Polymers Flashcards
What are the three types of polymers?
- Natural
- Synthetic
- Semi-synthetic
What are some examples of natural polymers?
- Proteins
- Polysaccharides
- Resins/gums
What are some examples of synthetic polymers?
- Plastics
- Elastomers
- Rubber
What is a polymer?
- long chain molecule
- constructed from many monomers that are covalently bonded together
- can be linear, branched, cross-linked or colloidal
What is a homopolymer?
Comprised of all the same monomers
What is a co-polymer?
Made up of different monomers
How do polymers end?
- there are end groups
- the nature of end groups allows method of synethsis to be analysed
- it is possible for there to be no end groups = rings
- useful for identifying a material
- ratio of end groups to in-chain groups allows measuring of polymer length
- reactive end groups allow further modification to control properties or add additional functionality
Why are end groups useful for identifying a material?
- type of end group is unique and will give a unique signature
- ususally more reactive so can give a unique signature
What are the 3 main types of polymer when classed by bulk properties?
- Thermoset
- Thermoplastic
- Elastomer
What are the properties of thermoset polymers?
- burns when heated
- irreversibly hardened once shaped
- generally inflexible
- chemical process called curing required
Examples of thermoset polymers
- epoxy resins
- silicones
- polyurethanes
What are the properties of thermoplastic polymers?
- melts when heated
- can be remelted and reshaped
- generally more flexible
- no chemical curing required
Examples of thermoplastic polymers
- polystyrene
- nylon
- polycarbonate
What are the properties of elastomer polymers?
- can be thermoplastic or thermoset
- has viscosity (can flow) and eslasticity (return to orignial shape)
Examples of elastomer polymers
- rubbers
HDPE
- high density polyethylene
- high crystalline content
- less transparent than LDPE
- stiffer and harder than LDPE
- higher density
- very low/no branching
- less gas permeable
LDPE
- low density polyethylene
- low crystalline content
- more transparent than HDPE
- forms good films
- lower density
- highly branched
- more gas permeable than HDPE
Define crystallinity
The regions of atomic ordering where intramolecular folding/stacking of adjacent chains occur
What properties does crystallinity affect?
- crease
- thermal behaviour
- transparency
- stiffness
- impact resistance
- tensile strength
- young’s modulus
What does a degree of 0 and 1 mean in crystallinity?
- 0 = completely amorphous
- 1 = completely crystalline
What is tensile strength
how much something will stretch before it breaks
* for polymers this increases with polymer chain length and crosslinking
* easier to stetch smaller things - less matter to move = less force required
How do we measure tensile strength?
-ε = l/L
* ε = strain (use this instead of stretch in solids
* I = fractional change after stretching
* L = initial starting length
For a solid how do we work out the force used to stretch it?
-σ = F/A
* σ = stress (used instead of force)
* independant of sample size
* F = force
* A = distance stretched
What does Young’s Modulus describe?
How stiff a solid is
How to measure Young’s Modulus?
E = -σ/ε
* plot stress over strain and the gradient is the answer
* Divide the gradient by 1000 to be in GPa units
* To be in N/m^2 multiply the gradient by 1000000
What is the force for a spring?
F = -kx
* k = force/spring constant
* x = distanced stretched (i.e. I)
How do you work out k in F = -kx?
- plot force (y-axis) by distanced stretched (x-axis)
- the gradient of the line is k
