Quantification of Long Chain-Branched Homopolymers by Multidetection Size Exclusion Chromatography: A Critical Assessment

MEIRA, G.R.; VEGA, J.R.; CASTIGNOLLES, P.

Roma

Simposio; XVI ISSS - International Symposium on Separation Science - Recent Advancements in Chromatography and Capillary Electromigration Techniques; 2010

This work highlights several problems associated with the determination of the distributions of molar masses (MMD) and of number of long branches per molecule (LBD) via Size Exclusion Chromatography (SEC) with triple detector systems, i.e.: with a Differential Refractometer (DR), a Specific Viscometer (SV), and a Light Scattering (LS) detectors. At any fixed hydrodynamic volume, the molar mass increases with the level of branching. Thus, even with perfect fractionation according to hydrodynamic volume, whole distributions of molar masses and of chain topologies are instantaneously present in the detector cells. For quantitative estimates of the global MMD and LBD, such instantaneous distributions must be narrow. This condition was not verified in the case of branched polyacrylates however, since dispersities above 2 were observed at the high molar mass limit [1]. A simulated example will be presented to quantify errors on the estimated MMD and LBD. The chromatograms will be simulated on the basis of known results from a polymerization model on the MMDs of each of the different chain topologies (with each topology characterized by the number of trifunctional branching points per molecule). The Zimm-Stockmayer (Z-S) equations [2] will be used together with known values of the exponent that interrelates the g and g’ contraction parameters and of the universal calibration parameters. Apart from the insurmountable problems associated with the chromatographically-complex nature of branched homopolymers, the following additional sources of error will be evaluated: a) presence of band broadening; b) uncertainties in the constants required in the data treatment; c) deviations caused by application of the Z-S model onto branched radical homopolymers obtained by chain transfer to the polymer and by polymerization with terminal double bonds; d) the low sensitivity of SV and LS detectors to low molar mass material; and e) errors at the chromatogram tails caused by the signal ratios LS/DR and SV/DR. The ultimate aim of the investigation is to provide recommendations on the expected accuracies of the various possible SEC configurations and associated data processing.