Study of the separation principle in size exclusion chromatography

Size exclusion chromatography (SEC) separates polymer molecules according to their size in dilute solution, but what size to use has been a matter of debate for 35 years. In 1967, Benoit and co-workers found an excellent correlation between elution volume and a dynamically based molecular size, the hydrodynamic volume V-H, for a wide range of species and large-scale molecular architectures. However, both theory and simulations assume a thermodynamic separation principle. This assumption is based on experimental observations that elution volumes are independent of flow rates. Thus, one might surmise that the radius of gyration R-g is a more appropriate size measure for purposes of universal SEC calibration, although the calculations of Casassa and co-workers on star polymers suggest some other thermodynamically based size may be required. In an attempt to resolve this dilemma, we undertook an extensive study of SEC with three groups of nearly monodisperse polymers-linear polystyrenes, linear polyethylenes, and polyethylenes with several types of long chain branching. Measurements of R-g, molecular weight M, intrinsic viscosity [eta] (V-H = [eta]M), and the elution volume on these various samples demonstrate that R-g correlates the partitioning of linear chains more satisfactorily than does V-H, while the reverse is true for branched chains. Extending the theoretical calculations of distribution coefficients for star polymers to other types of branched architectures may be necessary to settle the matter.