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NCM 1 > Polymers > Flashcards

Polymers Flashcards

1
Q

Name the 3 possible structures of co-polymers

A

Random (ABBBAABA)
Alternating (ABABABA)
Di-block (AAAAnBBBBn)

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2
Q

Name and draw the 3 different polymer architectures

A

Linear
Branched - lots of linear connected
Network - loads of branches over layed

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3
Q

Draw a methyl group

A

CH3 - C - CH3

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4
Q

Draw an Ester carbonyl group

A

O-C=O

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5
Q

Draw an amide group

A

H O
| - ||
N. C

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6
Q

Draw a nitrile group

A

C
|||
N

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7
Q

What is a free radical?

A

A highly reactive molecule/atom with an unpaired electron

Demoted by • on atom

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8
Q

Describe the addition polymerisation technique

A

1 - initiation to produce free radicals (chemical, thermal decomp, ionising)
2 - propagation (increase in Mw)
3 - termination (deduction in Fr)
A medium is required for reaction to occur

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9
Q

How can early termination increase Mw of the polymer chain?

A

Lots of free radicals cause lots of reactions = short chain when free radical combines with free radical
Termination reduces free radicals, therefore, long chains likely = high Mw

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10
Q

Explain the gel effect and why it happens

A

Gel effect - when reaction rate increases at end of reaction. Viscosity is high = low chain mobility = low termination.

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11
Q

Describe the differences between step-growth and addition polymerisation (condensation polymerisation)

A

Addition polymer - monomers add in sequence to end of polymer
Step growth - polymer chains combine to create really long chains
Both- polymer grows because of Fr, Adding heat increases reaction rate, termination occurs when Fr reduce (usually when cyclic chains are formed)

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12
Q

Why is a high degree of polymerisation favourable?

A

Usually means long chain length

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13
Q

Describe the first four classes of monomers and the polymer structures they create

A

Mono - short linear chains
Di- long linear chains
Tri- branched chains
Tetra (4 Fr) - network polymers

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14
Q

Draw the chain length distribution curve during polymerisation

A

Bell curve with majority of chains having medium Mw

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15
Q

How do you work out Mw of a polymer?

A

Use gel permeation chromatography
Put dissolved polymer through a column containing with beads - where distribution of pore size = distribution of chain length, and then into a detector

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16
Q

How can you determine Mw from viscosity?

A

Viscosity = K x Mw ^A, use a Viscometer to measure
As long chains are difficult to move.
K and A depend on: solvent used, temperature, concentration

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17
Q

How does a solvent affect a polymer?

A

Polymers curl up into spherical shapes when in solution
Good solvent - allows chain to open up
Bad solvent - causes further coiling

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18
Q

Describe a u-tube viscometer

A

1- Dissolve polymer in solvent and pour into tube until volume A is full
2- use suction to get liquid into B
3- release from B and measure time to cross between C & D markers
4- repeat procedure with pure solvent
Viscosity = (solution t - solvent t)/ sol t

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19
Q

What are steric repulsions and what do they mean?

A

Force similar atoms to maximise distance between them - when temp is low then molecule in trans confirmation = lowest energy form

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20
Q

Detail transformations a molecule experiences through 360° rotation

A 
Trans - 0°
Gauche + - 60°
Gauche - - 120°
Trans - 180°  etc etc 
If temp high enough, all present and means polymer is coiling
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21
Q

What are the different ways in which polymer addition can occur?

A

Head - head
Tail - tail
Head - tail - favoured as allows regularity, controlled by catalyst

Isotactic - sequential order
Syndotactic - every other in diff orientation
Atactic - random orientations

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22
Q

Describe and explain the stages on the glass transition graph

A

1 - glass stage (before Tg) - solid state, doesn’t flow
2- leathery stage (just below Tg) - chains begin to soften
3- rubber - very flexible and compliment but still not flowing
4 - viscous liquid - mobile chains, plastic behaviour & flows under own weight

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23
Q

Define the Tg point

A

Temp range that material transitions from glassy to liquid state

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24
Q

What happens to the polymer relevant to the Tg point

A

Above Tg - polymer ordered, full transformational changes, mobile chains
Below Tg - disorders & frozen polymer, confirmation interchange is very slow
*as temp increases, chain mobility incre, viscosity decre

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25
Q

How does long range motion occur in a polymer

A

When there’s free volume in the polymer, converted confirmation changes can occur (gauche&trans together) leading to long range motion

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26
Q

How can a polymer that’s brutally fractures reconnect?

