CHARACTERIZATION OF LOW-MOLAR-MASS POLYMERS BY GRADIENT POLYMER ELUTION CHROMATOGRAPHY .3. BEHAVIOR OF CRYSTALLINE POLYESTERS UNDER REVERSED-PHASE CONDITIONS

Gradient polymer elution chromatography (GPEC) can be used to characte rize polyester resins according to molar mass and chemical composition . In the third part in this series, the application of GPEC on crystal line polyesters was studied. In contrast to amorphous polyesters, crys talline polyesters were found to exhibit non-reproducible chromatograp hic behaviour in GPEC under certain conditions. The cause of this phen omenon was found in the dominance of precipitation (crystallization) a nd redissolution effects in the total retention mechanism. Crystalline polyesters were found to crystallize on the column after precipitatio n in contrast to amorphous polyesters, where no real solid-phase is fo rmed. Varying injection volume, flow-rate or precipitation medium affe ct the morphology of the precipitate, giving rise to a different redis solution behaviour. From the minor effects of increasing sample load a nd gradient steepness, it was concluded that separation is mainly gove rned by thermodynamics that determine at what % solvent during the gra dient, the melting point drops below the environmental temperature rat her than by redissolution kinetics. Raising the system temperature abo ve 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 differ ence in redissolution behaviour between amorphous and crystalline resi ns was used to separate blends of both types of resins by combined elu ent and temperature programming. (C) 1997 Elsevier Science B.V.