Polymer Characterization by SEC with a Concentration Detector. Errors in the Average Molar Masses due to Contamination by a Minority Constituent

VEGA, J.R.; BEREK, D.; MEIRA, G.R.

Torun

Simposio; 29th. International Symposium on Chromatography (ISC 2012); 2012

Size exclusion chromatography with a differential refractometer (SEC/DR) is the standard technique for determining the number- and weight-average molar masses (Mn and Mw) of synthetic polymers [1]. Simulated and experimental examples were carried out to evaluate the errors in the determination of such averages when a main polymer (A) is contaminated by small amounts of a minority polymer (B). The simulations considered different molar masses and variable amounts of A and B; as well as fractionation columns of different selectivity and resolutions. The band broadening (BB) function was represented by an exponentially-modified Gaussian distribution [2], and a gradual loss of resolution was modeled by increasing the BB parameters. The experiments included several blends of narrow polystyrene standards with different relative concentrations, and measured with columns of different resolutions and fractionation ranges. Three situations were analyzed. The best situation arose when A and B exhibited quite different molar masses and a large concentration ratio B/A. In this case, both constituents were well separated and monitored by DR. The worst situation arose when A and B exhibited similar molar masses, and B became completely disguised in the main chromatogram. In an intermediate situation, B produced a bulge in a chromatogram tail. When such bulge was overlooked, then the estimated average molar masses of A greatly differed from their true values. As expected, enlarged errors were observed for increasing amounts of B and/or for increasing BB parameters. In some cases, the use of a second detector (e.g. a UV sensor) is helpful for a better discrimination of the constituents. The ability of a numerical procedure for detecting the presence of B was also evaluated.