The objective of this article is to investigate the permeation characteristics of several polydispersed macromolecules through the asymmetric and the particulate poly (ethylene-co-vinyl alcohol) (EVAL) membranes. The solutes in examination include two nonionic solutes, dextran and polyethylene glycol (PEG), and one anionic solute, sodium polyacrylate (SPA). The ultrafiltration results indicate that the rejection coefficients increase monotonically with the molecular weights of test solutes permeating through the asymmetric EVAL membranes. This is consistent with the behavior of normal commercial ultrafiltration membranes. For the particulate membranes, the rejection curves show different trends. In most cases, there exist two local minima in the rejection curves, having the shape of “W”. This unusual behavior is found to be associated with the dual-pore character of the particulate membrane. To this context, a parallel composite membrane model is proposed to explain this specific rejection in terms of rejection coefficients for the small and the large pores.
