Topic 7- Polymers Flashcards
What is the Eden Project made from?
Ethylene tetrafluoro ethylene (ETFE)
Does ETFE have high or low density?
Low density
Important feature of ETFE?
Optically transparent.
Why are polymers used a lot for buildings?
Cheap and easy to produce.
What are polymers and plastics?
Polymers are materials with large macro-molecules, of which plastics is just on group.
What 6 useful properties do plastics have that mean they are widely used?
- Good thermal/electrical insulation.
- Low density/
- Easy to manufacture and low cost.
- Useful as adhesives (including composites).
- Good transparency.
- Durable
When do plastics have poor durability?
Under UV light.
What is the main characteristic of thermoplastic polymers?
Melt when heated so can form different shapes.
What is the main characteristic of elastomers?
Very high strain to failure (carbon chains are curled up so will stretch out).
What is the main characteristic of thermosets?
Crosslinks mean they will not melt when reheated, so will burn.
What is the main characteristic of Polysiloxanes?
No carbon backbone - alternating string of oxygen and silicon atoms.
What is the plastic most commonly used in building and construction?
PVC
What is the main component in polymers?
Carbon
What are the three organic elements in polymers? What is the fourth (non-organic?) element?
Carbon (C), Hydrogen (H), Oxygen (O).
Silicon (Si)
How big are polymer molecules in comparison to hydrocarbons?
Gigantic (macro-molecules).
What is the bonding in a polymer?
Atoms bound by covalent bonds to form long and flexible chains, secondary bonds between chains.
Are polymers crystalline or amorphous?
Amorphous or partially crystalline.
What is the name of the repeated unit in a polymer chain?
‘mer’ unit.
What does polymer mean?
Many parts
What is polymerisation?
Process of reacting polymer monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains.
What is one mer of polyethylene (PE)?
H-C-H H-C-H
What is one mer of polyvinyl chloride (PVC)?
H-C-H H-C-Cl
What is one mer of polypropylene (PP)?
H-C-H H-C-CH3
What are the two determining characteristics of polymers?
Low stiffness and low melting points.
What does a large molecular ‘weight’ mean?
Very long chains.
What are linear polymers? Give three examples.
Single chains which are flexible. ‘Mass of spaghetti’. Van der Waals between molecules.
Polyethylene (PE), PVC, Polystyrene
What are branched polymers?
Side branches off chains. Packing is less efficient so lower density. These will be less crystalline than linear polymers.
Which two types of polymer molecular structure are thermoplastics (melt when reheated?
Linear and branched.
Which two types of polymer molecular structure are thermosets (burn when reheated)?
Cross-linked and network.
What are cross-linked polymers? Give an example.
Adjacent chains that are joined at various positions by covalent bonds.
Rubbers.
Why are cross-links often non-reversible?
Due to chemical reactions.
What is a network polymer?
A heavily cross-linked polymer.
What is the order of increasing strength for polymer molecular structure?
Linear, branched, cross-linked, network.
What are copolymers? Give two examples/
Composed of two ‘mer’ units. E.g. ABABAB.
Polyamides and polyesters.
Why are the properties of copolymers often tailored and improved?
When more than one monomer is used, and irregular chain structure will result. This can be helpful to modify the intermolecular bonds.
What are the two main types of polymerisation?
Condensation polymerisation.
Free radical polymerisation.
What does condensation polymerisation do?
Eliminates a small molecule (H2O, HCl) by reacting a hyrdroxyl (-OH) and carboxylic acid (-C=O-OH) group to give an ester.
What does free radical polymerisation do?
Produce an addition reaction. Something with a dangling (incomplete) bond.
What are the four categories of copolymer?
Random, Alternating, Block, Graft.
What is the structure of a random copolymer? What random copolymers are used in tyres?
ABBABABBAAB.
Synthetic rubbers such as styrene butadiene (SBR).
What is the structure of an alternating copolymer?
ABABABABAB.
What is the structure of a block copolymer?
BBBBBAAAAA.
What might block copolymers be useful for?
Creating self-constructing fabrics with potential utility in semiconductor arrays.
What is the structure of a graft copolymer?
Chains of different mers grafted onto side of a longer chain.
