Polymers Flashcards
(102 cards)
1
Q
What is a polymer?
A
Many monomer units
covalently bonded together
2
Q
What is a homopolymer?
A
-A-A-A-
polyA / poly(A)
3
Q
What is an alternating copolymer?
A
-A-B-A-B-A-B-
poly(A-alt-B)
4
Q
What is a block copolymer?
A
-A-A-A-B-B-B-
poly(A-b-B)
5
Q
What is a statistical copolymer?
A
-A-B-A-A-A-B-A-B-B-A-
poly(A-stat-B)
6
Q
What is step polymerisation?
A
Generally produces heteroatom polymer backbone
7
Q
What is chain polymerisation?
A
Generally produces homoatom polymer backbone
8
Q
What is the exception for chain polymerisation?
A
Ring-opening polymerisation
Produces heteroatom polymer backbone
9
Q
What is Raoult’s law?
A
Relative lowering of vapour pressure
of a dilute solution
containing a non-volatile solute
proportional to mole fraction of solute
10
Q
What are thermoplastic polymers?
A
Hard —> Soft (Heat)
Soft —> Hard (Cool)
Physical, reversible change
Melt-processable
∴ potentially recyclable
11
Q
What are thermoset polymers?
A
Soft —> Hard (Heat)
Irreversible
Becomes hard & rigid
∵ inter-chain cross-linking reaction
12
Q
What does isotactic mean?
A
All R groups
pointing in same direction
13
Q
What does sydiotatic mean?
A
Alternating R groups
14
Q
What does atactic mean?
A
Random distribution R groups
15
Q
What are the features of isotactic & syndiotatic polymers?
A
Ordered microstructure
Efficient interchain packing
⇒ high crystallinity & density
16
Q
How can isotactic & syndiotatic polymers be synthesis?
A
Often with
Ziegle-Natta catalysts
17
Q
What are the features of atactic polymers?
A
More typical structures
Reduced packing efficiency
⇒ low crystallinity & density
Often get inferior materials
18
Q
What is the number-average equation?
A
Mn = ΣnM / Σn
19
Q
What is the weight-average equation?
A
Mw = ΣWM / ΣW = ΣnM(^2) / ΣnM
20
Q
What is the z-average equation?
A
Mz = ΣnM(^3) / ΣnM(^2)
21
Q
What does the molecular weight distribution show?
A
Mn biased towards low mol. wt. species
Mw biased towards high mol. wt. species
Mz rarely used
Mn < Mw < Mz for all real polymers
22
Q
What is the dispersity?
A
Dispersity = Mw / Mn
23
Q
What are the trends in dispersity?
A
Mw / Mn = 1
Ideal, perfectly monodisperse polymer
All chains have exact same length
< 1.20 = near monodisperse
> 1.20 = polydisperse
24
Q
What is the degree of polymerisation?
A
Mean no. monomer units per polymer chain
25
What is the equation for degree of polymerisation?
DP = Mn(polymer chain) / mol. wt. monomer
DP < 10 = oligomer
26
What is a method to determine Mn?
End group analysis
27
What are experimental methods to determine Mw?
Static light scattering
Needs dust-free solution
Neutron scattering
Expensive
X-ray scattering
Need high Z atoms
28
What is the experimental method to determine the whole MWD curve?
Gel permeation chromatography
Measures hydrodynamic volume of polymer coils
29
What happens during GPC?
Low mol. wt. polymers
tend to get lost in microporous beads
∴ high mol. wt. polymers pass through first
30
What is the disadvantage of GPC?
Requires calibration
Not all polymers have known calibration
Usually v. expensive
Often not available for polymer of interest
30
How can you get the mol. wt. Mv viscosity average?
Measure solution viscosity
Descent time between meniscus A & B
Viscosity (𝜂) ⍺ time of decent
31
What is the equation for viscosity?
[𝜂] = k x Mv*
32
What are the physical properties of polymers?
Not fully extended in solution
most exist as random coils in dilute solution
∵ entropically favourable
more conformations possible
33
What is the equation for the radius of gyration?
Rg(^2) = 2(nL(^2))
34
What is the structure of polymers in the solid state?
Real polymers
can be 100% amorphous
cannot be 100% crystalline
35
What is the equation of Dc?
Dc = (Area of Bragg peaks / total area under curve) x 100
36
What is required for high crystallinity?
Efficient chain packing
No bulky side groups
37
When is high crystallinity more likely?
Regular chain structure
eg. isotactic / syndiotactic
Stiff chain
eg. aromatic spacer group
conjunction
Strong inter-chain forces
eg. H-bonds
38
What is the Tm?