A

If the two parts are placed near to each other, and heated to above the Tg, long range motion can occur (concerted confirmational changes) which mitigates over the boundary.

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27
Q

Define reptation time and libration

A

Reptation time - time for entire polymer chain to be translated
Librarion - only partial rotation and partial trans/gauche interchanges

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28
Q

Name the 10 factors that affect Tg and briefly describe how they affect it

A

*low rotation = high Tg
Chain flexibility - high stiffness = low rotat
Chain length - small chain = incre rotat
Comp - diff mixtures have diff Tg
Cross-linking - high dens = low rota
Structure - large pendant group = low rot
Steric effects - flexible bonds = high rot
Plasticiser - acts as a lubricant = high rot
Polar pendants - their intermolecular forces = low rot
Mw - high Mw = long chain = low rot

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29
Q

How can TMA be used to measure compression/expansion?

A

Still have motor unit connected to rod.
Connect a linear variable displacement transducer (displacement measure) to the rod and apply force to sample - measure compression vs load
Place rod on sample (with no force) and measure reverse displacement to measure expansion

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30
Q

How can you use TMA to determine Tg

A

Apply constant load to sample, and then heat the sample - when there is some displacement = Tg

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31
Q

How can TMA be used to measure swelling?

A

Set the rod on the sample surface - suburge in water and measuring displacement

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32
Q

What are spherulites and how do they form?

A

Form when semi-crystalline materials cool down and harden

Spherical as they grow rapidly until impingement

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33
Q

Describe the 2 phase polymer model

A

ordered Lamella sheets with an amorphous layer in the middle of them

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34
Q

Describe the 3 phase polymer model

A

Amorphous layer in between lamellae sheets.

Cilia and tie chains present as well as short medium and long lamellae chains

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35
Q

Draw the structure of strain induced crystallisation

A

Rod like centres, separated by circular lamaella over growth

36
Q

What does a high degree of crystallinity generally lead to?

A

High solvent resistance & high Tg

37
Q

Describe the 3 ways to measure the Xc of a sample

A
  1. Use density gradient column and note relative flotation level to give relative density. Then do density when polymer is 100% crystalline (from WAXS data)/sample density
  2. WAXS (wide angle X-ray scattering) - measure material when 100% crystalline and then 100% amorphous. Area under crystalline graph/total area under graphs = Xc
  3. Use DTA (only one that can be performed on contaminated sample). DTA measures the samples temperature relative to a standard sample to detect phase changes and work out composition (using two thermocouples)
38
Q

How do polymer chains form crystals?

A

They must pack closely together in a regular 3D structure, the chains never touch because of the Van dear Walls forces between them.

39
Q

When can a polymer crystallise?

A

When the chains are mobile (above Tg)

40
Q

What’s the difference between a LDPE structure and a HDPE structure?

A

LDPE has lots of branching, limiting close packing and thus free volume - means it’s low density, low Xc and high Tg
HDPE has little free volume = high density, high Xc

41
Q

Which polymers have linear backbones? What does this mean?

A

Ethers, esters, and amides. Means they have low free volume = high density & Xc

42
Q

Why could polymers not crystallise?

A

Irregular units, high free volume, quenched so don’t have enough time to move into a regular structure

43
Q

What are the three orientations of phenyl rings and what do they do in relation to Xc?

A

Ortho - 60° rotation, amorphous - causes Xc to decrease
Meta - 120° rotation
Para - 180° rotation, semi crystalline, can cause Xc to decrease or stay the same

44
Q

How do polar groups affect Tg & Tm?

