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 compos ition (the transition point where size-exclusion and adsorption interaction s completely compensate each other) is currently used to separate functiona l oligomers and block copolymers. We have extended the concept of this crit ical elution chromatography to the cases of statistical copolymers as well as porous stationary phases with heterogeneous surfaces (viz., surfaces wit h both inert and active groups). The theory provides the quantitative condi tion for statistical copolymer chains to have a single adsorption-desorptio n transition point. The random copolymers with narrow chemical composition distribution (CCD) always posses such a point. For non-random copolymers th is condition includes the comparison between the mean length of chain segme nts connecting the attractive walls of the pore and the chemical correlatio n 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 a s hypothetical homopolymer chains with a single energy of interaction betwe en the effective monomer units and the active groups at the surface. The re lationship between this energy and the microstructure of copolymer chains h as been also analyzed. If copolymer has a broad CCD, each compositionally h omogeneous 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 chai n effectively behaves as a homopolymer chain at these points. This property was used in the past for the separation of block copolymers according to t he length of blocks comprised of the monomer units of a specific type.