Determination of Molecular Weight and Molecular Weight Distribution

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Determination of Molecular Weight and Molecular
Weight Distribution
Polymer Structure Synthesis

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Cryoscopy

• Freezing point of a solvent is lowered when a
  non-volatile solute is dissolved in it.
• Freezing point of the solution will be depressed.
• This phenomenon of depressing the freezing point
  of a liquid by the addition of a solute is known
  as Cryoscopy.
• The extent of the freezing point depends on the
  number of solute molecules dissolved per unit
  volume of solution and its independent of the
  size and nature of the solute molecule.

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• So, Number average molecular wt. is measured by
  this technique.
• If depression of the freezing point is denoted
  by , Its related to the
  Number-average Molecular wt. and concentration
  through following equation.
• Where, C is concentration of the solution, ? the
  density of the solvent, R the universal gas
  constant Tf the freezing point of the solvent, B
  is a constant and ?Hf is the heat of fusion of
  the solvent.

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• Graphical representation.
• To find out the freezing point, solvent or
  solution is carefully supercooled to a
  temperature that is about 5C lower than the
  freezing point.
• Once the freezing points of the solvent and
  solutions Tf different concentrations are known,
  ?Tf can be calculated for all these
  concentrations.
• Drawback of this method is, measurement of
  temperatures.
• Highly precised temperature sensors are required.
• This method can measure molecular wt. upto
  30,000.

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Ebulliometry

• This technique is based on boiling point of
  solutions.
• Boiling point of solutions is higher than the
  pure solvents. This phenomenon is used to
  determine the molecular wt. of polymers.
• Where ?Hv and Tb are the heat of vaporization and
  boiling of the solvent.

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• There are two types of ebulliometers.
• In first type, the solution of the polymer whose
  molecular wt. is to be determined is heated in a
  boiler. The solution is then brought in contact
  with a sensor. Another in pure solvent.
• In second type, both of solution and pure solvent
  are heated in identical containers and uniform
  boiling is achieved.
• It can also be used to determine the molecular
  wt. upto 30,000.

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Membrane Osmometry

• It is most widely used technique to determine
  number average molecular wt.
• It is based on the phenomenon of osmosis.
• What is Osmotic pressure?
• ? is the osmotic pressure.

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Vapor Phase Osmometry

• This technique is based on the principle that at
  a given temperature the vapor pressure of a
  solution is less than that of the pure solvent.
• The relationship between the vapor pressure
  difference of solvent and solution,
• and number average molecular wt. is given as
• Experimental method.