A

Eg Cl, OH, C=O - facilitate secondary forces between lamaella structure, increase Tg and Tm

45
Q

Name two nylons, draw their structure and describe their differences (in relation to solubility, Tg and Tm)

A

PA6 -H - 6 carbons - double bond O
N

PA6,6 - H - 6C - N - double bond O - 6C - =O
N H
Longer chains between amide groups (HN - C=O) = lower % H bonds per chain = lower Tg and Tm
Longer chain length = reduced solubility

46
Q

What is tacticity in PP

A

Tacticity is the arrangement of units within a polymer chain.
In PP must control addition of methyle unit groups onto chain or random spacing will occur - this disrupts crystallinity of structure ( and increases free volume) which decreases physical properties

47
Q

How can a catalyst aid tacticity

A

Catalysts control the orientation and addition of unit cells creating a repeating pattern = increased Xc and mechanical properties.
Eg. Methyle groups rotate 120° around backbone which allows for closer packing and increased Xc

48
Q

What is creep?

A

Creep is the increase in strain when a constant stress is applied over time. In polymers, creep can only occur above Tg as poly,era must be mobile and exhibit long range motion - some materials show elastic behaviour and can recover from creep

49
Q

Describe the two stages of crystallisation

A
  1. Nucleation - attachment of chain to nucleation agent = heterogeneous nucleation or chains become entangled = homogeneous nucleation. Nucleation rate generally lower with high temp.
  2. Growth - diffusion of crystallising chains across melt interface, these attached to nucleated surface. Growth rate higher with high temp as higher rate of diffusion.
50
Q

What is the polymer solidification model?

A

Start with a coiled chain with short chain lengths -> trans sections straighten out -> further chain alignment to form crystals

51
Q

How does crystallinity affect mechanical properties?

A

High Xc means the chains can’t slide over each other as easily = higher strength but low ductility
Low Xc means low strength but high ductility (up to 400%)

52
Q

What is the microstructure of semi-crystalline materials during deformation?

A

Amorphous and lamellae regions in layers. Amorphous chains straighten out -> lamaella regions align -> lamaella regions separate -> lamaella and amorphous regions align in direction of tensile axis.
Chains must be mobile (near or above Tg) or material will be too brittle

53
Q

What is DTA and what does it measure?

A

Heating plate heats 2 samples (sample and reference). Thermocouples record temp changes within the sample - determines if state change is exo/endo thermic and can be used to determine Xc

54
Q

How is enthalpy change calculated from a DTA graph?

A

From the area under/above the melting peak
If negative melting peak - endothermic
If positive melting peak - exothermic

55
Q

What is the structural difference between thermoplastic and thermoset polymers?

A

Thermoset polymers have crosslinks with in the material which decrease free volume and mobility = increased Tg

56
Q

How are thermoset polymers created?

A

Heat long fibres with epoxy resin, heat polymerises the resin = crosslinked system

57
Q

What type of polymer is rubber and how is it created?

A

Sulphur added to mixture, then heated at 300°C for 3 hours -> crosslinked system
This prevents plastic deformation as chains can’t slip (and dislocations can’t move through material) = hyper elastic material above Tg (60°C)

58
Q

What are thermoplastic elastomers?

A

Have same properties as rubber but can be repressed easily as crosslinks are thermally reversible

59
Q

How are crosslinks added to elastomers?

A

Copolymer created in diblock structure, one monomer in glassy state at operating temp - gives elastic and highly mobile monomer but with added strength

60
Q

Describe thermal degradation of a polymer via main chain scission

A

Happens when subject a polymer to high and sustained temperature, above Tm bonds in main chain break (scission process) = lower Mw

61
Q

Describe thermal unzipping

A

Follows initial thermal degradation, then endchain groups depolymerise into original monomers (90%)
Usually creates free radicals

62
Q

What is a free radical?

A

Uncharged, highly reactive molecule with an unpaired valence e-

63
Q

What dictates if a polymer goes through scission or unzips

A

Pendant groups within the polymer

64
Q

What is side group elimination?

A

Thermal dehydrochlorination leaving only C = C bonds, occurs slowly above Tg and leave hydrochloric acid
* scission of main chains at pendant groups can occur at the same time

65
Q

What are most plastic bottles made from, and how do you recycle them?