Temp. where crystalline regions
within polymers 'melt'
Becomes completely amorphous
39
What is the Tg?
Temp. of onset
of significant segmental chain motion
40
What is the effect of Tg?
For 100% amorphous polymers
T < Tg polymer glass
T > Tg polymer rubber
41
What is the effect of structure on Tg?
Decreases w/ increasing length pendant side chain
poly(methacrylate) always higher than poly(acrylate)
∵ methyl group causes greater side chain stiffness
42
What is the Fox equation?
Tg = Tg(max) - (k/Mn)
43
How can you raise/lower Tg?
Make statistical copolymer
Raises or lowers
Add plasticiser
Lowers
44
What is the Tg equation for copolymers?
1/Tg* = F1/Tg1 + F2/Tg2 + ...
45
What is the assumption for the Tg of a copolymer?
No interactions between copolymers
46
What are the elastomer properties?
Must stretch rapidly to high elongation
w/ little E dissipated as heat
Must have high tensile strength
& high modulus
when fully expended
Must retract rapidly
Must recover original dimensions
47
What are the molecular requirements for elastomers?
High mol. wt.
for mechanical strength
T > Tg
for high segmental ability
Must be amorphous in unstretched state
Must contain network of cross links
48
What does esterification depend on?
Monomer functionality
Monofunctional = low mol. wt. species
Bifunctional = linear polymer
Multifunctional cross-linked gel
49
What are the simplifying assumptions of step polymerisation?
Reactivity of functional group
independent of molecule size
No side reactions or impurities
50
What is rate of polymerisation for step polymerisation?
RoP = rate contant x [reactants in RDS]
RDS = second step
51
What is the effect of acid catalysed polymerisation?
Catalysed by strong acid
[HA] constant
∴ can assume 1:1 diol/diacid stoichiometry
52
What is the degree of polymerisation of diesters produces by step polymerisation
DP = C0 / C = 1 / (1-p)
Mn = M x DP
53
What effects the molecular weight of polyester?
Presence of impurities
Use high temp./long reaction times
to maximise p
must drive eqmb. to right
Non-stoichiometry of bifunctional monomers reduces DP
54
What are the general characteristics of chain polymerisation?
Each polymer chain grows fast
Chain reaction by radiation, initiator or catalyst
Chain growth involves kinetic chain of reactions
55
How do chain polymerisation reactions proceed?
Via monomer addition to active centre
56
When is chain polymerisation useful?
Vinyl monomers
57
What are the advantages of free radical polymerisation?
Robust technique
applicable to many vinyl monomers
Wide range operating conditions
works well in water
58
What are the disadvantages of free radical polymerisation?
Poor selectivity
radicals v. reactive species
Relatively poor control over MWD
59
What are free radicals usually generated by?
Chemical initiators
UV radiation
Gamma radiation
60
What are the common classes of chemical initiators?
Peroxides
Azo compounds
61
What is the equation for half life?
t1/2 = (log e 2) / Kd
62
What is the initiator efficiency?
Fraction of radicals
available for polymerisation
f increases w/ temp.
typically 0.6 - 1.0
63
What is the cage effect?
Free radicals can recombine
rather than w/ monomer
Reduces initiator efficiency
64
What gives the rate of production of radicals?
Rate of initiator, Ri
corrected using f
65
What are the types of termination for free radicals?
Combination
Disproportionation
66
What is the dominant termination mechanism?
Combination more common for
stryenes & acrylates
Disproportionation w/
methacrylates
up to 80% at 80°C
67
What is the rate of polymerisation?
RoP ⍺ [M][I](^1/2)
68
What is the kinetic chain length?
Dk = mean no. monomer units
consumed per radical active centre
Dk ⍺ [M][I](^-1/2)
69
What is the effect of termination on Dp?
Solely combination
Dp = 2Dk
Solely disproportionation
Dp = Dk
70
What affects the molecular weight of the polymer?
If [M] increases
rate increases
polymer mol. wt. increases
If [I] increases
rate increases
polymer mol. wt. decreases
71
What is the effect of intramolecular transfer of polyethylene/PVC?
Usually get short chain branching
reduces crystallinity
& lowers density
72
What is the retarder?
Species that react w/
radical to form product
incapable of adding further monomer
73
What is an initiator?
V. effective retarder
Usually added to monomers
to prevent spontaneous polymerisation
during long-term storage
Can be easily removed by distillation
74
What coupling is favoured for chain polymerisation?
Head-to-tail
Favoured for both steric & electronic
75
What is anionic polymerisation?
