FORMATION OF AIR-CAST CELLULOSE-ACETATE MEMBRANES .1. STUDY OF MACROVOID FORMATION

Macrovoid formation is reported for air-cast cellulose acetate membranes (i.e. under formation conditions where no convectional intrusion of precipitant can take place). Polymer chain length (molar mass) was found to play an important role in this process. Increase of polymer chain length by a factor of 2.0-2.7 suppressed macrovoid formation. It is proposed that reptational diffusion of the high-molar-mass polymer was too slow to allow for a lean-phase nucleation and a subsequent macrovoid formation. Decrease of the thermodynamic quality of the initial solvent/non-solvent mixture also helped to suppress macrovoid formation. Although instantaneous demixing was observed for all of the systems studied (type II membranes according to Reuvers [1]), it was not found to be necessarily accompanied by macrovoid formation. Membranes formed from the high-molar-mass polymer showed less skinning on both surfaces and better hydraulic permeabilities than those formed from polymers of lower molar mass.