A

PET - degraded when reacts with hydrochloric acid
& PVC - goes through thermal dehydrocholrination
Means that contaminated sources can’t be recycled

66
Q

Describe hydrolysis degradation

A

Polyamide mixing with water causes a reverse step growth polymerisation which returns polymer to original monomer & reduces mechanical properties

67
Q

How do you prevent hydrolysis degradation?

A

Heat monomer pellets prior to processing (via heating - too hot = thermal degradation occurring), slow process

68
Q

What is biodegradation?

A

Biological enzymes break down bonds of polymer

Initial degradation is to amorphous surfaces - reduces Mw = increased chain mobility = further crystallisation

69
Q

What is photo degradation?

A

EM radiation degrades polymers by causing crosslinking, free radical creation and terminating chains early

70
Q

How is a polymer tested against environment weathering?

A

Placed in heated water bath containing uv tubes, samples are subjected to simulation of hydrolysis, thermal and photo degradation

71
Q

What is the relationship between exposure time and mechanical properties ?

A

Longer exposure time = more degradation, first 100 hours little degradation due to processing

72
Q

How are polymers protected against UV?

A

Some organic additives absorb UV radiation (so reduces polymer absorption)
Protective films decrease penetration distance and intensity exposure, confining degradation to surface layers - protective layer will degrade over time

73
Q

How do you detect polymer degradation?

A

During degradation Mw decreases - by measuring Wt of polymer you can detect amount of degradation occurring

74
Q

What is TGA, how does it work and what can it detect?

A

Thermogravometric analysis - Gas chamber, which contains a sensor and baffles, on a mass balance and inside a furnace.
Sample placed in non-reacting crucible and heated, gas passed around sample to get rid of reactants (baffles prevent eddy currents forming and modifying test). Correction curve must be ran to account for gas effect on sample upthrust.

75
Q

How can TGA be used in submarines?

A

Submarine cladding contains multiple elastomers so are easily deformed by sonar (no reflections), and filled with carbon black to prevent infrared analysis. Means TGA must be used to analysise composition of cladding - as cladding degrades weighs less = different comp

76
Q

What needs to be done to 1) a thermoplastic and 2) a thermoset polymer for them the be reprocessed?

A

Thermoplastic - only needs heat and pressure to flow, and one to form
Thermoset - needs de-polymerising to monomer form and then reprocessed (due to crosslinks)

77
Q

What products can’t vacuum forming be used for?

A

Can’t be used for curved products - can only be used for simple ones

78
Q

Name 2 polymers that can be used in vacuum forming

A

Pmma, pvc

79
Q

Describe the vacuum forming process

A

Heat polymer above tg (so it softens), vacuum applied to make sure polymer completely covers mould, cool polymer so it hardens

80
Q

What products can rotational moulding be used to produce?

A

Large, hollow products

81
Q

Describe the rotational moulding process

A

Put polymer powder into rotating vessel, heat above Tg until liquid (rotation makes polymer fill sides of vessel) and cool to stabilise polymer
*Like centrifugal casting

82
Q

Describe the extrusion technique and what products it can create

A

Put polymer pellets into screw extruder, heat barrel to heat polymer, polymer forced through due to create shape, then polymer cooled to stabilise.
Used to make rods and sheets

83
Q

Describe the injection moulding technique

A

Polymer pellets heated above Tg, pressure used to inject polymer into a mould, polymer cooled to stabilise (crystals form in orientation of pressure)

84
Q

How is a polymer fibre produced?

A

Polymer heated to form solution, then filtered and pressurised, put through spinneret and then cooled

85
Q

Name and describe the four methods of polymer shape stabilisation.

A

Melt - polymer is heated and then cooled
Wet - dissolved in solvent and then precipitated in non- solvent
Dry - dissolved in solvent and then solvent evaporated away
Gel - progressive solution of fibre in non-solvent

86
Q

What is the molecular orientation of grains in fibres?

A

Polymer chains coil up in solution - spinneret caused extension of chains in direction of movement

87
Q

What name is given to polymers with high molecular orientation

A

Lyotropic

NCM 1 (3 decks)

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