Lecture Problem 4'6: Molecular Weights of Polymers

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Lecture Problem 4.6 Molecular Weights of
Polymers

• Zane Kealey
• January 31st 2007

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What does molecular weight tell us about the
polymer?

• Technically a mass
• Related to viscosity and osmotic pressure
• Higher molecular weight, higher melting
  temperature
• 100g/mol -gt gases or liquids
• 1000g/mol -gt waxy solids
• 10000g/mol and up -gt solid (high) polymers

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Problem Statement 4.6

• Molecular weight data for some polymer are
  tabulate here. Compute
• (a) the number-average molecular weight, and
• (b) the weight-average molecular weight.
• (c) If it is known that this materials
  number-average degree of polymerization is 477,
  which one of the polymers listed in Table 4.3 is
  this polymer? Why?
• (d) What is this materials weight-average degree
  of polymerization?

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Problem Statement 4.6
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xi versus wi

• xi the number fraction
• The number of polymer chains that fall within a
  certain molecular weight range divided by the
  total number of polymer chains.
• wi the weight fraction
• The mass of polymer chains that fall within a
  certain molecular weight range divided by the
  total mass.

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Number-average molecular weight.
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(b) Weight-average molecular weight
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(c) Identify polymer using number-average degree
of polymerization

• nn number-average degree of polymerization
• Average number of mer units per polymer chain
• nn Mn / m, where m is the molecular weight of a
  mer unit
• nn 477 and Mn 47720 g/mol, therefore m
  100.042 g/mol

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(c) Identify polymer using number-average degree
of polymerization

• Table 4.3 lists common mer units
• Molecular weight of each mer unit can be found by
  summing molecular weights of constituent atoms
• In this case well ignore the molecular weight
  contributions of the terminal groups

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Mer Unit Molecular Weights
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(c) Identify polymer using number-average degree
of polymerization

• Most likely PTFE (Teflon) or PMMA (Plexiglass)
• Differentiate by color
• Teflon white
• PMMA clear
• Unlikely to use this procedure to identify a
  polymer in real life

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(d) Weight-average degree of polymerization

• nw weight-average degree of polymerization
• Average number of mer units per polymer chain
• nw Mw / m, where m is the molecular weight of a
  mer unit
• m 100.042 g/mol and Mw 53720 g/mol,
  therefore nw 537
• Different from nn because calculated using
  different average molecular weight, Mw vs Mn

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Questions?
Plexiglass cgi.ebay.com
Teflon www.astrosurf.com