Recent Developments in Size Exclusion Chromatography of Polymers

MEIRA, G. R.

Porto de Galinhas

Simposio; XIV Simposio Latinoamerico de Polímeros; 2014

ABPol

Size Exclusion Chromatography (SEC), also known as Gel Permeation Chromatography (GPC), is the main analytical technique for determining the Molar Mass Distribution (MMD) of high polymers. Its main advantage is that, irrespective of the chemical nature, the same column system enables the fractionation (by hydrodynamic volume) of many different synthetic polymers. Its main drawback is that even under perfect fractionation by hydrodynamic volume, accurate MMDs are only possible in the very specific case of flexible linear homopolymers.1 This presentation aims at quantifying the errors in the MMD estimates when long chain-branched homopolymers are analyzed by SEC with triple detection; i.e., when the chromatograph is fit with a Differential Refractometer, a Light Scattering sensor, and a Specific Viscometer. The work was carried out under the auspices of the IUPAC project: ?Data Treatment in SEC and Other Techniques of Polymer Characterization. Correction for Band Broadening and Other Sources of Error?, Chair G. Meira, http://www.iupac.org/web/ins/2009-019-2-400. Randomly-branched homopolymers are synthesized by chain- or step-growth polymerizations involving monomers of functionality 3 or higher; and (more often) by radical polymerizations of bifunctional monomers with chain transfer to the polymer. For accurate MMDs, the local dispersity (i.e., the instantaneous value of Mw/Mn in the detector cells) should be close to unity along the chromatogram, and deviations with respect to unity are indicative of bias. For radical polymerizations of bifunctional monomers with inter- and intra-molecular chain transfer to the polymer, the experimental article by Gaborieau et al.2 observed local dispersities as high as 2 at the low molar mass limit of the chromatograms. However, these rather worrying biases were in contrac¡st with the recent simulation results by Netopolík3 and Clementi et al.,4 where negligible biases are predicted in the MMD estimates. (In contrast, large biases are to be expected in the branching function; i.e., the function that relates the local average number of branches per molecule with the local molar mass.) At a much higher experimental cost, Two-Dimensional chromatography can improve the characterization of some specific chromatographically-complex polymers. In Chang et al.,5 a slow fractionation by Temperature-Gradient Interaction Chromatography was combined with a fast fractionation by SEC with triple detection, in order to estimate the MMD and local number of branches per molecule in mixtures of anionically-prepared star-branched polystyrenes.