Part 5 Flashcards

1
Q

What are polymers?

A
  • macromolecules made of repeating units

- units are linked by covelant and physical bonds

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2
Q

Give examples of natural and synthetic polymers

A
  • DNA, proteins, starch, silk, gelatin

- Polyethylene, PMMA, Nylon 6-6, PDMS

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3
Q

How is the physical behaviour of polymers defined?

A

By N, the degree of polymerisation

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4
Q

What are the different types of polymer:

  • topology
  • composition
  • stereochemistry
A
  • linear or branched
  • homopolymeric or copolymeric
  • isotactic, syndiotactic, atactic
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5
Q

How do we model a polymer?

A
  • 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
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6
Q

What is the entropy of a polymer?

A

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

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7
Q

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

A
  • 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
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8
Q

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

A
  • 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
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9
Q

What does polymer self interaction cause?

A
  • polymer swelling

- distant points on the chain interact with each other

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10
Q

How are solvent effects on polymers quantified?

A
  • 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
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11
Q

What is the theta temperature?

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