Characterization of Random Copolymers by Liquid Chromatography under Limiting Conditions of Adsorption

Abstract

Liquid chromatography under limiting conditions of adsorption (LC LCA) can be considered as a new tool to perform, simultaneously, molar mass and composition determination of statistical copolymers and to separate copolymer blends as a function of copolymer composition or microstructure. The suitability of LC LCA has been demonstrated for two series of statistical copolymers, poly(styrene-co-methyl methacrylate), poly(S/MMA), varying either in molar mass or composition. The binary eluent consisted of a mixture of n-hexane and THF, containing either 26 or 30 vol.% of n-hexane, respectively, for low and high LC LCA. Two parameters, the ratio of peak area over height and the ratio of peak area of chromatograms obtained by LC LCA and size exclusion chromatography (SEC), were employed to evaluate statistical copolymer elution through LC LCA. From the peak analyses, macromolecule recovery was complete over the entire composition and the molar mass ranges. LC LCA and SEC were further compared, and off-line fractionation was subsequently performed using a combination of both methods, with different statistical copolymer solutions having narrow, broad, and bimodal composition distributions.