Size Exclusion Chromatography (SEC) is a commonly used technique to determine the molecular size and weight of polymeric products. The retention of components is based on the size in solution so that the largest molecules are excluded from the stationary phase pores and elute earlier in the chromatogram. The smaller molecular weight components enter the stationary phase pores and, as a result, elute later in the chromatogram. SEC is utilized to qualitatively determine the molecular weight differences between two lots of polymers or quantitatively determine the number, weight, Z average molecular weights and polydispersity. Both aqueous and organic mobile phases are utilized in separate separations. Available detectors include differential refractive index (RI) and multi-angle light scattering (MALLS). With the RI detector, molecular weights are determined by standard curves generated with polymers of well-defined molecular weight. With the MALLS detector, molecular weights are determined directly based on the light scattering.

SEC is utilized to determine the molecular weight and molecular weight distribution of a number of polymers. Typically, molecular weight distribution and polydispersity are directly related to physical properties of the polymer. A routine analysis is to compare a lot of material that performs to specification to a lot that does not. The results of the analysis are often used to optimize process parameters.

SEC is applicable to the characterization of a number of polymer systems including:

• Dendrimers and other water soluble polymers
• Organopolysiloxanes
• General purpose and high impact polystyrene (GPPS, HIPS)
• Acrylonitrile-butadiene-styrene (ABS) polymer
• Polycarbonate (PC)
• Polyurethane (PU, TPU)

Examples

• Determination of the extent of polymerization.
• Quantification of residual monomers.
• Final product quality control.
• Isolation and purification of high molecular weight species.
• Quantification of high molecular weight species.