Reversed phase liquid chromatographic retention behavior of polystyrene homopolymers

Summary

Isocratic and gradient elution high performance liquid chromatographic measurements of the retention behavior of polystyrene homopolymers with molecular weights ranging from 2 kD to 390 kD were performed using mixed methylene chloride-methanol mobile phases of varying composition and a C-18 chemically bonded stationary phase supported on either 100 Å or 300 Å mean pore diameter silica. Isocratic measurements of the capacity factor, k′, for different molecular weight homopolymers as a function of the volume fraction of methylene chloride, ϕ, permit determination of the critical composition, ϕ_c, which renders k′=1 and the local slope, S=−∂lnk′/∂ϕ_c of the lnk′-ϕ isotherm at ϕ=ϕ_c, and also the dependence of ϕ_c and S on the degree of polymerization, M. Linear gradient elution measurements of ϕ_c and S were also performed and compared to the corresponding isocratic measurements. The general retention behavior and the dependence of ϕ_c and S on M compare favorably to the predictions of the theory of homopolymer retention and fractionation developed by Boehm, Martire, Armstrong, and Bui (BMAB). Comparison is also made between the present work and the experimental observations of other workers on related chromatographic systems involving hompolymer retention.