Membrane and Desalination Technologies - TCE Moodle Website

72 J. Ren and R. Wang
structure will be solidified due to the vitrification of the polymer-rich phase at point B. If the
coagulation concentration approaches the point C (in Fig. 2.19) from the pure nonsolvent, the
gelation time will also increase. The influence of the gelation time on the membrane
morphology is also shown in Fig. 2.19. At the very short gelation time, the system (S2!B)
solidifies quickly after the first step of phase separation (A!S1), the membrane will have a
fine pore structure and the original characteristics given by the initial demixing mechanism
can be kept. For the NG process, a membrane morphology of closed cells would be formed
because of rapid stop of the demixing during the initial stages. With the time going on, the
nuclei would grow and touch each other forming interconnected pores. The SD demixing
would induce the formation of an interconnected pore structure from the beginning, which
might evolve into closed cell structure overtime (44). Because of the coarsening phenomenon,
the final membrane morphology cannot be completely attributed to the original demixing
process. For the NG and SD demixing processes, when the concentration of the
coagulation bath is or lower than point C shown in Fig. 2.19, the nascent membrane
morphology cannot be solidified and eventually two fully separated layers may be obtained.
4.2.5. Membrane Surface Formation of Porous Membranes
The formation process of the membrane surface for porous membranes can be illustrated
with a ternary phase diagram as shown in Fig. 2.20. According to solvent evaporation or
solvent–nonsolvent exchange, the surface polymer composition will reach the vitrification
boundary directly (path 1) and consequently a dense membrane surface will be formed. When
the polymer solution under the skin layer crossed the binodal curves, a porous membrane
morphology under the skin layer is formed. The resultant membranes will show intrinsic
I
A
path1
A
F
path2
A path3
P(polymer)
B
S (solvent) C
N(nonsolvent)
Fig. 2.20 Schematic diagram of membrane surface formation process for DIPS.
G