Molar Mass Distributions of Copolymers by Multi-Detection Size Exclusion Chromatography

CLEMENTI, C.G. GUTIERREZ, M.M. YOSSEN, J.R. VEGA

Los Cocos

Simposio; XII Simposio Argentino de Polímeros - SAP 2017; 2017

Laboratorio de Materiales Poliméricos (LaMaP) / Instituto de Investigación y Desarrollo en Ingeniería y Procesos y Química Aplicada

Size exclusion chromatography (SEC) is a widely-used technique for estimating the molar mass distribution (MMD) and the average molar masses of synthetic and natural polymers. In standard SEC, a concentration sensitive detector (such as a differential refractometer -DR-, or an UV sensor) is employed. In such cases, a molar mass calibration is required in order to transform elution volumes into molar masses. In general, SEC exhibits several drawbacks, such as: band broadening, concentration effects, baseline uncertainties, and errors in the molar mass calibration. The simultaneous utilization of multiple detectors (e.g., DR + UV + light scattering -LS-), could help to determine the MMD without requiring a previous calibration. Unfortunately, data treatment is difficult due to the low signal-to-noise ratio at the chromatograms tails. In particular, copolymers are considered as chromatographically-complex polymers, because at any elution volume a variety of molar masses and chemical compositions coexist in the detector cell, thus turning difficult the estimation of the MMDs. Additionally, a copolymer can be contaminated with the corresponding parents homopolymers, and therefore a blend of homopolymers and copolymers must be characterized.In this work, a novel data treatment is proposed for estimating the MMD and the chemical composition of a copolymer from SEC-(DR+UV+LS). To this effect, the following procedure is proposed: (i) the LS chromatogram is utilized for correcting the DR and UV signals for band broadening; (ii) a deconvolution method is used for determining the concentration and the chemical composition from the corrected DR and UV chromatograms; and (iii) the MMD of each species is estimated on the basis of a molar mass calibration (which is in turn obtained by a proper interpolation of the molar mass calibrations of the corresponding parent homopolymers). The proposed data treatment method was utilized for characterizing a styrene-methyl-methacrylate copolymer synthesized through a solution polymerization. With the proposed method, accurate MMD estimations are obtained by analyzing the whole sample, without requiring previous fractionation by solvents.