Polymers Flashcards
What is the difference between Thermoset and Thermoplastics?
Thermosets have cross-links between chains while Thermoplastics do not.
Describe some properties of Thermoplastics
- Discrete chains
- Held together by secondary bonds (VDWs and Hydrogen)
- Melted and reformed Recyclable
- Generally softer and more flexible
- Common types- Polyethylene,Polystyrene, Nylon and PVC
Describe some properties of Thermoset plastics
- Chemically linked chains
- Don’t melt but degrade
- Good strength and stiffness in high temp.
- Good chemical resistance
- Made by mixing components (liquid or soft) which cross link and harden on reaction
- Common types- Epoxies and Polyesters
What are Elastomers?
- Technically thermosets
- Much fewer cross links than normal thermosets
- Cross links produced by Vulcanisation (adding sulphur)
- Chains are more flexible
Crystalinity in Polymers
Thermoplastics can either be amorphous or semi-crystalline
Thermosets are amorphous
Amorphous Thermoplastics
- No long-range strucural order
- Random glassy structure provides strength
- No major secondary bonds between chains
- Relatively stiff polymers required
- Polystyrene, Perspex, PVC and Polycarbonate
Melting point in Amorphous Thermoplastics?
None. At a certain temp. the polymer transisitons from hard solid to soft and rubbery
Amorphous Thermoplastic flexibility
- Chain flexibility determined by chemical structure
- Flexibility decreases with C=C and C-O-C and big side groups Increases with a non-C backbone
Semi-crystalline Thermoplastic
- Random regions of crystal and amorphous
- Chain folding and stacking
- Secondary bonding creates strength
- Definite melting point
- Chains extend from the crystaline to the amorphous regions providing mechanical strength
- Polyethylene, Polypropylene, Nylon
What factors affect the melting point of Semi-crystalline Thermoplastics
Flexibility of chains
Chemical interactions between chains (more H bonds, higher melting temp.)
Factors affecting crystallinity in Semi-crystalline Thermoplastics
- Slow cooling rate increases
- Branching decreases
- Impurities decrease
- Below a critical molecular weight decreases
Orientation of Fibres in polymers
This happens in the drawing process where polymer chains orient themselves along the fibre (Anisotropy)
Define Anisotropy
The property of being directionally dependent as opposed to Isotropy, which implies identical properties in all directions
Why would Polymers have a low Young’s Modulus?
Chains are not parallel to stress in bulk PE
There is a considerable amount of amorphous material
What is a Copolymer?
A polymer whose properties are improved when two or more polymers are combined
What are the types of Copolymers?
- Random- ABBBABAABBAAA- When two monomers have the same reactivity
- Alternating- ABABABABABABABA- When A-B is more reactive than A-A or B-B
- Block- AAAAABBBBBAAAAA- When A-B is less reactive than A-A or B-B
- Graft- When a second polymer is grafted onto the first in a seperate reaction
What are the properties of the Copolymers?
Alternate and random have properties that are the average of A and B
Block and graft show combined properties of A and B
How do you increase toughness in Polymers?
Add rubber particles to the polymer
The two types of Polymerisation?
Addition and Condensation
What is the Addition process?
- Need a monomer with a double or triple bond
- Activate a polymer chain with a free radical
- Double bond opens up and monomers join together
- e.g. Polyethylene, PVC and Polystyrene
What is the Condensation process?
- Involves reactions between functional groups at both ends of the monomer
- Acid+Alcohol gives (Poly)esters
- Amine+Acid gives (Poly)amides, Nylons
- If there are more than two functional groups on a monomer we can get branching, producing a thermoset
Addition vs. Condensation
Addition is easier to produce long chains, faster and has cheaper monomers
Condensation occurs easily under ambient conditions, Monomers often liquids so they are useful adhesives and it can also produce thermosets
