What are polymers?
- macromolecules made of repeating units
- units are linked by covelant and physical bonds
Give examples of natural and synthetic polymers
- DNA, proteins, starch, silk, gelatin
- Polyethylene, PMMA, Nylon 6-6, PDMS
How is the physical behaviour of polymers defined?
By N, the degree of polymerisation
What are the different types of polymer:
- topology
- composition
- stereochemistry
- linear or branched
- homopolymeric or copolymeric
- isotactic, syndiotactic, atactic
How do we model a polymer?
- as a freely jointed chain, a random walk
- bonds don’t interact with each other but can cross one another
- end to end distance is defined by the mean squared model
- the size of an ideal chain increases with the degree of polymerisation
What is the entropy of a polymer?
The probability of finding a particular distance (r) given a degree of polymerisation (N)
- relationship follows a gaussian probability
Why does increasing the polymer length r from equilibrium cost energy?
- For a given N , increasing/decreasing r, reduces the number of available configurations.
- Hence increasing the length of a polymer from its equilibrium size requires a force.
- force in entropic and changes with temperature
How is the force required to move a polymer from equilibrium measured?
- To measure this force a laser is used to move a colloid towards and away from a wall that it is attached to by a strand of DNA in an optical trap
- A very small force is required
- The current model is good if stretching is below 50%, beyond this the equations do not follow experimental findings
What does polymer self interaction cause?
- polymer swelling
- distant points on the chain interact with each other
How are solvent effects on polymers quantified?
- mean field theory used to describe this interaction
- X< 0.5 good solvent conditions, size dominated by excluded volume effects r approx. N^0.6
- X=0.5 theta condition, ideal chain as excluded volume effects are cancelled by polymer solvent interactions r approx. N0.5
- X> 0.5 Bad solvent conditions, chain collapses into a globule to minimise contact with the solvent N ^ 0.3333
What is the theta temperature?
- For X=0.5 the theta temperature is the temperature at which the transition between the globule and coil occurs
