Methods for measurement of number-average molar mass

5 Methods for measurement of number-average molar mass 

The number-average molar mass, Af , of a polymer is of great importance and there are several methods for its measurement. The common feature underlying each of these methods, is that they measure the number of polymer molecules in a given mass of polymer. The methods which involve measurement of the colligative properties of dilute polymer solutions are applicable to most polymers and will be considered in Sections 3.6-3.8. Additionally, for certain polymers it is possible to measure A/ by end-group analysis and examples of such measurements will be described in Section 3.9. 

Colligative properties are those properties of a solution of a non-volatile solute which in the limit of infinite dilution, depend only upon the number of solute species present in unit volume of the solution and not upon the nature or size of rtose species. Thus the colligative properties of polymer solutions enable Af to be measured for linear and branched homopolymers and copolymers with equal ease. The four important colligative effects are the osmotic pressure of the solution, the lowering of solvent vapour pressure, the elevation of solvent boiling point and the depression of 

TABLE 3.4 The relative magnitudes of the colligative effects for polymers dissolved in benzene at a concentration of 10gdm The values are given in practical units and have been calculated assuming ideal solution behaviour 

M (gmol ) Osmotic pressure (mm benzene) Lowering of vapour pressure (mm Hg) Elevation of boiling point (K) Depression of freezing point (K) 

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