Characterization of polystyrene-b-polyisoprene diblock copolymers by liquid chromatography at the chromatographic critical condition

In the chromatographic separation of macromolecules with a porous stationar y phase, the retention is determined by both size exclusion and interaction mechanisms. At the chromatographic critical condition, the effects due to the two separation mechanisms compensate each other, and the retention of h omopolymer molecules becomes independent of molecular weight. Liquid chroma tography at the critical condition has attracted much interest for the char acterization of block copolymers since it might permit the characterization of individual blocks of a block copolymer by making one block chromatograp hically "invisible". In this study, we critically examine this method using two sets of styrene-isoprene block copolymers designed to have one block l ength constant while varying the other block length. For these block copoly mer systems we found that a block cannot be made completely "invisible" at the critical condition of its homopolymer, and the retention of block copol ymers is affected to some extent by the length of the "invisible" block und er its chromatographic critical condition.