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 chro matography (GPEC) to obtain information on molar mass and chemical com position. 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 pronounce d dependencies of molar mass and concentration on cloud points, solubi lity effects in GPEC were-studied under chromatographic conditions, us ing 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 stee l pre-column filter were compared for the use as an inert medium. A no n-porous glass column was shown to be the best choice in this respect. By comparison of the results of the various polyester fractions on th e glass and a C-18 column, respectively, it was shown that the separat ion on C-18 is solely determined by sorption effects. The observed cor respondence between values of the %-solvent at the peak-end of the who le, unfractionated polyester on an inert and a sorbing column, alone, is therefore no evidence for solubility governing retention in the hig h molar mass part of the chromatogram. A comparison with measurements of maximum solubility under static equilibrium conditions of four diff erent polyester fractions in various S/NS combinations revealed that c oncentrations of the eluting fractions on the glass column are conside rably lower than maximum solubility. This can be explained by kinetic effects, influencing re-dissolution.