Project Details:

Data treatment in size exclusion chromatography of polymers

Project No.:2003-023-2-400
Start date:2004-01-01
End date:2007-12-01
Division:Polymer Division
Objective:

In spite of the ample use of Size Exclusion Chromatography (SEC) for measuring the molar mass distribution (MMD) of polymers, there are huge variations in the results obtained by different laboratories, when analyzing the same polymer sample. One of the possible reasons for such variations may be the applied data treatment. Thus, it seems indispensable to standardize the SEC data treatment.

The IUPAC Mission is "to provide leadership, facilitation, and encouragement of chemistry, promoting the norms, values, standards, and ethics of science and the free exchange of scientific information". The standardization of the SEC data treatment through a prestigious and non-profit organization such as IUPAC seems the ideal solution for producing more uniform and accurate estimates of the MMD among the different chemistry laboratories of the world.

Objectives:

  • Develop some novel, more practical, and more efficient procedures for the correction of BB and other sources of error; in order to improve the accuracy of the measured MMD and other polymer molecular characteristics.
  • Compare and critically evaluate the different classical and novel data treatment methods, with the final aim of recommending a standard procedure that may be useful for general SEC user and for the instrument manufacturer.
    • Description:

    SEC is the main analytical technique for determining the MMD of synthetic polymers. In spite of its ample use, the accuracy of the technique is relatively low, as it is verified by the dispersion of results obtained in round-robin tests. For good quantitative results, not only the equipment must be in good physical shape. Also required are a careful experimental work and a judicious data treatment.

    Due to BB, the polydispersity is overestimated when calculated from a mass signal and a calibration curve, but is generally underestimated when obtained from the combined mass and "molar mass" signals. This bias can be important if the sample MMD is narrow or multimodal. So far, it is rather surprising that no instrument or standards manufacturer offers any practical BB correction software. Possible reasons for this are: a) the BB function is difficult to estimate; and b) the theoretically exact solution, requires an ill-posed deconvolution operation. Many approximate solutions have been proposed to overcome the classical deconvolution approach, e.g.: a) to assume simple analytical functions for the kernel and the mass chromatogram; b) to propose semi-empirical corrections based on the chromatogram shapes; and c) to modify the molar mass calibration rather than correcting the raw chromatograms. All of these approximations offer different limitations, advantages and drawbacks. The final project aim is to standardize a method that is simple to apply, while still providing reasonable estimates. Some of the items to be analyzed in relation to the BB correction problem are: a) kernel determination; b) robust deconvolution; c) non-deconvolution methods; d) oligomer analysis; e) use of light scattering and viscosity detectors; and f) solution via more fundamental physico-chemical models (rather than on phenomenological models).

    Apart from the BB problem, other sources of error can be significantly more serious for their effect on the MMD estimates. Examples of these are: a) polymer degradation; b) column saturation, especially at high molar masses; c) insufficient sensitivity of the light scattering (LS) and intrinsic viscometer sensors to low molar mass material; d) chromatogram baseline selection; e) detector nonlinearities; and f) propagation of errors when signals ratio are required (e.g.: to calculate the instantaneous composition or molar mass). The first aim here is to carry out a propagation of error studies that will highlight the significance of these effects on quantifying the MMD or other characteristics such as degrees of branching or copolymer composition. This will determine the main items to be cared for when estimating such properties.

    The present project is a continuation of an earlier project titled: "Band Broadening Correction in SEC" (Coordinator: Jörg Baumgarten); the main outcome of that inital project was a publication by J.L. Baumgarten, J.-P. Busnel, and G.R. Meira (2002), "Band Broadening in Size Exclusion Chromatography of Polymers. State of the Art and Some Novel Solutions", 25th. Anniversary Issue, J. Liq. Chrom. & Rel. Technol., Vol. 25, N° 13-15, pp. 1967-2001. That project was part of the activities of the former IUPAC Working Party IV.2.2: "Molecular Characterization of Commercial Polymers" (Coordinator: Du+AWE-an Berek).

    Progress:
    The final report was presented at Polychar-15 World Forum on Advanced Materials, in Rio de Janeiro, Brazil, 16-20 April 2007, and later published in Macromolecular Symposia Vol 258 covering the event. The report is "Band Broadening Function in Size Exclusion Chromatography of Polymers: Review of Some Recent Developments", Macromol. Symp. 2007, 258, 186–197 [doi: 10.1002/masy.200751221]
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