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CHEM342 > Hasell - case studies and basics > Flashcards

Hasell - case studies and basics Flashcards

1
Q

What are crosslinks?

A
  • a small region in a macromolecule from which at least four chains emnate
  • formed by reactions involving sites or groups on existing macromolecules or by interactions between existing macromolecules
  • normally irreversible
  • usually don’t melt or dissolve
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2
Q

what are the three types of polymer structures?

A

linear, branched, crosslinked

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3
Q

what material types are crosslinks characteristic of?

A

thermosetting materials

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4
Q

what is ‘curing’?

A

the crosslinking of thermosetting resins

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5
Q

what are thermoplastics?

A

normally linear
can be melted and remoulded by heating

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6
Q

examples of thermoplastics?

A

polyethene, polyvinyl chloride, polystyrene

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7
Q

what are thermosets?

A

usually crosslinked and insoluble
can’t be melted or remoulded by heating
stronger due to crosslinks
need to be moulded into final form before curing (reactive injection moulding)
difficult or impossible to recycle

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8
Q

what is DCPD

A

dicyclopentadiene

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9
Q

use of DCPD

A

to product automobile body parts

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10
Q

properties and description of DCPD

A

good impact and corrosion resistance

1) first forms a linear polymer (thermoplastic) by ROMP
2) then crosslinked by reacting the other double bond (thermoset)
3) done by reaction injection moulding

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11
Q

what is ROMP?

A

ring-opening metathesis polymerisation

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12
Q

what are the two arrangements for polymer chains?

A

amorphous, crystalline

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13
Q

what is amorphous v crystalline arrangement?

A

amorphous chains are randomly ordered (wet spaghetti), crystalline chains are arranged in an orderly manner (dry spaghetti)

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14
Q

what type of arrangement are lamallae?

A

crystalline

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15
Q

what is the result of crystallinity?

A

INCREASES:
density (crystals closer together)
solvent resistance (harder for solvent to enter in-between molecules)
opacity (the crystallites scatter more light)

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16
Q

what does crystalline melting point depend on?

A

1) molecular weight (will be a mixture)
2) crystallite size
3) presence of co-monomers
4) impurities (solvents, plasticisers, monomers)

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17
Q

How is crystal melting temperature denoted?

A

Tm

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18
Q

How is crystal melting temperature measured?

A

DSC

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19
Q

How is degree of crystallinity denoted?

A

Xc

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20
Q

How is degree of crystallinity measured?

A

PXRD or DSC

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21
Q

What does PXRD measure?

A

angle and intensity of scattering from the sample

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22
Q

eq for Xc?

A

Intensity of the crystalline regions/ (intensity of the crystalline + amorphous regions)

intensity measured from the area under the curves of the PXRD graph

amorphous = broader peaks along the bottom

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23
Q

what is the glass transition temp?

A

only for the amorphous phase of polymers
above the Tg the polymer is rubbery (chains can move over each other)
below the Tg the polymer is glassy (chains lack thermal mobility) - will shatter like glass if cold

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24
Q

how is Tg measured?

A

DSC

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25
Q

How does Tg change with chain length?

A

longer chains = higher Tg

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26
Q

what is considered high temp for high performance polymers?

A

greater than 150C

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27
Q

what are high performance polymers?

A

retain desirable mechanical, thermal, and chemical properties when under harsh conditions (high temp, pressure, corrosive chemicals)

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28
Q

Is polyimide (PI) amorphous or crystalline?

A

amorphous

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29
Q

Is polyethylenimide (PEI) amorphous or crystalline?

A

amorphous

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30
Q

is PES amorphous or crystalline?

A

amorphous

31
Q

is PTFE amorphous or crystalline?

A

crystalline

32
Q

is PEEK amorphous or crystalline?

A

crystalline

33
Q

is polyamide (PA) (Nylon) amorphous or crystalline?

A

crystalline

34
Q

is PMMA amorphous or crystalline?

A

amorphous

35
Q

is PETG amorphous or crystalline?

A

amorphous

36
Q

is PVC amorphous or crystalline?

A

amorphous

37
Q

is HDPE/LDPE amorphous or crystalline?

A

crystalline

38
Q

what is the structure of Teflon?

A

same as polyethylene but 4F instead of 4H

39
Q

Describe properties of Teflon

A

1) very unreactive (strong C-F bonds)
2) repels water and hydrocarbons (due to fluorination)
3) very resistant to solvents and corrosion so used for scientific and medical components
4) semicrystalline

40
Q

Disadvantages of Teflon?

A

1) no known solvents for it
2) too high viscocity to melt, doesn’t flow but decomposes if heated more
3) to be shaped it needs to be granulated and compressed at high temp

41
Q

Tm, Tg, and relative crystallinity of Teflon?

A

Tm = 320-330C
Tg = 120-130C
Xc = high (60-80%)

42
Q

what does it mean if a polymer has both Tm and Tg?

A

semicrystalline

43
Q

what polymer is PTFE?

A

teflon (polytetrafluoroethylene)

44
Q

Describe PEEK

A

1) semicrystalline
2) high temp thermoplastic
3) made by step condensation at ~300C in polar aprotic solvents

45
Q

Tm and Tg of PEEK?