How does the extent of cross-linking between chains influence the material?
Stronger attractive forces between chains leads to a stronger, less flexible polymer (e.g. nylon).
What will make a polymer flexible?
If chains are able to slide past each other easily (VdW bonding).
What do thermoplastics have a greater ability to form?
(Semi-) crystalline regions.
Are thermoplastics ductile? Why?
Yes, long period of plastic deformation.
Why are thermosets difficult to recycle?
Do not soften on reheating once formed.
What percentage of mers are cross-links in thermosets?
10-50 %.
Which type of polymer provides shape ‘memory’/retention?
Elastomers - linear with some cross-linking.
What is ‘heat shrink’?
A property of elastomers to insulate electrical cables.
What is molecular weight Mw?
Mass of a mole of chains.
Which three properties are linked to molecular weight?
Stiffness, tensile strength and viscosity.
How does tensile strength vary with number of repeating units?
Tensile strength increases with the number of repeating units.
What is the approximate critical length needed before strength increases?
Approx. 100 but only around 40 for nylons.
How are spherlites formed?
Folded chain lamellae grow and spread out to reach more crystallisable material. The ends of the lamellae twist producing spherelites.
What is the optical feature of amorphous polymers?
Transparent in pure state.
How does % crystallinity vary with molecular weight?
Greater the molecular weight, greater the % crystallinity.
In what temperature range will crystallinity occur?
Between Tg and Tm.
What is formation rate of crystals a product of?
Nucleation rate and growth rate.
When in crystallisation growth rate a maximum?
Just below Tm.
When is nucleation rate maximum?
Just above Tg.
When is overall crystallisation formation rate maximum?
About halfway between Tg and Tm.
What is the equation for total volume of a polymer?
V = Vc + Va
What is the equation for mass of a polymer?
W = Wc + Wa
What is the equation for density of a polymer?
pV = pcVc + paVa
What is the equation for mass fraction of crystalline regions?
xc = Wc/W = pcVc/pV
What two things can dramatically affect the mechanical properties of polymers?
The molecular structure.
Temperature at which it is operating.
What will the structure of a brittle polymer consist of?
Heavily crosslinked and chains aligned.
What happens when force is applied to a crystalline region of a polymer? (Semicrystalline)
Crystalline regions align themselves with direction of force.
What is crazing? (Linear)
When molecules are drawn out on a very small scale producing strong strands which bridge microcracks. E.g. clear ruler turns white when you bend it.
What happens when a polymer is cold drawn?
Molecules draw out and align so material is much stronger in draw direction than before. Annealing possible.
How does cold drawing affect the properties of a polymer?
Increases elastic modulus and tensile strength.
Reduces ductility.
How does the structure rearrange when an elastomer is stretched?
Chains are straight but still cross-linked.
What does the mechanical state of a polymer depend on?
Glass transition temperature, Tg.
What is the Tg for Polyethylene? What is its mechanical state at room temperature?
3 degrees C.
Ductile, low E.
What is Tg for PMMA? What is its mechanical state at room temperature?
60 degrees C.
Brittle/ high E.
How does the amount of side groups affect Tg and Tm?
More side groups mean less ease of rotation, therefore melting point and glass transition temperatures are higher.
What is Tg and Tm for Polyvinyl Chloride and Polyethylene?
Polyvinyl chloride:
Tm = 220 degrees C Tg = 100 degrees C
Polyethylene:
Tm = 115 degrees C Tg = -18 degrees C
What happens to stiffness, strength and ductility of PMMA as you reduce the temperature?
Strength and stiffness increase.
Ductility decreases.
What are the three differences between injection moulding for thermoplastics and thermosets?
- Thermoplastic: screw root tapered. Thermoset: screw root straight.
- Thermoplastic: mold is cooled. Thermoset: mold is heated.
- Thermoplastic: two hoppers (resin and colorant). Thermoset: one hopper (just resin)
What is important to consider when injection moulding thermosets?
To melt the resin but not let cross-links form until it is in the mold.
What are four advantages of injection moulding polymers?
- Complex shapes.
- Rapid manufacturing.
- Thousands of samples can be manufactured (hence polymers are cheap).
- Very cheap once infrastructure in place.
