Membrane and Desalination Technologies - TCE Moodle Website

Preparation of Polymeric Membranes 75
Fig. 2.23 Membrane structures for P84 copolyimide asymmetric membranes (15 wt% P84, H 2O/
NMP: 4.8/95.2 (w/w); coagulant: water; casting temperature: 25 C).
fingering, which could advance quickly into the casting solution (Fig. 2.22b, d). Additionally,
the loose bases of macrovoids were also the main transport channels for nonsolvent and
polymer-lean phase to go into the viscous fingering and displace the casting solution. Hence,
the finger-like structure of porous membranes was a result of hydrodynamically unstable
viscous fingering that was developed when the casting solution was displaced by a polymer
poor phase. The schematic diagram of macrovoid formation is shown in Fig. 2.24. Analogue
to the viscous fingering in the polymer system in the Hele–Shaw cells, the advance of viscous
fingering for a non-Newtonian polymer solution in the Hele–Shaw cells can be described with
Darcy’s Law (80):
v ¼ b2
rP; ð27Þ
12 eff
where v is the finger tip velocity, b is the plate gap of the Hele–Shaw cell, eff is the shear
dependent viscosity of the viscous polymer solution and rP is the pressure gradient.
Because of the phase inversion process, the viscous finger phenomenon in the immersion
precipitation is different from traditional viscous fingering for two immiscible phases.
Compared with the viscous fingering in the polymer system in the Hele–Shaw cells, the
advance of viscous fingering in the immersion precipitation is controlled by three factors:
the osmosis pressure between the polymer-poor phase and the coagulant, the viscosity of the
dope solution and the approaching ratio (which can directly reflect the competition between
the phase inversion process and viscous fingering). When poor coagulants are used or some
solvents were added to the coagulation bath, the exchange rate of solvent and nonsolvent
between the coagulation bath and the polymeric system will be very low and the delay
demixing process occurs. Compared with a pure strong coagulation bath, its osmosis pressure