycled
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5
Q

Amorphous Thermoplastics

A

polymer chains are disordered even in solid state

  • can be recycled like thermoplastics
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6
Q

Semi-crystalline thermoplastics

A

contains both amorphous and crystalline regions or domains

  • they can only be reprocessed when heated above T m crystalline domains as above T g only the amorphous domains become mobile.
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7
Q

Amorphous domains

A

disordered chains with a glass transition temp (T g )

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8
Q

Crystalline domains

A

ordered chains with a melting point (T m )

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9
Q

Properties of linear/ branched polymers

A

soluble in some solvent

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10
Q

Properties of crosslinked/ network polymers

A

not soluble but can interact with solvent - they may swell which depends on crosslink density (thermosets with a high crosslink density may not swell at all)

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11
Q

Low density polyethylene (LDPE)

A
  • comprised of chains with random branching therefore chain-packing/ crystallinity is inhibited by branching (e.g. plastic bags, food wrappers, etc)
  • LPDE is less dense than HDPE so has a lower melting point and is soft and flexible
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12
Q

High density polyethylene (HDPE)

A
  • comprised of chains with only a few branches therefore chains pack easily and the degree of crystallinity is high.
  • HDPE has high density therefore a higher melting point and is hard, tough and rigid (e.g. plastic milk bottles)
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13
Q

Homopolymers

A

only one type of monomer

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14
Q

Copolymers

A

two or more types of monomer

  • random
  • alternating
  • block (diblock, triblock, graft or comb block)
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15
Q

What are the conditions for Size Exclusion Chromatography (SEC)?

A
  • in a good solvent for the polymer
  • under ideal consitions where there are no enthalpic interaction between polymer and packing materials
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16
Q

What is K SEC a function of?

A

The loss of conformational entropy when a polymer chain enters a pore

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17
Q

How do SEC separate polymers?

A

In terms of the size of the polymer chain in solution relative to the size of the pores.

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18
Q

Examples of physical properties of polymers

A

they can be
- tough or brittle
- strong and durable
- rigid and inflexible
- flexible and rubbery
- hard or soft

(Properties vary with temperature thus allowing polymers to be processed)

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19
Q

Below T g

A

Polymer chains are effectively frozen and it is described as glassy.
Motion is restricted to short-range vibrations and rotations.
The polymer is hard, flexible and brittle.

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20
Q

Above T g

A

Chains have enough thermal energy to allow long range molecular motions so chains can move past each other.

Polymer chains are rubbery, can flow as a viscous liquid and be processed.

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21
Q

What determines the T g of a polymer? (stiff)

A

T g increases with increasing chain stiffness - stiffness is induced in polymer chains that have aromatic units in the polymer backbone or with bulky substituents.

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22
Q

What determines the T g of a polymer? (forces)

A

T g increases with increasing forces of intermolecular attraction.

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23
Q

T g of chain segments in a thermoset?

A

No T g with almost no mobility due to high crosslink density.

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24
Q

T g of chain segments in an elastomer?

A

Has a T g and retains mobility due to low crosslink density.

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25
Q

T g and T m in semi-crystalline polymers

A

T m is always higher than T g

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26
Q

Why don’t we get 100% crystalline polymers?

A
  • Melted polymer chains will have a disordered, random conformation.
  • As polymer melt cools to T m , it starts to crystallise which occurs at T c , the crystallisation temp.
  • It takes a long time for all chains to unravel, align and form a 100% crystalline polymer.
27
Q

What sort of control is crystallisation under?

A

Kinetic rather than thermodynamic - the degree of crystallinity obtained depends upon crystallisation conditions.

28
Q

What will crystallinity be like if we rapidly reduce temperature?

A

Degree of crystallinity will be low as the chains have little time to rearrange into ordered crystalline regions before T g is reached and all chains become rigid in place.

29
Q

What are semi-crystalline polymers used for?

A

Making fibres for clothing and textiles, as well as bulk plastic products.

30
Q

Chain packing influence on T g

A

The ability of chains to pack in a regular arrangement influences the degree of crystallinity and may inhibit crystallinity altogether thus forming an amorphous polymer.

