Mj. Han et D. Bhattacharyya, CHANGES IN MORPHOLOGY AND TRANSPORT CHARACTERISTICS OF POLYSULFONE MEMBRANES PREPARED BY DIFFERENT DEMIXING CONDITIONS, Journal of membrane science, 98(3), 1995, pp. 191-200
A polymer solution film, which is cast from a homogeneous polysulfone
(PS) solution (1.5 wt%) in dimethylformamide (DMF), demixes by liquid-
liquid phase separation due to nucleation of polymer-poor phase with s
orption of water vapor from atmosphere. The separated two liquid phase
s continue to grow until the polymer-rich phase solidifies. However, t
he polymer-rich phase in the liquid-liquid phase-separated solution ca
n demix again before precipitation of the phase. The demixing of the p
olymer-rich phase on the top layer of the film is induced by kinetical
ly fast demixing conditions, such as rapid mass transfer between nonso
lvent (water) and solvent (DMF) in a nonsolvent bath. Morphology of so
lidified membranes shows that when a membrane structure is established
by water vapor sorption alone, the liquid-liquid phase separated solu
tion film forms closed cell-like structures in a whole cross section i
ncluding surface region and has round pores on the top surface. Even t
hough pore sizes on the surface of the membrane are more than 1 mu m,
those pores do not work as active pores for membrane performance. A sk
in structure of a membrane precipitated by fast or instantaneous demix
ing in a water bath includes nodules or polymer aggregates which can b
e distinguished from the cell-like structures formed by the liquid-liq
uid phase separation due to nucleation of the polymer-poor phase. The
membrane including nodules in the skin region has higher surface area
and broader pore size distribution than the membrane which consists of
the cell-like structures. The crack-like pores on the surface of the
former contribute to the permeation characteristics of the membrane.