Here is an overview of the two main methods that are used to determine Molecular Weight (Mw):
Gel permeation chromatography (GPC) is a chromatographic method in which particles are separated based on their size, or in more technical terms, their hydrodynamic volume. Molecules are separated from one another based on differences in molecular size. GPC is performed in organic mobile phases and is used mostly for synthetic polymers. GFC (gel filtration chromatography) is defined as sized based separation performed in an aqueous mobile phase and is typically applied for protein analysis or for water soluble polymers. A broader term which encompasses both of these separation methods is size exclusion chromatography (SEC).
GPC is accomplished by passing the sample through a porous media. Larger molecules can not access some of the pores and exit the column more rapidly. Smaller molecules penetrate into more of the porous structure and elute at longer retention times.
GPC is the most commonly used method for polymer molecular weight and polydispersity index. This process is done by first analyzing a series of standards of known molecular weight. The retention time for these standards is then used to create a calibration curve.
Laser light scattering (LLS) is one of the most powerful and widely-used techniques to study the molecular weight distribution of a polymer.
Typically the eluent of the SEC column is allowed to pass through both a refractive index detector (that gives measures for the concentration in the solution as a function time) and through a laser scattering cell. The scattered intensity is measured as a function of time under a small angle with respect to the laser beam. Under these conditions the laser signal together with the concentration data can easily be translated into a curve that yields both the Mn and the Mw, the molar mass weighted by number and by weight respectively. The combination of those two data gives information on the linearity of the polymer.
The technique is sometimes complemented or combined with viscometry and polystyrene standards are available for validation of the results.
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