Properties of asymmetric polyimide ultrafiltration membranes. I. Pore size and morphology characterization

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Abstract
Properties of asymmetric UF membranes made by solution casting of aromatic polyimides as a function of fabrication conditions are reported. The characteristic properties investigated include: the overall porosity, hydraulic permeability, the equivalent pore size of the skin layer (via a newly developed general diagram), and morphological features shown by the scanning electron microscope. It was found that as casting solution concentration and/or evaporation period was increased, the average pore size decreased while the skin thickness increased. At low casting solution concentrations the membranes were highly porous and the precipitated polymer phase had a granular structure consisting of aggregates of precipitated polymer micelles. While at high concentrations marcrovoid porosity was reduced but the precipitated polymer phase had a spongy structure. The interstitial openings of the granular skin structure of a membrane made from 15% polymer solution with no evaporation as revealed by SEM showed pore size values that were close to those calculated through pore models.