Polymers Flashcards
1
Q
Crystallisation
A
Driving force is g = h - ts
S is reduced when the temperature is decreased as a crystalline structure is achieved
System is in equilibrium when G is a minimum
Refers to the alignment of polymer chains at specific distances (occurs due to strong intermolecular forces
polymers usually only partially disentangle so they will only partially crystalise
2
Q
Mechanism of Crystallization
A
Thermodynamically favourable when the magnitude of enthalpy (latent heat) is very negative and the change is greatest than T*S
3
Q
Tacticity
A
Structural property of polymers that refers to the way groups are arranged along a hydrocarbon
- Isotactic
- Syndiotactic
- Atactic
4
Q
Isotactic
A
- Have pendant groups which are located on the same side of the hydrocarbon chain
- Usually arrange themselves in higher ordered structures with high level of order (semi-crystalline)
- All groups have the same steric order and usually reaches 65% crystallinity
5
Q
Syndiotactic
A
- Have pendant groups that are arranged in an ordered and regular pattern around the hydrocarbon chain
- Direction of the group alternates
- Semi-crystalline
6
Q
Atactic
A
- Random pattern of pendant group arrangement
- The polymers lack translational symmetry making them amorphous
- Are not crystalline
- Not the same steric order
7
Q
Lamellar Structure in Polymer Crystals
A
- Polymer chains must fold in order to form crystals
- Lamellar crystallites are never 100% crystalline due to folded regions
- Fold length is calculated from L (long period) through SAXS
8
Q
SAXS
A
- Small Angle X-ray Scattering
- Measures the lamellar crystal thickness by analysing peak positions
- follows Braggs law: n(lambda) = 2dsin(theta)
9
Q
WAXS
A
- Wide Angle X-ray Scattering
- Used to measure degree of crystallinity
- Mass fraction of crystal = Xc = Ac/(Aa + Ac)
- Peaks will refer to diffraction peaks. If its amorphous there will be no peaks and if its amorphous + crystalline, there will be many peaks present
10
Q
Effect of pressure on lamellar structure
A
Increasing pressure increases crystallisation at low supercooling
11
Q
Annealing Effect
A
- Refers to the thermal treatment post crystallisation at temperatures higher than the crystallisation temperature
12
Q
Crystallisation Under Stress
A
- Under certain processes, crystallisation occurs under lots of stress (from strain)
- Extends the chain molecule which reduces the number of possible conformation
- Entropy = S = kbln(resistance)
13
Q
Spherulite
A
- Formed from melt or concentrated solution
- Folding is stacked and radiates from a nucleus centre
- Single lamellas grow into hendrides which are groups of lamellas radiating from a central nucleaus
- There is a big range of orientations with amorphous regions between the lamellar crystalline branches - semi crystalline
- Growth of spherulites strongly depends on crystalisation temperature - is at 0 at Tm and Tg and max in between these two temperatures
14
Q
Crystal Growth
A
- Refers to the temperature dependence for short range transport
- Below Tf, there will be no mobility in the polymer chains
- As Tg is approached, the transport term severely limits crystallisation
- Crystallisable material is moved to the growth face (where the existing nucleus is) and non-crystallisable material is moved away from the growth face
15
Q
Growth of Spherulites
A
- r = vt (spherulite radius = growth rate * time)
- The spherulites grow linearly with time at a constant pressure
- Polymer chains with more molar mass take a longer amount of time to crystallise
- When density increases, crystallisation will also increase so crystallisation rate increases during heterogenous nucleation
- Also increases with epitaxy (which is the ability for a layer of a material to conform to the same crystal structure as the layer underneath
16
Q
Heterogenous nucleation
A
- Fluid materials are in contact with a surface or different medium (like cold air)
- Enhances nucleation at the interface
17
Q
Melting of polymers
A
- Slow melting is better as fast melting does not give the system enough time to reorganize
- Equilibrium at Tm means that Delta G = 0 so Tm = Delta Hf/Delta Sf (enthalpy of fusion/entropy of fusion)
- Transition from crystalline solid to a fluid phase is a first order transition which occurs at Tm
18
Q
Hoffman-Weeks Plot
A
- Tm vs Tc is generally linear and Tm > Tc
- Tm = Tc if heating and and cooling cycles were done infinitely
- Equilibrium temperature (Tm o) is the temperature where crystallisation takes place through an infinitely slow process and the crystal obtained is formed by fully extended chains
- Crystallisation temperature dictates the amount of crystaline component. Higher the crystalline temperature, the larger the crystalline component. More crystalline component will melt at higher temperatures
19
Q
Gibbs-Thompson Equation
A
- At T < Tm o, Delta G f is not 0
- At Tm there is no change in free energy for the idealized boundaryless crystal since melting and crystalisation are equally probably.
