The mechanism of copolymer retention in interactive polymer chromatography. II. Gradient separation

The molecular-statistical theory of polymer solutions in confined media is applied to the conventional chromatographic theory of gradient elution. This approach leads to the prediction of the special mode of interactive polymer chromatography: gradient elution at critical point of adsorption. We demonstrate theoretically and experimentally that under appropriate conditions elution of each compositionally homogeneous fraction of copolymer occurs at the critical mobile phase composition. This critical mobile phase composition depends only on the local structure of the copolymer chain and is independent of its molecular weight. As a consequence, gradient elution produces the chemical composition distribution of the copolymer. The theory provides the quantitative conditions for statistical copolymer chains to have a single transition point. Equations describing relationships between the critical eluent composition and the chemical composition and microstructure of macromolecules are developed. The experimental verification of the theory was performed by the normal phase isocratic and gradient elution of chlorinated polyethylene with various chemical compositions (chlorine content) and molecular weights. To the best of our knowledge, these experiments, for the first time, demonstrate the existence of the adsorption-desorption transition point for statistical copolymers. The gradient separation at the critical point of adsorption can be applied also to other complex polymers containing various types of molecular heterogeneity.