Determination of the Composition of Statistical Copolymers by Liquid Chromatography Under Limiting Conditions of Adsorption

Abstract

Liquid chromatography under limiting conditions of adsorption (LC LCA) was applied to determine the composition of statistical poly(styrene-methyl methacrylate) (PS/ MMA) copolymer standards. Two series of copolymers were prepared and characterized. The first series varies with respect to the styrene/methyl methacrylate ratio in the copolymer, at a constant molar mass (220,000 ± 30,000 g/mol), while the second has molar masses from 20,000 to 250,000 g/mol at a molar styrene fraction of 0.5 ± 0.02. The copolymers were fractionated using size exclusion chromatography (SEC) and LC LCA. In the case of LC LCA, two different ratios of THF to n-hexane were employed. Low and high LC LCA were differentiated according to the composition of the binary eluent, the lowest n-hexane content corresponding to low LC LCA. The influence of the molar mass on the peak distance, defined as the difference of retention volume between SEC and LC LCA, was negligible in the studied range compared to composition effect. A calibration curve was subsequently established and correlated the peak distance to the copolymer composition. The application range of the calibration curve was found to vary with the percentage of n-hexane contained in the eluent and corresponded to 0.1–0.4 and 0.2–0.6 molar fraction of styrene for low and high LC LCA, respectively. Using these calibration curves, the P(S/MMA) copolymer composition distribution calculation was estimated for one sample. Therefore, the limiting condition method can be coupled on-line or off-line with SEC to perform a simultaneous characterization of the compositions and molar mass distributions.