CRITICAL RETENTION BEHAVIOR OF POLYMERS - A STUDY ON THE INFLUENCE OFSOME PRACTICAL PARAMETERS

Liquid chromatography under critical conditions is an important tool f or the microstructural characterization of telechelic polymers and blo ck copolymers. Until now, only little information on the practical asp ects of this technique is available. The influence of some important p ractical parameters was investigated, using polystyrene. Critical cond itions depend strongly on the type of column packing. The solubility o f polymers under critical conditions for different solvent-non-solvent combinations differs to a great extent. For different solvent-non-sol vent pairs on reversed-phase systems, a roughly constant eluent streng th in terms of the Hildebrand solubility parameter, under critical con ditions is found. Temperature can be a useful tool for fine-tuning cri tical conditions. On normal-phase systems, however, the retention of p olystyrene changes non-monotonously with temperature, which limits the use of temperature variations. It is not possible to obtain exact mol ecular mass independence on any of the investigated systems, which can not be ascribed to chemical differences. This makes the validity of t he current theories on critical conditions questionable. Especially fo r the higher-molecular-mass polystyrenes, peak broadening increases si gnificantly when going from size exclusion conditions to. critical con ditions. This phenomenon can limit the application of liquid chromatog raphy under critical conditions to a certain molecular mass range. The composition of the solvent in which polystyrenes are dissolved prior to injection, has to be exactly the critical solvent composition, in o rder to suppress zone splitting as much as possible. For higher-molecu lar-mass polystyrenes this effect cannot be completely prevented.