Part 5

Question 1

What are polymers?

Answer 1

• macromolecules made of repeating units

- units are linked by covelant and physical bonds

Question 2

Give examples of natural and synthetic polymers

Answer 2

• DNA, proteins, starch, silk, gelatin

- Polyethylene, PMMA, Nylon 6-6, PDMS

Question 3

How is the physical behaviour of polymers defined?

Answer 3

By N, the degree of polymerisation

Question 4

What are the different types of polymer:

• topology
• composition
• stereochemistry

Answer 4

• linear or branched
• homopolymeric or copolymeric
• isotactic, syndiotactic, atactic

Question 5

How do we model a polymer?

Answer 5

• 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

Question 6

What is the entropy of a polymer?

Answer 6

The probability of finding a particular distance (r) given a degree of polymerisation (N)
- relationship follows a gaussian probability

Question 7

Why does increasing the polymer length r from equilibrium cost energy?

Answer 7

• 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

Question 8

How is the force required to move a polymer from equilibrium measured?

Answer 8

• 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

Question 9

What does polymer self interaction cause?

Answer 9

• polymer swelling

- distant points on the chain interact with each other

Question 10

How are solvent effects on polymers quantified?

Answer 10

• 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

Question 11

What is the theta temperature?

Answer 11

• For X=0.5 the theta temperature is the temperature at which the transition between the globule and coil occurs