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Materials Science 1 > Polymers > Flashcards

Polymers Flashcards

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1
Q

where do polymers come from in terms of raw material?

A

crude oil

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2
Q

what are polymers?

A

polymers are materials with large macro-molecules, of which plastics is just one group - long chains of hydrocarbons with sometimes other atoms like oxygen and silicon

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3
Q

what are some general properties of polymers?

A
  • good thermal/electrical insulation
  • low density
  • easy to manufacture and low cost
  • useful as adhesives (including composites)
  • good transparency
  • durable
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4
Q

what is the main difference between thermoplastics and thermosets?

A

thermoplastics - can be re-heated

thermosets - cannot be re-heated

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5
Q

what is an example of an elastomer?

A

rubber (can undergo higher strain)

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6
Q

what are polysiloxanes?

A

alternating oxygen and silicon atoms

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7
Q

what is the bonding in polymers?

A

atoms are bounded by covalent bonds to form long and flexible chains, secondary bonds between chains (VdW)

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8
Q

what is the structure of polymers?

A

amorphous or partially crystalline materials

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9
Q

what is teflon?

A

a polymer made from a hydrocarbon chain with fluorine - used in frying pans

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10
Q

what are the repeated units called in a polymer?

A

mers

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11
Q

what is polymerization?

A

the chemical process that causes a large no. of monomers to combine to form the polymer

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12
Q

why do polymers have low stiffness and low melting points (generally)?

A

because although they have a strong covalent backbone, chains will often bond by weaker secondary attraction

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13
Q

what determines the mechanical properties of the material?

A

molecular structure and molecular weight

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14
Q

what are linear polymers?

A
  • single chains
  • flexible
  • ‘mass of spaghetti’
  • van der walls between molecules
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15
Q

what are branched polymers?

A
  • side branches
  • packing is less efficient
  • lower density
  • less crystalline than linear polymers
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16
Q

what are cross-linked polymers?

A
  • adjacent chains joined at various positions by covalent bonds, e.g rubbers
  • cross-links are often non reversible due to chemical reactions
  • when heavily cross linked, may be referred to as a network
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17
Q

order the molecular structures of polymer from lowest to highest strength?

A
  • network
  • cross-linked
  • branched
  • linear
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18
Q

how can we tailor the properties of polymers?

A

using different ‘mer’ units

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19
Q

what is condensation polymerisation?

A
  • elimination of small molecule like H2O or HCl etc.

- reaction between an hydroxyl (-OH) and carboxylic acid (-COOH) to make an ester and water

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20
Q

what is free radical polymerisation?

A
  • produces an addition reaction

- the 3 steps are: initiation, propagation, termination

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21
Q

what are the three main classes of polymers?

A
  • thermoplastics
  • thermosets
  • elastomers
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22
Q

what are some properties of thermoplastics?

A
  • they soften on heating
  • linear/branches chains (minimal cross linking)
  • weak intermolecular bonding
  • greater ability to form (semi-) crystalline regions
  • ductile: long period of plastic deformation
  • e.g PP, PE, PS
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23
Q

what are some properties of thermosets?

A
  • only degrade on heating
  • react and harden (e.g epoxy resin)
  • heavily cross-linked (10-50% of mers)
  • do not soften on heating once formed, difficult to recycle
  • brittle: minimal period of plastic deformation
  • e.g vulcanised rubber, epoxies, urea formaldehyde, polyester resin, polyesters
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24
Q

what are some properties of elastomers?

A
  • linear with some cross-linking, which provides shape ‘memory’ / retention
  • useful for heat shrink, to insulate electrical cables and structural bearing pads in construction
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25
Q

what is the molecular weight Mw in terms of polymers?

A

the molecular weight Mw, is the mass of a mole (fixed number) of chains

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26
Q

what is linked to molecular weight?

A
  • stiffness
  • tensile strength
  • viscosity
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27
Q

what does a large chain length in a polymer result in?

A

increased tensile strength (longer chains are more likely to entangle and can ‘anchor’ better)

28
Q

what can occur when solidifying a polymer (thermoplastic)?

A
  • you can form amorphous regions where they have solidified randomly which is encouraged by random copolymerisation, bulky side groups and chain branching
  • some semi crystalline regions where the chains are lined up (caused by annealing), these crystalline regions are resistant to solvent
29
Q

why do amorphous/non crystalline polymers look like in their purest state?

A

transparent

30
Q

what is the nucleation rate?

A

rate at which nucleation occurs (crystals forming)

31
Q

how do you work out the overall crystallisation rate of a polymer?

A

nucleation rate x growth rate

about halfway between Tg and Tm

32
Q

out of crystalline and amorphous polymers, which takes up more volume?

A 
amorphous = more volume
crystalline = less volume
33
Q

what is a semi-crystalline polymer?

A

a polymer with amorphous and crystalline regions (we can never have a polymer with purely crystalline regions)

34
Q

what is the equation for the total volume of the polymer?

A

V = Vc + Va

where Vc and Va are the volumes of the crystalline and amorphous regions respectively

35
Q

what is the equation for the total mass of the polymer?

A

W = Wc + Wa

where Wc and Wa are the masses of the crystalline and amorphous regions respectively

36
Q

what is the mass fraction equation of crystalline regions and how else can it be rewritten?

A

Xc = Wc / W = ρc x Vc / pV

OR Xc = ρc (ρ - ρa) / ρ (ρc - ρa)

37
Q

why do samples have considerably different crystallinities?

