Characterization of low-molar-mass polymers by gradient polymer elution chromatography III. Behaviour of crystalline polyesters under reversed-phase conditions

摘要

Gradient polymer elution chromatography (GPEC) can be used to characterize polyester resins according to molar mass and chemical composition. In the third part in this series, the application of GPEC on crystalline polyesters was studied. In contrastto amorphous polyesters, crystalline polyesters were found to exhibit non-reproducible chromatographic behaviour in GPEC under certain conditions. The cause of this phenomenon was found in the dominance of precipitation (crystallization) and redissolution effects in the total retention mechanism Crystalline polyesters were found to crystallize on the column after precipitation in contrast to amorphous polyesters, where no real solid-phase is formed. Varying injection volume, flow-iate or precipitation medium affect the morphology of the precipitate, giving rise to a different redissolution behaviour, From the minor effects of increasing sample load and gradient steepness, it was concluded that separation is mainly governed by thermodynamics that determine, at what % solvent during the gradient, the melting point drops below the environmental temperature rather than by redissolution kinetics. Raising the system temperature above the depressed melting point of the polyester was shown to give rise to highly reproducible, normal elution behaviour governed by sorption, since the formation of a crystalline phase was prevented. The difference in redissolution behaviour between amorphous and crystalline resins was used to separate blends of both types of resins by combined eluent and temperature programming.