Absolute molar mass determination in mixed solvents. 1. Solving for the SEC/MALS/DRI “trivial” case

Abstract

Size-exclusion chromatography (SEC) with on-line static light scattering, specifically multi-angle static light scattering (MALS), and differential refractometry (DRI) detection remains the premier method by which to determine absolute, calibrant-independent molar masses of polymers. The method is restricted to the use of either neat solvents or solvents with a small amount of additive. In mixed solvents, preferential solvation (i.e., the enrichment, within the solvated volume of the polymer in solution, of one solvent over the other as compared to the solvent ratio outside said volume) leads to errors in the areas of the MALS and DRI chromatograms, as the solvent baseline does not accurately represent the solvent contribution to these detectors’ peaks. A seemingly trivial way by which to overcome this problem is through the use of an isorefractive solvent pair. This “trivial” solution is complicated by the fact that the solvents in the pair must be miscible with each other in all proportions; the individual solvents as well as the mix must be able to fully dissolve the analyte; the solvents must possess sufficient optical contrast with the solution so as to generate an adequate detector signal; the solvent mix must be compatible with the chromatographic stationary phase, such that enthalpic contributions to the separation are minimal and analyte recovery from the columns is quantitative; and the difference in the Rayleigh factors of the solvents can be ignored. Herein, we present the analysis of narrow dispersity polystyrene (PS) and poly(methyl methacrylate) (PMMA) samples, across a four-fold range in molar mass, using SEC/MALS/DRI in a mix of tetrahydrofuran (THF) and methyl isoamyl ketone (MIAK), solvents which are shown to be isorefractive with each other at the temperature and wavelength of the experiments. Molar mass averages and dispersities are demonstrated to be statistically independent of solvent composition and to correspond well to the values in neat THF. The experiments were augmented by the use of on- and off-line quasi-elastic light scattering and of off-line MALS and DRI, to study the effect of solvent composition on polymer size in solution and on dilute solution thermodynamics. Additionally, 1H nuclear magnetic resonance spectroscopy was used to study the effect of tacticity on the insolubility of PMMA100 in 100% MIAK. We believe this constitutes the first example of obtaining accurate molar masses of polymers by SEC/MALS/DRI employing mixed solvents. The value of these experiments to other forms of macromolecular liquid chromatographic separations is also noted.