The mechanism of copolymer retention in interactive polymer chromatography. I. Critical point of adsorption for statistical copolymers

A comprehensive analysis of interactive polymer chromatography is presented. Isocratic liquid chromatography of polymers at the critical eluent composition (the transition point where size-exclusion and adsorption interactions completely compensate each other) is currently used to separate functional oligomers and block copolymers. We have extended the concept of this critical elution chromatography to the cases of statistical copolymers as well as porous stationary phases with heterogeneous surfaces (viz., surfaces with both inert and active groups). The theory provides the quantitative condition for statistical copolymer chains to have a single adsorption-desorption transition point. The random copolymers with narrow chemical composition distribution (CCD) always posses such a point. For non-random copolymers this condition includes the comparison between the mean length of chain segments connecting the attractive walls of the pore and the chemical correlation segment characterizing the randomness of the copolymer microstructure or the surface of this pore. If the critical transition point exists, then the copolymer chains behave as hypothetical homopolymer chains with a single energy of interaction between the effective monomer units and the active groups at the surface. The relationship between this energy and the microstructure of copolymer chains has been also analyzed. If copolymer has a broad CCD, each compositionally homogeneous fraction has its own adsorption-desorption threshold. The partial transition points have been established for the copolymers with blocky or alternating microstructure. Only a portion of the copolymer chain effectively behaves as a homopolymer chain at these points. This property was used in the past for the separation of block copolymers according to the length of blocks comprised of the monomer units of a specific type.