Hexafluoroisopropanol as size exclusion chromatography mobile phase for polyamide 6

The present study deals with the use of hexafluoroisopropanol (HFIP) as size exclusion chromatography (SEC) mobile phase for polyamide 6 (PA6). Contradictory conclusions relating to the use of HFIP as SEC mobile phase for polyamides are found in the literature. By using a multi-detector SEC apparatus equipped with on-line viscometer and multi-angle light scattering we have studied the chromatographic artifacts and the validity of the universal calibration (UC) in HFIP for different PA6 samples (hydrolytic and anionic, monofunctional or bifunctional activator). Appropriate SEC columns and optimized experimental conditions allow most of the chromatographic artifacts to be avoided, even in neat HFIP. The use of a salt in the mobile phase, namely 0.01 M tetraethylammonium nitrate (TEAN), slightly increases the elution volume for both PA6 and PMMA polymers. Under the right conditions, the UC substantially holds for PA6. The validity of the UC is not linked to the presence of TEAN in the mobile phase. With some PA6 samples, namely those anionically synthesized using a bifunctional activator, aggregation becomes a problem and the molar mass in neat HFIP is overestimated. Addition of TEAN prevents any aggregation of the above anionically synthesized PA6. In contrast, the use of a different salt, namely potassium trifluoroacetate, increases the extent of aggregation.