A

Tg = 143C
Tm = 343C

46
Q

Advantages of PEEK

A

1) temperature stable to be used upto 250C
2) good chemical resistance except to strong acids
3) can be melt processed

47
Q

What are the applications of PEEK?

A

for bearings, piston parts, HPLC columns, valves, electrical insulation, medical implants

bc it doesn’t wear down easily

48
Q

What are aramids?

A

aromatic polyamides

49
Q

describe kevlar

A

1) aramid
2) not melt-processable - decomposes below its melting point
3) requires conc sulfuric acids and specialist equipement for processing
4) very strong temp resistance ~500C
5) normally spun into fibres
6) high strength and lightweight

50
Q

why does kevlar have such strong temp dependence?

A

the chains are all directed into the trans conformation, cis would be sterically hindered by the phenol hydrogens

trans conf allows hydrogen bonding between linear chains

51
Q

applications of kevlar

A

bulletproof vests and military armour, ropes, sails and sport equip, fireproof clothing, in composites with epoxy resin

52
Q

describe kapton

A

1) heterocyclic polyimide - strong, good thermal and chemical resistance
2) soluble intermediate but insoluble product
3) product is thermoset so doesn’t melt and is hard to process
4) stable upto 400C

53
Q

describe kapton

A

1) heterocyclic polyimide - strong, good thermal and chemical resistance
2) soluble intermediate but insoluble product
3) product is thermoset so doesn’t melt and is hard to process
4) stable upto 400C
5) linear

54
Q

why is the intermediate of kapton soluble and the product isnt?

A

intermediate = polyamic acid: the O=C-NH is bent so gives some flexibility

55
Q

why is Kapton a linear polymer but also an insoluble thermoset?

A

the electron acceptors are the carbonyl groups on the ONE SIDE of the rings, the nitrogen atoms inside are the electron donors

the other carbonyl groups the other side of the ring pull electron density away from the acceptor units

the polyimides stack to arrange so the electron acceptor is above the electron donor, allowing the electron donor/acceptor groups to interact with the adjacent other group

56
Q

applications of kapton?

A

1) thermal and electrical insulation
2) circuit boards
3) x-ray diffraction windows (doesn’t diffract)
4) insulating heat shields when coated with Al for spacecraft
5) thermal blankets

57
Q

Describe phenol-formaldehyde

A

1) chemically stable and inflammable
2) Bakelite = electrically insulating and resistant to damage
3) forms a 3D network
4) once melted cannot be molded - only cut

58
Q

how is phenol-formaldehyde synthesised? MECHANISM

A

1) e- from O on the phenol goes back into the bond while the double bond next to it attacks the H2C of the methanol
2) proton transfers and methanol is now attached to the double bond next to the phenol whihc has been regenerated
3) attached methanol is protonated until the H2O leaves it and Ch2+ remains attached
4) the H2C+ is attacked by another new phenol molecule to add the whole molecule to it so it is a double ring structure

ACID CATALYSED SO REACTS AT ORTHO OR PARA POSITIONS

59
Q

Describe epoxy resins?

A
  • the epoxy group is strained and very reactive which allows reaction with many hydrogen donors
  • often use multifunctional (so not linear) amines to crosslink
  • most are based on bisphenol A and epichlorohydrin
60
Q

How are epoxy resins made?

A

1) bisphenol is made from 2 phenols and acetone (condensation reaction)
2) bisphenol A then reacts under basic conditions with epichlorohydrin

61
Q

How is the degree of polymerisation controlled with epoxy resins?

A

by controlling the ratio of bisphenol A to epichlorohydrin

62
Q

why do do multifunctional amines act as crosslinkers?

A

they have two sets of NH2 so reaction can occur at any of the 4 proton positions to produce a highly crosslinked structure

63
Q

describe rubber

A
  • natural rubber is polyisoprene
  • synthetic rubber can be made from isoprene, butadiene, or often copolymer of styrene and butadiene
    -vulcanised to crosslink
64
Q

describe vulcanisation

A
  • the reactive sites are unsaturated groups, sulfur bridges for inbetween these (can be one or many S atoms)
65
Q

how does vulcanisation with sulfur change the properties of rubber?

A

makes the linear rubber into a cross-linked thermoset

66
Q

How does the amount of crosslinking change the properties of rubber?

A

lower crosslinking = elastic & durable
higher crosslinking = hard and brittle

67
Q

Why is S8 unstable without crosslinker?

A

depolymerises back into the rings bc of the radicals on each end

68
Q

applications for phenol-formaldehyde?

A

replacement for ivory
electrical items, jewellery

69
Q

applications of epoxy resins?

A

glues

70
Q

applications of rubber?

A

car tyres, soles of shoes, rubber bands

71
Q

at what temperature does S8 melt and ring-open to polymerise in inverse vulcanisation?

A

over 159C

72
Q

describe inverse vulcanisation

A

1) 50% or more S8 = polymeric sulfur crosslinked by small organic molecule
2) first crosslinker reported was DIB
3) no solvents, simple synthesis, atom efficient

73
Q

applications for inverse vulcanised polymers

A

1) more stable LiS batteries
2) IR- transparent lenses
3) recyclable/repairable crosslinked polymers (vitrimers)
4) heavy metal capture
5) thermal/electrical insulation
6) antimicrobial materials

CHEM342 (8 decks)

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