Absolute molar mass determination in mixed solvents. 2. SEC/MALS/DRI in a mix of two “nearly-isovirial” solvents

Abstract

The accurate determination of polymer molar mass (M) averages and distributions via size-based separation methods employing mixed solvents remains one of the great macromolecular characterization challenges. This is due to the possibility for preferential solvation, whereby the region in the immediate vicinity of the polymer in solution becomes enriched in one of the solvents as compared to this solvent's fractional amount in the mix. In such cases, the chromatographic baselines of differential detectors such as light scattering photometers and differential refractometers no longer provide a quantitative reflection of the solvents' contribution to the heights of individual peak slices, thereby introducing error into molar mass calculations. The problem is addressed here through the use of a “nearly-isovirial” solvent pair. The second virial coefficient (A2) of both solvents being nearly equal for the polymers analyzed means that preferential solvation is obviated. The accuracy of this approach is shown via analysis by size-exclusion chromatography with on-line multi-angle static light scattering and differential refractometry detection (SEC/MALS/DRI), for a trio of narrow-dispersity polystyrene (PS) standards covering a nearly 40-fold range in M. In a mix of two nearly isovirial solvents, namely tetrahydrofuran (THF) and toluene, calculated molar mass averages and distributions are shown to be essentially identical across the range of solvent ratios. Polymer size is likewise shown to be constant with solvent ratio in this scenario. This is contrasted with the same polymers dissolved in a mix of the non-isovirial, non-isorefractive solvents THF and dimethylformamide (DMF). Results from this latter set of experiments show the large error in calculated M that results from preferential solvation, which can be as high as ≈ 1 × 105 g mol−1 for an 8 × 105 g mol−1 narrow-dispersity polystyrene. It is determined that, for a 25:75 THF:DMF mix, the solvent ratio in the immediate vicinity of the polymers examined “flips” to ≈75:25 THF:DMF due to preferential solvation.