- A critical length will be achieved (l*) which will make a secondary nuclealus stable so delta Gn = 0
20
Q
Copolymer
A
- Consists of two or more types of monomers
- If a polymer is made of a crystallisable polymer and non-cystallisable polymer
21
Q
Random copolymers
A
- don’t crystallise
- generally amorphous
- if it is a block or graft polymer, it can partially crystallise
22
Q
pressure and Tm
A
- As pressure increases there is an increase in the supercooling at processing temperature (Tm increases)
- The addition of small molecules (like solvents and monomers) interact with the macromolecular chains through Van der Wall forces or Hydrogen Bonds
- Causes depression of the melting temperature of the polymer solid
- Higher pressure induces higher melting temperature which allows the polymer to crystalllise at higher T
23
Q
Amorphous state of polymers
A
- State which does not have any long-range order
- Lack of order caused by highly irregular polymer chain and the kinetics of the formation of the solid exceed the rate at which crystallisation may occur
- Does not give rise to typical features of crystals but still characterised by glass transition temperature
24
Q
Glass transition temperature
A
- Temperature where amorphous polymer changes from hard/glossy state to a soft
- At T < Tg - glass (chains are still frozen in position
- At T > Tg - rubber-like (chains are allowed movement)
- Tg is only associated with the amorphous portion. Properties of the material change as a function of temeprature
- Acts as a second-order thermodynamic transition
25
Free Volume
1. Molecules have thermal motion and this motion refers to spheres oscillating in a cage of neighbours.
2. oscillations create a "free volume" higher than the empty space characteristic of random close packing
3. Any significant movement of the polymer chain is generated by rotation about the single bonds connecting the atoms in the chain
4. Polymer undergoes a series of kinetic units. The moves are independent and involves the cooperative movement of a number of consecutive chain atoms
5. Free volume is generated as extra space generated by the random molecular arrangement rotations
26
Specific Volume
volume occupied by a unit of mass of a material
27
Factors affecting Tg
1. Chain Flexibility & molecular structure
2. Steric Hindrance
3. Branching
4. Cross-linking
5. Polymer Molar Mass
6. Additives
7. Chemical Composition
28
Chain Flexibility & Molecular Structure (Tg)
1. The more flexible the backbone, the lower the Tg of the polymer
2. Steric hindrance is the introduction of a side group. As the side group gets bigger and bulkier, the Tg goes up
3. Adding a flexible side group will lower the Tg
4. Side groups stops the backbone from rotating. Less flexible, higher Tg
29
Effect of Branching (Tg)
1. Branching leads to the restriction of segmental mobility (increase Tg) and the increase of end groups and free volume (lower Tg)
2. Hard to predict what will happen but small branching will likely decrease Tg and higher branching will increase Tg
30
Effect of Crosslinking (Tg)
1. Increasing cross-linking density increases Tg
2. Closslinking will reduce the available free volume which will increase Tg
31
Thermosetting
1. Polymers that increase in crosslinking as it is heated
2. Becomes permanently hard when heated above a critical temperature
3. Does not soften upon reheating as they are crosslinked
32
Thermoplastic
1. Soften when heated above Tg
2. Can be shaped and after cooling, it'll harden again
33
Elastomers
1. Exhibit large reversible elongation under a small-applied stress. No energy dissipation
2. Hyperelasticity - stretch and still go back
3. Typically have very low stiffness and low Young's modulus
4. Derived from liquids of polymer molecules - they are able to deform readily with little energy dissipation because they retain teh liquid property that molecules can change shape regularly
5. Polymer molecules are cross-linked so their macroscopic properties are solidlike. The bonds are not stretched but are being uncoiled.
