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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
How does long range motion occur in a polymer
When there’s free volume in the polymer, converted confirmation changes can occur (gauche&trans together) leading to long range motion
26
How can a polymer that’s brutally fractures reconnect?
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.
27
Define reptation time and libration
Reptation time - time for entire polymer chain to be translated Librarion - only partial rotation and partial trans/gauche interchanges
28
Name the 10 factors that affect Tg and briefly describe how they affect it
*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
29
How can TMA be used to measure compression/expansion?
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
30
How can you use TMA to determine Tg
Apply constant load to sample, and then heat the sample - when there is some displacement = Tg
31
How can TMA be used to measure swelling?
Set the rod on the sample surface - suburge in water and measuring displacement
32
What are spherulites and how do they form?
Form when semi-crystalline materials cool down and harden | Spherical as they grow rapidly until impingement
33
Describe the 2 phase polymer model
ordered Lamella sheets with an amorphous layer in the middle of them
34
Describe the 3 phase polymer model
Amorphous layer in between lamellae sheets. | Cilia and tie chains present as well as short medium and long lamellae chains
35
Draw the structure of strain induced crystallisation
Rod like centres, separated by circular lamaella over growth
36
What does a high degree of crystallinity generally lead to?
High solvent resistance & high Tg
37
Describe the 3 ways to measure the Xc of a sample
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
How do polymer chains form crystals?
They must pack closely together in a regular 3D structure, the chains never touch because of the Van dear Walls forces between them.
39
When can a polymer crystallise?
When the chains are mobile (above Tg)
40
What’s the difference between a LDPE structure and a HDPE structure?
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
Which polymers have linear backbones? What does this mean?
Ethers, esters, and amides. Means they have low free volume = high density & Xc
42
Why could polymers not crystallise?
Irregular units, high free volume, quenched so don’t have enough time to move into a regular structure
43
What are the three orientations of phenyl rings and what do they do in relation to Xc?
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
How do polar groups affect Tg & Tm?
Eg Cl, OH, C=O - facilitate secondary forces between lamaella structure, increase Tg and Tm
45
Name two nylons, draw their structure and describe their differences (in relation to solubility, Tg and Tm)
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
What is tacticity in PP
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
How can a catalyst aid tacticity
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
What is creep?
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
Describe the two stages of crystallisation
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
What is the polymer solidification model?
Start with a coiled chain with short chain lengths -> trans sections straighten out -> further chain alignment to form crystals
51
How does crystallinity affect mechanical properties?
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
What is the microstructure of semi-crystalline materials during deformation?
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
What is DTA and what does it measure?
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
How is enthalpy change calculated from a DTA graph?
From the area under/above the melting peak If negative melting peak - endothermic If positive melting peak - exothermic
55
What is the structural difference between thermoplastic and thermoset polymers?
Thermoset polymers have crosslinks with in the material which decrease free volume and mobility = increased Tg
56
How are thermoset polymers created?
Heat long fibres with epoxy resin, heat polymerises the resin = crosslinked system
57
What type of polymer is rubber and how is it created?
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
What are thermoplastic elastomers?
Have same properties as rubber but can be repressed easily as crosslinks are thermally reversible
59
How are crosslinks added to elastomers?
Copolymer created in diblock structure, one monomer in glassy state at operating temp - gives elastic and highly mobile monomer but with added strength
60
Describe thermal degradation of a polymer via main chain scission
Happens when subject a polymer to high and sustained temperature, above Tm bonds in main chain break (scission process) = lower Mw
61
Describe thermal unzipping
Follows initial thermal degradation, then endchain groups depolymerise into original monomers (90%) Usually creates free radicals
62
What is a free radical?
Uncharged, highly reactive molecule with an unpaired valence e-
63
What dictates if a polymer goes through scission or unzips
Pendant groups within the polymer
64
What is side group elimination?
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
What are most plastic bottles made from, and how do you recycle them?
PET - degraded when reacts with hydrochloric acid & PVC - goes through thermal dehydrocholrination Means that contaminated sources can’t be recycled
66
Describe hydrolysis degradation
Polyamide mixing with water causes a reverse step growth polymerisation which returns polymer to original monomer & reduces mechanical properties
67
How do you prevent hydrolysis degradation?
Heat monomer pellets prior to processing (via heating - too hot = thermal degradation occurring), slow process
68
What is biodegradation?
Biological enzymes break down bonds of polymer | Initial degradation is to amorphous surfaces - reduces Mw = increased chain mobility = further crystallisation
69
What is photo degradation?
EM radiation degrades polymers by causing crosslinking, free radical creation and terminating chains early
70
How is a polymer tested against environment weathering?
Placed in heated water bath containing uv tubes, samples are subjected to simulation of hydrolysis, thermal and photo degradation
71
What is the relationship between exposure time and mechanical properties ?
Longer exposure time = more degradation, first 100 hours little degradation due to processing
72
How are polymers protected against UV?
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
How do you detect polymer degradation?
During degradation Mw decreases - by measuring Wt of polymer you can detect amount of degradation occurring
74
What is TGA, how does it work and what can it detect?
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
How can TGA be used in submarines?
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
What needs to be done to 1) a thermoplastic and 2) a thermoset polymer for them the be reprocessed?
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
What products can’t vacuum forming be used for?
Can’t be used for curved products - can only be used for simple ones
78
Name 2 polymers that can be used in vacuum forming
Pmma, pvc
79
Describe the vacuum forming process
Heat polymer above tg (so it softens), vacuum applied to make sure polymer completely covers mould, cool polymer so it hardens
80
What products can rotational moulding be used to produce?
Large, hollow products
81
Describe the rotational moulding process
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
Describe the extrusion technique and what products it can create
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
Describe the injection moulding technique
Polymer pellets heated above Tg, pressure used to inject polymer into a mould, polymer cooled to stabilise (crystals form in orientation of pressure)
84
How is a polymer fibre produced?
Polymer heated to form solution, then filtered and pressurised, put through spinneret and then cooled
85
Name and describe the four methods of polymer shape stabilisation.
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
What is the molecular orientation of grains in fibres?
Polymer chains coil up in solution - spinneret caused extension of chains in direction of movement
87
What name is given to polymers with high molecular orientation
Lyotropic

NCM 1 (3 decks)

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