Characterization of low-molar-mass polymers by gradient polymer elution chromatography .2. Solubility effects in the analysis of polyester resins under reversed-phase conditions

Polyester resins can be characterized by gradient polymer elution chromatography (GPEC) to obtain information on molar mass and chemical composition. The separation can be influenced by size-exclusion, sorption and solubility (precipitation/re-dissolution) effects. The influence of solubility effects was investigated in this study. Due to pronounced dependencies of molar mass and concentration on cloud points, solubility effects in GPEC were-studied under chromatographic conditions, using inert column packings and low polydispersity fractions obtained by size-exclusion chromatography. The investigated system was a reversed-phase system, using water and tetrahydrofuran as solvent/non-solvent (S/NS) combination, Bare silica, non-porous glass and a stainless steel pre-column filter were compared for the use as an inert medium. A non-porous glass column was shown to be the best choice in this respect. By comparison of the results of the various polyester fractions on the glass and a C-18 column, respectively, it was shown that the separation on C-18 is solely determined by sorption effects. The observed correspondence between values of the %-solvent at the peak-end of the whole, unfractionated polyester on an inert and a sorbing column, alone, is therefore no evidence for solubility governing retention in the high molar mass part of the chromatogram. A comparison with measurements of maximum solubility under static equilibrium conditions of four different polyester fractions in various S/NS combinations revealed that concentrations of the eluting fractions on the glass column are considerably lower than maximum solubility. This can be explained by kinetic effects, influencing re-dissolution.