CHARACTERIZATION OF LOW-MOLAR-MASS POLYMERS BY GRADIENT POLYMER ELUTION CHROMATOGRAPHY .1. PRACTICAL PARAMETERS AND APPLICATIONS OF THE ANALYSIS OF POLYESTER RESINS UNDER REVERSED-PHASE CONDITIONS

The application of gradient elution techniques such as gradient polyme r elution chromatography (GPEC) for the characterization of low-molar- mass polymers has been known for a long time. Until now, however, prac tical applications have been lacking. The objective of this study was to find out to what extent GPEC can be useful to provide information o n the (micro)structure of these materials. The influence of some pract ical parameters in GPEC under reversed-phase conditions with tetrahydr ofuran and water as the solvent-non-solvent combination, was investiga ted using a co-polyester resin. A gradient steepness less than 1%/min hardly improves the separation at the cost of a much longer analysis t ime. Increasing the column length or temperature only significantly im prove the separation in the low-molar-mass part of the chromatogram. A sample load of up to 1 mg does not influence the separation. This ind icates that separation is probably dominated by sorption (adsorption a nd/or partitioning) rather than solubility effects. Injection volumes exceeding 10 mu l give rise to additional peak broadening due to a sam ple solvent effect. For different polyesters, an almost linear depende nce was found between the reciprocal square root of the molar mass and the percentage of solvent at the point of elution of an oligomer. Thi s dependence was used to calculate average molar masses for one polyes ter. The M(w) thus calculated, agreed well with absolute methods. The oligomer distribution obtained using GPEC was in excellent agreement w ith the theoretical distribution, thus demonstrating the feasibility o f GPEC for providing information on reaction kinetics. By comparison o f GPEC results for two co-polyester resins, evidence for differences i n chemical composition distribution could be obtained.