Chapter 4 Polymer Structures Supplemental Problem 181

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Chapter 4 Polymer Structures Supplemental
Problem 181

• M. Scott Kimbrell
• 1-31-07

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The Problem

• A sample of polymer has become separated from
  much of its manufacturing data. It is believed
  that the weight-averaged degree of polymerization
  (nw) is 407.
• The sample is either pure polyethylene or a 7525
  polyethylene-polypropylene copolymer.
• a) Calculate the weight-averaged molecular
  weight.
• b) Calculate the number-averaged molecular
  weight.
• c) Assuming part b is correct, calculate the
  number-averaged degree polymerization.
• d) Assuming the given data are correct, is it PE
  or a PE-PP blend?

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The Formulas
Number-average molecular weight
Weight-average molecular weight
Mer molecular weight
Number-average degree of polymerization

Weight-average degree of polymerization
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a) Determine weight-averaged molecular weight.
(0.3045g/g)(11050g/mol) (0.3045g/g)(11050g/mol
) (0.3045g/g)(11050g/mol) 12,850.15
g/mol.
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b) Determine number-averaged molecular weight.
Use
Knowing
and
But how do we know xi???
We determine .
.
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c) Find the number-averaged degree polymerization.
12,850.2 g/mol
and find 31.573 g/mol.
We know
407
Then 402.25.
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d) Is it PE or a PE-PP blend?
We know the true 31.573 g/mol.
Use to find for both pure
polyethylene and the blend
(1)(28.054 g/mol) 28.054 g/mol
(.75)(28.054 g/mol) (.25)(43.089 g/mol)
31.813 g/mol
Since the true is closer to , it
is a PE-PP blend!
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Implications / What weve learned

• Materials properties predictions
• As molecular mass increases, viscosity ?, will
  increase.
• Melting temperature is raised with increasing
  molecular weight.
• Polymers with short chains (100 g/mol) are
  liquids/gases.
• Those with weights of 1000 g/mol are waxy
  solids.
• Solid polymers can have molecular weights between
  10,000 g/mol to several million g/mol!
• Dont be afraid of a conversion between wi and
  xi!!