Polymers Flashcards
What is a Thermoplastic polymer?
- A semi-crystalline and amorphous polymer with no cross-linking
- When heated melts a little bit and you can reshape it
What is a Thermoset polymer?
- An amorphous polymer that has cross-linking which means that when heated it doesn’t change shape
- it is irreversibly hardened
What is the Glass transition temperature?
- the temperature where a polymer transitions from a hard glassy composition to a soft rubbery material
What is the main difference between Thermoplastic and Thermoset polymers in terms of structure?
Cross-linking
When are polymers most viscoelastic?
Close to their glass transition temperature
What is an elastomer?
Thermoset polymers that are above their glass transition temperature which means that they have a certain level of viscoelasticity
What factors influence crystallinity?
Chain length, chain branching, interchain boning
How does chain length influence crystallinity?
longer chains mean a higher level of crystallinity
How does chain branching influence crystallinity?
The more chain branching means less crystalline regions
this is where low density and high density come into play
high density have less branching
low density have more branching
How does interchain bonding influence crystallinity
hydrogen bonding increases crystallinity (this doesn’t mean cross-links)
- side chains reduce crystallinity
What techniques are used to make polymers?
Condensation and addition
Give a brief outline of how condensation polymers are made?
Two monomers are reacted together and a small molecule is eliminated
All proteins are condensation polymers
water is always the small molecule that is released
Give a brief outline of how addition polymers are made?
Free radical additions, it’s basically addition but one of the monomers needs to have a free radical
What is a free radical?
An atom/molecule/ion that has an unpaired outer shell electron
What are the general properties of a non-biodegradable polymer?
Generally, high MW polymers that do not degrade in the body
What are problems that are faced by non-biodegradably polymers?
- Leaching of plasticisers and additives
- surface reactions and absorption of proteins
- enzymatic degradation - but this is on a very small scale
What are plasticisers?
A substance added to polymers to make them more flexible
Give me a bit of information about polyethylene
A bioinert and hydrophobic polymer
yield strength too low for load-bearing applications
yield strength increases with molecular weight
What are the different types of polyethylene used for?
UHMWPE - cup part of a hip replacement
HDPE and LDPE - catheter tubes and drains
give me a little bit of information about Polysiloxanes
another bioinert polymer
very chemically stable and unreactive
very hydrophobic
can be an elastomer (catheter tubes etc), gel (boob job) or adhesive (silicone medical adhesive)
Give some examples of polymers designed to be biodegradable and how do they work?
polylactide and polyglycolide
they are condensation polymers and degrade by hydrolysis
what is hydrolysis?
a chemical break down due to a reaction with water
what is the product from the degradation of polyglycolide?
glycolic acid
what is the product form the degradation of polylactide
lactic acid
What is a typical application of polyglycolide and polylactide?
Scaffolds for tissue engineering
What is a hydrogel?
An insoluble water-swollen network
superabsorbent polymer
How are hydrogels made?
Chemical gelation and physical gelation
What is chemical gelation?
Using radiation as an initiator to add cross-links to polymer chains
What is physical gelation?
Occurs when polymer chains are bonded through various ionic interactions, hydrogen bind and through molecular entanglements or the nature of the hydrophobicity of the material
What are the properties of a hydrogel?
Weak in comparison with other polymers
mechanical properties altered by crosslinking density of the polymer, polymerisation conditions during network formation and hydrogel swelling
How do polymers degrade?
By enzymatic cleavage, hydrolysis, enzymatic cleavage and dissolution
What is potentially an exciting use for a hydrogel?
it could release entrapped molecules and therefore be used for drug delivery
Give some examples of hydrogels
collagen (natural), alginate (seaweed, natural), poly(ethylene glycol) and poly(acrylic acid)
How is alginate used?
In tissue engineering - used to grow bone