Chain polymerisation
w/ anionic active centre
Repulsive anionic ends
cannot react w/ each other
∴ no intrinsic termination step
76
What is a living polymer?
Suitable vinyl monomer
polymerised in non-reactive solvent
in absence of impurities
77
What are the requirements for living polymers?
Rigorous purification of
monomer, solvent & initiator
to remove H2O
Dry reaction vessel
Need X to be EWG substituent
to stabilise anionic charge
78
What is the criteria for a narrow MWD for anion polymerisation?
Ki >> Kp
Monomer conc. & temp.
uniform throughout reaction solution
No side reactions/branching
occurs during propagation
No protic impurities are present
79
What is the equation for the number average degree of polymerisation?
DP = no. mol. monomer / no. mol. Initiator
DP = [M]/[I]
80
What is the equation for the number average molecular weight?
Mn = m(monomer) / no. mol. initiator
81
What is the difference between radical & anionic polymerisation?
Radical
Broad distribution chain length
∵ termination
Anionic
Narrow distribution chain length
∵ fast initiation & lack of termination
82
What are the characteristics of living polymers?
Linear evolution mol. wt.
w/ monomer conversions
Low dispersity
Mw/Mn = 1.1 - 1.2
Can make block copolymers
via sequential monomer addition
83
What are the synthetic advantages of anionic polymerisation?
Homopolymers w/ narrow MWD
can be prepared over wide range mol. wt.
by varying monomer/initiator molar ratio
Can cap living polymers
w/ specific end groups
Well defined AB diblock (or triblock) copolymers
can be synthesised by addition
monomer B to living polymer A
84
What are the disadvantages of anionic polymerisation?
Synthetically demanding
Unsuitable for many functional monomers
containing labile protons
85
How can monomer functionality be masked?
W/ protecting group
that can selectively removed after polymerisation
86
What is ring-opening polymerisation?
Can get PEO w/ v. narrow MWD
Mw/Mn < 1.10
Can synthesis PS-PEO diblock copolymers
by sequential monomer addition
PS must be synthesised 1st
∵ oxyanion chain end of living PEO
not reactive enough to polymerise styrene
87
What is the solution viscosity of star polymers?
Relatively weak temp. dependence
88
What is bulk polymerisation?
Just monomer & initiator
89
What are the properties of bulk polymerisation?
ΔH(polymer) < 0
Chain polymers v. exothermic
difficult to control heat evolved w/ vinyl monomers
Highly viscous
stirring problems even @ low conversion
Local heat spots may occur
can get degradation/decolourisation
& MWD can be v. broad
Usually only used for less exothermic step polymerisation
90
What are the properties of solution polymerisation?
Solvents act as diluent
& dissipates heat of polymerisation
Lower viscosity
easier to stir solution
Can get chain transfer of radical to solvent
Mn decreases
/ have to remove solvent to isolate polymer
toxic & expensive
91
What is precipitation polymerisation?
Monomer + initiator
Dilute in reaction medium
but polymer insoluble
Least important
92
What are the advantages of precipitation polymerisation?
Better control over thermal dissipation
No viscosity problem
93
What are the disadvantages of precipitation polymerisation?
Poor mol. wt. control
Difficult to obtain high mol. wt.
94
What is dispersion polymerisation?
Precipitation polymerisation
in presence suitable polymeric stabiliser
Stabiliser absorbs into precipitating polymer nuclei
& prevent further growth
via steric stabilisation
95
What are the properties of dispersion polymerisation?
Microscopic particles of polymers
suspended in solvent
∴ do not aggregate further
Good control over
heat dissipation & viscosity
Can be conducted in aqueous/non-aqueous media
V. high solids w/ low viscosity
sine particles
96
What is the formulation of suspension polymerisation?
Water-immiscible monomer
Water-insoluble initiator
Water-soluble polymeric stabiliser
Water
97
What is an example of a polymeric stabiliser?
Poly(vinyl alcohol)
Cellulosic derivatives
98
What are the properties of suspension polymerisation?
Efficient stirring important
for disperse polymer beads
Low solution viscosity
99
What is the formulation of emulsion polymerisation?
Water-immiscible monomer
Water-soluble initiator
Surfactant
Water
100
What are micelles?
Surfactant forms
losse colloidal structure
101
What are the properties of emulsion polymers?
Low monomer solubility in aq phase
Initiator not soluble in monomer droplets
99% polymerisation inside monomer-swollen micelles
No more than 1 polymer radical per micelle
∵ rapid radical annihilation
reduces rate termination
relative to rate of propagation
Good thermal & viscosity control
V. high monomer conversion