34
Effect of polymer molar mass (Tg)
1. Increasing molar mass reduces the mobility of the polymer
2. Tg will increase
35
Effect of Additives (Tg)
1. Adding plasticisers often used to lower Tg so polymer is easier to process
2. Plasticizers are small molecules like pollutants. Polymers may undergo a lack of flexibility due to evaporation of plasticizers
3. Water acts as a plasticizer for H-bonding so Tg can be decreased below room temperature
4. Adding low molar mass additives like Poly(p-phenylene oxide) will increase Tg
36
Effect of Chemical Composition (Tg)
1/Tg = W1/Tg1 + W2/Tg2
37
Glassy State
1. Polymers which can be cooled below its melting temperature without crystallising
2. Rigid and brittle material as chains are immobile and frozen in space
38
Block polymer
1. Usually consists of an ABA triblock copolymer
2. Phases separates in domains that are gassy and elastic
39
Homo-polymer
1. Made up of only a single type of monomer
40
Random copolymer
1. Refer to adding a fraction of polymer B to a chain a polymer A which destroys the ordering which will reduce Tm
2. Tg is less affected so Tg and Tm becomes close
41
Entropy
1. Favours phase separation as it maximises the number of spatial arrangements available to both A and B
2. Number of ways of arranging the chains
42
Interaction Energy
1. Energy change when mixing 2 polymers
2. Interaction energy can promote or inhibit mixing but usually favours phase separation for polymer blend
43
Delta G (mix)
1. Delta G (mix) = Delta G (interaction) - TdeltaS
2. Delta G (mix) > 0 for phase separation and negative for mixing
44
Lattice Model
1. Separates space into smaller pieces with the assumption that the species have equal volumes
45
Flory-Huggins Theory
1. Explains the energy of interactions only considering first neighbour interactions
2. If the polymer chain and solvent will mix or phase seperate, Gibbs energy must be considered
DeltaG (mix) = RT(n1ln(volume fraction 1) + n2 ln (volume fraction 2) + n1 (volume raction 2) (mole fraction))
46
Flory Huggins Interaction Parameter
X = 1/2 - C/theta (1 - theta/T)
1. X > 1/2 : poor solvent - solvent will precipitate (phase seperation). Adopts a collapsed conformation so it will interact as little as possible with the polymer
2. X < 1/2 : Good solvent so polymer chain is extended
3. X = 1/2 : Borderline between good and bad solvent. Polymer has no preference in interacting with itself or the solvent. Adopts a random coil orientation
47
Copolymers in solution
1. If Xa > 1/2 and Xb > 1/2 : collapsed state
2. If Xa > 1/2 and Xb < 1/2 : polymer will microphase separate in solution
3. If Xa < 1/2 and Xb < 1/2 : both polymer chains are extended in solution
48
Michelle FOrmation
Xa > 1/2 (hydrophobic); Xb < 1//2 (hydrophilic)
Hydrophobic block collapsed in the centre and surrounded by hydrophilic extended block at the corona (outer part of the michllle)
49
Microphase Separation
1. Microphase separation will only occur if the A and B block are incompatible enough
2. It is still one phase but theres microseparation
3. Increasing the volume fraction of one component will limit the volume and lead to stronger segregation
50
Densimetry
1. Using a density gradient coloumn to determine crystallinity
2. Density gradient coloumn is set up with water and ethanol. More water at the bottom and the ethanol moved to the top
3. Glass calibration spheres of known density which will settle to the point in the coloun where the ethanol/water mixture has the same density as the sphere
4. Difference in density between the amorphous and crystalline region is about 20%
51
Densimetry Pros
1. Requires limited capital
2. Simple
3. requires only small samples
4. Very accurate
52
Thermal Analysis
1. Can be calometric (record exo and endothermic transition (melting, crystallisation, degradation) or Second order transitions (glass temperature transition
53
Densimetry Cons
1. Senstive to bubbles in sample and fillers
2. Assumes no interaction between the ethanol/water and the sample
54
DSC
1. Differential Scanning Calorimetry
2. Temperature of the sample holder is kept the same as the reference holder by continuous adjustment of the heater power
3. Provides enthalpy
4. Melting and glass transition is endothermic, crystallisaiton is exothermic
55
DTA
1. Differential Thermal Analysis
2. Sample and reference are heated by a single heat source and delta T is measured and converted to heat
3. Tsample > Tref : exothermic
4. Tsample < Tref : endothermic
56
Increasing enthalpy of fusion
1. Can be done by increasing number of hydrogen bonds
2. Nylon 6 melts at higher temp than nylon 12 due to higher density of hydrogen bonds which increases enthalpy
57
Decreasing entropy of fusion
1. Can be done by increasing rigidity of the backbone by introducing a phenyl ring as increased rigidity will increase Tm
