L12: Polymer Characterisation

Question 1

When is NMR useful in polymer characterisation?

Answer 1

• Arguably most important characterisation technique in synthetic chemistry
• Confirming that polymerisation has occurred
• End group analysis
• Determining extent of tacticity / monomer sequence in copolymers
• Also able to use a H-H decoupling technique -> switching off coupling to the CH protons -> tetrad sequence

Question 2

2 key difficulties in polymer characterisation:

Answer 2

• Statistical distribution of molecular weight -> can’t use straightforward techniques like chromatography
• Practical concerns such as forming a solution or handling/moving samples due to thick consistency

Question 3

Application of GPC in polymer characterisation:

Answer 3

• Gel permeation chromatography
• Solution of the polymer is separate out by size
• Can obtain concentration as a function of elution time
• Mw distribution can be obtained by comparing to linear calibration, calculated by elution of known Mw standards e.g. polystyrene (higher Mw pass through more quickly as they struggle to fit into pores, vice versa with shorter chains)

Question 4

Key disadvantages of GPC in polymer characterisation:

Answer 4

• Structural differences between calibration standard and sample can throw off readings -> use MALLS

Question 5

What is MALLS?

Answer 5

• Multi angle laser light scattering technique
• Same column/set up as GPC
• Instrument at end uses multiple lasers at angles to measure differential refractive index of polymeric material -> unique for a given polymer structure
• How does response of polymer change with given concentration? -> refractive index

Question 6

Applications of DSC in polymer characterisation:

Answer 6

• Differential Scanning Colorimetry
• Provides Tg, Tm and Tc with associated energies
• Requires 2-8 mg of polymer
• Measures energy input needed for these respective transitions using reference and sample pan on independent heaters

Question 7

Application of MALDI-ToF in polymer characterisation:

Answer 7

• Matrix assisted laster desorption ionisation - Time of flight mass spectrometry
• No fragmentation of polymer chains occurs (due to use of a very ‘soft’ ionisation technique)
• Result is a distribution where each peak corresponds to a polymer chain with given \
  ‘n’