A

different crystallinities are observed because branches disrupt chain folding

38
Q

what is crazing?

A

(like when you bend a ruler and it goes white)
crazing creates lots of little voids/cracks that scatter light which is why you cannot see through it - similar process when the polymer comes into contact with a solvent

39
Q

what is a technique used to improve yield strength of polymers?

A

cold drawing

40
Q

what is cold drawing?

A

the process of which polymers are stretched out:

  • molecules are drawn out and aligned
  • material much stronger in draw direction than before
  • increasing elastic modulus and tensile strength
  • reduces ductility
  • annealing possible
41
Q

what is the glass transition temp Tg?

A

the temp. at which you go from brittle to plastic behaviour

42
Q

what factors effect Tm and Tg?

A

chain stiffness (additional side groups)

  • branching (additional bulky side groups) increases the melting point
  • harder for chains to melt and move and slide past each other
43
Q

how else can side groups affect Tm?

A

very polar, electronegative side groups leads to significant intermolecular bonding forces so Tm increases

44
Q

if you increase the temperature of a polymer what happens?

A

it goes from behaving in a brittle way to a plastic way, ductility increases

45
Q

what is the equation for glass transition temp?

A 
Tg = Tg∞ - (ρyNaθ / αfMn)
where 
Tg = glass transition temp.
Tg∞ = glass transition temp. at infinite molecular weight
ρ = density
y = total number of ends per chain
Na = avogadro number
θ = contribution of chain end to free volume
αf = thermal expansion coefficient
Mn = number average molecular mass
46
Q

what are some advantages/disadvantages to injection mounding?

A
  • can make complex shapes
  • rapid manufacturing, used to make thousands and thousands of the same component
  • very cheap once infrastructure in place
  • expensive moulds
  • pigments in pellet form can be added to colour the polymer
47
Q

how does extrusion work in terms of processing polymers?

A
  • similar to injection moulding
  • polymer is heated to melt into liquid
  • force through a die or required cross sectional shape
  • cool extrusion
  • used to make linear sections
  • for high volume manufacture
  • e.g UVPC window frames
48
Q

can both thermoplastics and thermosets be injection moulded? what is the main difference?

A

yes - the mould is cooled for thermoplastics with 2 hoppers and heated for thermosets with only 1 hopper

49
Q

what is the process of thermoforming?

A
  • includes: vacuum forming, pressure forming and mechanical forming
  • using a sheet of polymer
  • heat is applied
  • formed against a one sided mould
  • for thermoplastics
  • cheap and easy to use
  • uses air pressure
  • e.g trays
50
Q

what is blow moulding?

A
  • used to make plastic bottles (PET)
  • parison extrusion
  • high temperature and stretching in the desired direction causes the molecules to polarize, line up and essentially crystallise to produce a bottle of superior strength
51
Q

what is compression moulding?

A
  • used for thermosets
  • a preheated preform is placed in the open mould
  • as the mould closes, the preform is compresses and starts to flow
  • the mould is almost closed and the resin is now a hot flowing fluid
  • the heated mould is held shut until the resin cures
  • then the mould is opened and the finished part is ejected
52
Q

what is an antimicrobial? (polymer additive)

A

to control the build up of bacteria, fungi and algae on the surface of the polymer (prevent mould) - used in sealants for bathrooms

53
Q

what is an antioxidant? (polymer additive)

A

for higher temp. stability

54
Q

what is an antistatic? (polymer additive)

A

to prevent the build up of static electric charge

55
Q

what are fibres? (polymer additive)

A

fibres are added to increase strength and stiffness

56
Q

what are plasticizers? (polymer additive)

A

added to increase flexibility

57
Q

what are lubricants? (polymer additive)

A

for easier moulding or for increasing adhesion and viscosity of the moulded parts

58
Q

what are UV stabilizers? (polymer additive)

A

for resistance to sunlight

59
Q

what are fillers? (polymer additive)

A

for economical reasons, to bulk it out

60
Q

what are flame retardants/smoke suppressants? (polymer additive)

A

it’s in the name

61
Q

what are conductive fibres or flakes? (polymer additive)

A

for special applications

62
Q

what are polysiloxanes? and what are their applications?

A
  • backbones consist of Si-O-Si units polymeric organosilicon compound commonly referred to as silicones
  • alternating oxygen and silicon atoms
  • most widely used silicon-based organic polymer
  • used in consumer applications, such as silicone caulk and adhesives, sealants in kitchens, showers and bathrooms
63
Q

what are some properties of silicone?

A
  • good electrical insulator
  • can be formulated to be electrically conductive
  • good flexibility
  • thermal stability over a wide range of (-100 to 250 degrees C)
  • ability to repel water and form watertight seals
  • resistance to oxygen, ozone and UV light
  • does not stick after cure (non stick, so easy to clean)
  • low chemical reactivity
  • low toxicity
  • does not support microbiological growth
64
Q

what are some applications of silicone in construction?

A
  • sealants and caulks
  • coatings
  • fire protection
  • glazing seals
  • in plumbing, silicon grease is typically applied to O-rings in taps and valves
65
Q

are the properties of plastic temperature dependent?

A

yes

66
Q

what must you remember to do if you want to calculate the number of branches from y?

A

minus 2 from y to get NUMBER OF POLYMER BRANCHES

Materials Science 1 (10 decks)

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