The retention behavior of diblock copolymers in gradient chromatography; Similarities of diblock copolymers and homopolymers.

The retention of binary diblock copolymers in solvent gradient interaction chromatography (SGIC) was investigated theoretically and compared with experimental results. The isocratic and gradient elution behavior of binary block copolymers can be described by the equations of an equivalent homopolymer having a suitable critical eluent composition and size to pore size ratio. For diblock copolymers the adsorption-desorption transition occurs at a limiting interaction energy, which is characterized by a limiting interaction parameter c, like the critical interaction parameter of homopolymers. Both, the limiting interaction parameter and the critical interaction parameter are independent of pore size. However, while for homopolymers the critical energy is also independent of molecular size, the limiting interaction parameter of block copolymers depends on the size and composition of the macromolecule. Therefore, even for block copolymers of a fixed composition a molar mass independent elution cannot be realized at isocratic conditions. Since binary block copolymers in solvent gradients elute very close to the stronger adsorbing homopolymer, separation of the block copolymer from the stronger adsorbing homopolymer becomes difficult. A slightly better separation is expected to occur in larger pores. The close elution of block copolymers and homopolymers in gradient elution was confirmed for poly(methy methacrylate)-block-poly(styrene)s in chloroform-THF gradients on a bare silica column.