CHARACTERIZATION OF THE SURFACE-LAYER OF INTEGRALLY SKINNED POLYIMIDE MEMBRANES - RELATIONSHIP WITH THEIR MECHANISM OF FORMATION

A series of dense films and membranes was prepared using different processes; their surface morphology was investigated by transmission electron microscopy using the replication technique. On the basis of TEM results, SEM observations of cross sections of membranes and gas-separation properties of membranes, two kinds of membrane morphology are evidenced in relation to their preparation conditions and final properties. The first one gives an ultrafiltration-type membrane. Large nodules are observed on the surface of the membranes. It is suggested that these are formed by liquid-liquid separation that occurs by nucleation and growth of the polymer-rich phase. The second one yields ps-separation-type membranes. No nodules are evident at the surface of these membranes. The skin is formed by a step that concentrates the initial solution. It is followed by the transformation of the solution in the glassy state. Liquid-liquid separation takes place below the first skin layers by nucleation and growth of the polymer-poor phase.