Molar mass characterization of cationic methyl methacrylate–ethyl acrylate copolymers using size-exclusion chromatography with online multi-angle light scattering and refractometric detection

Size-exclusion chromatography (SEC) combined with online multi-angle light scattering (MALS) and refractometric (RI) detection has been employed for the molar mass characterisation of water-insoluble cationic methyl methacrylate–ethyl acrylate copolymers (Eudragit RS and RL). Due to their positive charge, cationic polymers are particularly difficult to separate on a SEC column, in worst cases being completely adsorbed on the oppositely charged packing material. This work has examined how a careful addition of salt (LiCl) to the copolymer solution in ethanol decreases the electrostatic interactions, clearly seen as a decrease in elution volume from the SEC column as well as an improved recovery. At a certain level of ionic strength, typically about 50 mM, the copolymer recovery from the SEC column reached 100% and molar mass distributions corresponding to the complete sample could be obtained. The combined MALS/RI detection gives the opportunity to measure the absolute molar mass independent of recovery and retention. Thus, in this study, it turned out to be a favourable tool for tracing the changes in elution behaviour of the charged copolymer as the ionic strength was increased.