Hasell - case studies and basics Flashcards
What are crosslinks?
- 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
what are the three types of polymer structures?
linear, branched, crosslinked
what material types are crosslinks characteristic of?
thermosetting materials
what is ‘curing’?
the crosslinking of thermosetting resins
what are thermoplastics?
normally linear
can be melted and remoulded by heating
examples of thermoplastics?
polyethene, polyvinyl chloride, polystyrene
what are thermosets?
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
what is DCPD
dicyclopentadiene
use of DCPD
to product automobile body parts
properties and description of DCPD
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
what is ROMP?
ring-opening metathesis polymerisation
what are the two arrangements for polymer chains?
amorphous, crystalline
what is amorphous v crystalline arrangement?
amorphous chains are randomly ordered (wet spaghetti), crystalline chains are arranged in an orderly manner (dry spaghetti)
what type of arrangement are lamallae?
crystalline
what is the result of crystallinity?
INCREASES:
density (crystals closer together)
solvent resistance (harder for solvent to enter in-between molecules)
opacity (the crystallites scatter more light)
what does crystalline melting point depend on?
1) molecular weight (will be a mixture)
2) crystallite size
3) presence of co-monomers
4) impurities (solvents, plasticisers, monomers)
How is crystal melting temperature denoted?
Tm
How is crystal melting temperature measured?
DSC
How is degree of crystallinity denoted?
Xc
How is degree of crystallinity measured?
PXRD or DSC
What does PXRD measure?
angle and intensity of scattering from the sample
eq for Xc?
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
what is the glass transition temp?
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
how is Tg measured?
DSC
How does Tg change with chain length?
longer chains = higher Tg
what is considered high temp for high performance polymers?
greater than 150C
what are high performance polymers?
retain desirable mechanical, thermal, and chemical properties when under harsh conditions (high temp, pressure, corrosive chemicals)
Is polyimide (PI) amorphous or crystalline?
amorphous
Is polyethylenimide (PEI) amorphous or crystalline?
amorphous
is PES amorphous or crystalline?
amorphous
is PTFE amorphous or crystalline?
crystalline
is PEEK amorphous or crystalline?
crystalline
is polyamide (PA) (Nylon) amorphous or crystalline?
crystalline
is PMMA amorphous or crystalline?
amorphous
is PETG amorphous or crystalline?
amorphous
is PVC amorphous or crystalline?
amorphous
is HDPE/LDPE amorphous or crystalline?
crystalline
what is the structure of Teflon?
same as polyethylene but 4F instead of 4H
Describe properties of Teflon
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
Disadvantages of Teflon?
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
Tm, Tg, and relative crystallinity of Teflon?
Tm = 320-330C
Tg = 120-130C
Xc = high (60-80%)
what does it mean if a polymer has both Tm and Tg?
semicrystalline
what polymer is PTFE?
teflon (polytetrafluoroethylene)
Describe PEEK
1) semicrystalline
2) high temp thermoplastic
3) made by step condensation at ~300C in polar aprotic solvents
Tm and Tg of PEEK?
Tg = 143C
Tm = 343C
Advantages of PEEK
1) temperature stable to be used upto 250C
2) good chemical resistance except to strong acids
3) can be melt processed
What are the applications of PEEK?
for bearings, piston parts, HPLC columns, valves, electrical insulation, medical implants
bc it doesn’t wear down easily
What are aramids?
aromatic polyamides
describe kevlar
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
why does kevlar have such strong temp dependence?
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
applications of kevlar
bulletproof vests and military armour, ropes, sails and sport equip, fireproof clothing, in composites with epoxy resin
describe kapton
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
describe kapton
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
why is the intermediate of kapton soluble and the product isnt?
intermediate = polyamic acid: the O=C-NH is bent so gives some flexibility
why is Kapton a linear polymer but also an insoluble thermoset?
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
applications of kapton?
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
Describe phenol-formaldehyde
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
how is phenol-formaldehyde synthesised? MECHANISM
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
Describe epoxy resins?
- 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
How are epoxy resins made?
1) bisphenol is made from 2 phenols and acetone (condensation reaction)
2) bisphenol A then reacts under basic conditions with epichlorohydrin
How is the degree of polymerisation controlled with epoxy resins?
by controlling the ratio of bisphenol A to epichlorohydrin
why do do multifunctional amines act as crosslinkers?
they have two sets of NH2 so reaction can occur at any of the 4 proton positions to produce a highly crosslinked structure
describe rubber
- natural rubber is polyisoprene
- synthetic rubber can be made from isoprene, butadiene, or often copolymer of styrene and butadiene
-vulcanised to crosslink
describe vulcanisation
- the reactive sites are unsaturated groups, sulfur bridges for inbetween these (can be one or many S atoms)
how does vulcanisation with sulfur change the properties of rubber?
makes the linear rubber into a cross-linked thermoset
How does the amount of crosslinking change the properties of rubber?
lower crosslinking = elastic & durable
higher crosslinking = hard and brittle
Why is S8 unstable without crosslinker?
depolymerises back into the rings bc of the radicals on each end
applications for phenol-formaldehyde?
replacement for ivory
electrical items, jewellery
applications of epoxy resins?
glues
applications of rubber?
car tyres, soles of shoes, rubber bands
at what temperature does S8 melt and ring-open to polymerise in inverse vulcanisation?
over 159C
describe inverse vulcanisation
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
applications for inverse vulcanised polymers
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
