Definitions of Molecular Weight

• There are several ways to define molecular weight:
  
  
  1. - Number Average Molecular Weight
     
     
  2. - Viscosity Molecular Weight
     
     
  3. - Weight Average Molecular Weight
     
     
  4. - Z-average Molecular Weight
     
     
• There are multiple definitions because different techniques for measuring molecular weight yield different results. It is important to understand what each technique is measuring, and to understand how to use that information.
  
  
  

• Number Average Molecular Weight

  (Sperling- 78)(Rodriguez- 117)
  
  
  is determined by:
  
  
  1. Osmometry
  2. End-group titration
  3. Colligative properties
  
  
  example numbers to illustrate the above definition formula.
  
  

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• Viscosity Average Molecular Weight

  (Sperling- 78)(Rodriguez- 117)
  
  is determined by:
  
  
  1. Intrinsic Viscosity.
  
  
  

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  Weight Average Molecular Weight

  (Sperling- 78)(Rodriguez- 117)
  
  
  is determined by:
  
  

  1. Light scattering
  2. Small Angle Neutron Scattering (SANS)
  3. Sedimentation velocity [Hiementz-42]
     
     example numbers to illustrate the above definition formula.

  
  
  

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  Z-Average Molecular Weight

  (Sperling- 78)(Rodriguez- 117)
  
  
  is determined by
  
  

  1. Sedimentation equilibrium [Hiementz-42]
     
     

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Last Update- September 3, 1995- wld

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