CONTROL OF THE POROSITY OF ASYMMETRIC TPX MEMBRANES

In the present work, the effect of adding nonsolvent in the casting so lution on the porosity of asymmetric TPX (poly(4-methyl-1-pentene)) me mbranes was systematically investigated. A series of alcohols, with ca rbon number ranging from 2 to 14, was added in the casting solution (T PX/cyclohexane) to alter the porosity of two types of asymmetric TPX m embranes, prepared by using ethanol and 1-propanol as the coagulation medium. It was found that the effect of nonsolvent on membrane porosit y is different for the two types of membranes and the difference can b e reasoned by considering the exchange rate between the polymer solven t and the coagulation medium during membrane formation. The results in dicate that, for the membrane formation system with low exchange rate between coagulant and solvent, the membrane porosity is controlled by the coagulation value, defined as the volume of coagulant required to demix the casting solution. On the other hand, for the system with hig h exchange rate, the membrane porosity is not controlled by the coagul ation value but by the penetration speed of the coagulant front moving through the casting solution. (C) 1998 Elsevier Science B.V.