Structural Properties of the Poly(acrylonitrile) Membrane Prepared with Different Cast Thicknesses

Decreasing the thickness of cast polymer solution in a membrane-forming system may lead to structure transitio from finger- to spongelike structure. The shrinkage measurement of the cast poly(acrylonitrile) solutions reveal that the maximum value of shrinkage takes place when the macrovoids disappear from the structure. Th solute rejection properties of membranes also show a different behavior between below and above the structur transition thickness. This experimental finding is explicable on the grounds of porosity variation of th membrane skin. The scanning electron micrograph of a cross-section membrane shows a nodular structur near the precipitation-bath interface, the structure of which is a result of spinodal demixing that is influence by varying the thickness of the cast polymer solution. With the cast solution being regarded as two layer including a very thin top layer and a sublayer, closely packed spheres of polymer are considered for discussio of the thickness dependence of the structure evolution of the membranes.