Polymers Contaminated by Small Amounts of Admixtures: Errors in the Average Molar Masses when Measured by SEC with a Concentration Detector

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

Bahía Blanca

Simposio; Simposio Argentino de Polímeros; 2011

UNS-CONICET

Size exclusion chromatography (SEC) is the standard technique for determining the molar mass distribution (MMD) and the number- and weight-average molar masses (Mn and Mw) of synthetic polymers. On-line molar-mass sensitive detectors (such as light scattering sensors and viscometers) have contributed to improve the characterization of complex polymers (Striegel et al., 2009). However, most commonly, a single concentration-sensitive detector (e.g., a differential refractometer: DR) is employed.Drawbacks and pitfalls of SEC/DR include among others non-exclusion fractionation mechanisms, band broadening (BB), concentration effects, baseline uncertainties, and errors in the molar mass calibration (Berek, 2010). The analysis of polymer blends by SEC is a complex task because the fractionation mechanism does not discriminate according to composition. For example, block copolymers are typically contaminated by the parent homopolymers; and the homopolymer peaks can be either completely hidden in the main copolymer peak, or can eventually originate a bulge in the global chromatogram. The identification by SEC/DR of the existence of a minor contaminant in the main sample is only feasible when the contaminant peak elutes in a V-range different to that expected for the main sample constituent. Such identification is only possible when the contaminant is present in a relatively high concentration and when the fractionation columns exhibit high resolution.Based on simulated examples, this work evaluates the errors in Mn, Mw, and Mw/Mn as determined by SEC/DR, when the main polymer (A) is contaminated by small amounts of the secondary polymer (B). The simulations assume different column resolutions and amounts of admixture B.