Methods for testing relative molecular mass of polymers

There are many methods, the most commonly used are method 7, gel chromatography, and GPC

There are many methods to determine the molecular weight of polymers, the most common ones are the following 8 methods

1. End group analysis method. The molecular weight can be derived by measuring specific end group content through chemical analysis. The premise is that the polymer structure must be fully understood. It can also be used to determine the number of branch chains. The molecular weight generally cannot be too large using this method.

2. The boiling point increases and the freezing point decreases. This is a method that uses the colligative properties of dilute solutions to determine the molecular weight of solutes, and is a classic physical and chemical method. When a non-volatile solute is added to a solvent, the vapor pressure of the solution decreases, causing the boiling point of the solution to be higher than that of the pure solvent and the freezing point of the solution to be lower than that of the solvent. This method has never been used before, and it requires very high accuracy in measuring the temperature difference.

3. Membrane osmotic pressure. The method of measuring osmotic pressure through a semipermeable membrane should also be a physical and chemical method. It seems troublesome and has never been used.

4. Vapor phase osmosis (VPO). The molecular weight is determined by using the difference in saturated vapor pressure between the pure solvent and the solution with added solute. What is measured is the number average molecular weight. Never used it.

5. Light scattering/small-angle laser light scattering (LALLS). These two methods only differ in terms of instruments, data processing and light sources used, but the principles are similar. This method is commonly used, and the instruments have now developed to a certain level. It is the most effective method for testing the absolute molecular weight of polymers.

6. Ultracentrifugal sedimentation. It is very complex and was first used for the measurement of protein molecules. It is a relative method. Never used it.

7. Gel chromatography (GPC). Very commonly used, molecular weight is measured based on the different residence times of molecules of different sizes in the medium. It is a relative method and must be combined with other methods.

8. Viscosity method. It is too common. It is the most economical method to use a glass viscometer (Ubbelohde viscometer, Austrian viscometer) to increase the specific viscosity, then extrapolate the intrinsic viscosity number, and calculate the molecular weight according to the Mark-Houwink equation, and the recalculation is very good. . Water-soluble polymers generally use this method to measure molecular weight, which is also a relative method