Chapter 3 - Practical Aspects of Molecular Weight Measurements

Abstract

This chapter reviews the essentials of practical techniques for measuring average molecular weights and characterizing molecular weight distributions. Most of the procedures for measuring molecular weight rely on colligative solution properties. These properties include osmotic pressure, boiling point elevation, freezing point depression, and vapor pressure lowering. At given solute concentration, these relations show that the effect of the dissolved species on the chemical potential of the solvent decreases with increasing solute molecular weight. Membrane osmometry is the most sensitive and accurate colligative property technique. Two other techniques that are also used to measure molecular weight are not colligative properties in the strict sense. These are based on end-group analysis and on vapor phase osmometry. Both methods, which are limited to lower molecular weight polymers, are described in this chapter. The practical range of molecular weights that can be measured with membrane osmometry is approximately 30,000 to one million. Osmometers consist basically of a solvent compartment separated from a solution compartment by a semipermeable membrane and a method for measuring the equilibrium hydrostatic pressures on the two compartments. Osmotic equilibrium is not reached quickly after the solvent and solution first contact the membrane. Other colligative methods must be employed in range below about 30,000, and vapor pressure lowering can be considered in this connection.