Critical retention behaviour of polymers a study on the influence of some practical parameters

Liquid chromatography under critical conditions is an important tool for the microstructural characterization of telechelic polymers and block copolymers. Until now, only little information on the practical aspects of this technique is available. The influence of some important practical parameters was investigated, using polystyrene. Critical conditions depend strongly on the type of column packing. The solubility of polymers under critical conditions for different solvent-non-solvent combinations differs to a great extent. For different solvent-non-solvent pairs on reversed-phase systems, a roughly constant eluent strength in terms of the Hildebrand solubility parameter, under critical conditions is found. Temperature can be a useful tool for fine-tuning critical conditions. On normal-phase systems, however, the retention of polystyrene changes non-monotonously with temperature, which limits the use of temperature variations. It is not possible to obtain exact molecular mass independence on any of the investigated systems, which can not be ascribed to chemical differences. This makes the validity of the current theories on critical conditions questionable. Especially for the higher-molecular-mass polystyrenes, peak broadening increases significantly when going from size exclusion conditions to critical conditions. This phenomenon can limit the application of liquid chromatography 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 order to suppress zone splitting as much as possible. For higher-molecular-mass polystyrenes this effect cannot be completely prevented.