31
Q

Two factors affecting chain packing

A
  • Stereoregularity (tacticity)
  • Polymer Architecture (chain branching)
32
Q

Low density polyethylene (LDPE)

A
  • comprises of chains with random branching
  • chain packing / crystallinity is prohibited
  • LDPE is less dense, has a lower melting point and is soft and flexible
33
Q

What is LDPE used for?

A
  • food wrappers
  • shopping and rubbish bags
34
Q

High density polyethylene (HDPE)

A
  • comprises of chains with only a very few branches
  • chain packing is easy
  • HDPE is more dense, has a higher melting temp and is hard, tough and rigid
35
Q

What is HDPE used for?

A
  • plastic milk bottles
  • containers
36
Q

What is tacticity?

A

refers to the arrangement of side groups along a polymer chain

37
Q

Isotactic

A

all the repeating units have the same stereochemical configuration

38
Q

Syndiotactic

A

the repeating units have alternating stereochemical configurations

39
Q

Atactic

A

the repeating units have no regular stereochemical configuration

40
Q

What type of polymers can give rise to semi-crystalline polymers?

A

Isotactic and syndiotactic polymers since they have a regular orientation of side groups thus enabling regular packing

41
Q

Isotactic vs Atactic Polypropylene

A

Atactic propylene has no commercial uses - a soft waxy polymer that becomes sticky upon heating.

Isotactic propylene is a semi-crystalline polymer that is tough and has excellent chemical resistance – used for packaging, textiles, carpets, fibres, automotive parts, etc

42
Q

What can be used to measure thermal properties such as heat capacities (T g ), heats of fusion (T m ), enthalpies of reactions, etc

A

Differential Scanning Calorimetry (DSC)

43
Q

What does DSC measure?

A

How much more heat (heat flow) is required to keep the sample pan at the same temperature as reference plan.

  • Plot the difference vs temp
44
Q

What type of reaction leads to increase in heat flow?

A

Endothermic (e.g. T g and T m )

Exothermic transitions result in decreased heat flow (e.g. T c )

45
Q

What happens when polymers crystallise?

A

They give off latent heat - exothermic

46
Q

What happens when polymer crystals melt (T m )?

A

They absorb latent heat - endothermic

47
Q

What is an alternative to DSC as it is hard to detect small change in heat flow?

A

Dynamic Mechanical Analysis (DMA)
measures variations in mechanical properties such as
- force
- strain
- frequency
- time
- temperature

Many mechanical properties change suddenly at T g as polymer goes from a glassy solid to a rubbery liquid.
More sensitive than DSC.

48
Q

What is tensile strength?

A

it measures how difficult it is to break a substance when stress is applied to pull it apart.

49
Q

How to measure stress (σ)?

A

Stress (σ) = Force / Area

50
Q

How to measure strain?

A

Elongation / Original length

51
Q

What is strain?

A

a measure of how much the material has been stretched

52
Q

What is Young’s Modulus and how do you measure it?

A

It is a measure of stiffness.

Young’s Modulus = Stress / Strain

53
Q

Stress at the Yield point

A

the load is sufficient to cause polymer chains to slip past each other and yield occurs

54
Q

Elongation at Yield Point

A

how far you can stretch the polymer before yield failure takes place (chain slip)

55
Q

Stress at Failure (ultimate tensile strength)

A

polymer molecules can no longer maintain cohesive integrity and sample breaks

56
Q

Elongation at Break

A

how much the sample can stretch before failure occurs

57
Q

When does tensile strength generally increase?

A

With molar mass (chain length)

58
Q

What is a crosslinked/ network polymer?

A

one in which all polymer chains are chemically bonded together

59
Q

What happens when an elastomer absorbs and releases heat?

A

Shrinks when absorbs heat
Stretch when releases heat

Elastomers exhibit negative thermal expansivity.

60
Q

What happens when elastomers are stretched?

A

the chains become more uncoiled, and more ordered thus sometimes resulting in crystallisation - exothermic (heat is released)

61
Q

What happens when solvent is added to low cross-link density network of a gel?

A

highly swollen gel

62
Q

What happens when solvent is added to high cross-link density network of a gel?

A

less swollen gel

63
Q

What is the main distinguishing feature between elastomers and thermosets?

A

The crosslink density

elastomer = low crosslink density
thermoset = high crosslink density

Materials Chemistry 1 (4 decks)

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