Membrane and Desalination Technologies - TCE Moodle Website

Preparation of Polymeric Membranes 77
demixing, which can influence the membrane morphology strongly. The membrane morphology
changed from finger-like to sponge-like structure with an increase in the approaching
ratio (57). In order to describe the change in the membrane structure more accurately, the
relative sponge-like thickness b was defined as the ratio of the thickness of the sponge-like
layer and that of the total membrane. The relationship between the approaching ratio a and
relative sponge-like thickness b is shown in Fig. 2.25. (57). The critical approaching ratio a *
reflected the sharp change of membrane structure from finger-like to sponge-like. For the
P84/NMP/H2O casting solution (15 wt% P84; coagulant: water; casting temperature: 25 C),
the critical approaching ratio a * was about 0.67. In this figure, the membrane structures were
classified into three parts: Part I: 0 a < a , the membrane was mainly a finger-like
structure; Part II: the membrane would change from finger-like to sponge-like structure at
the short period including a . Part III: a < a < 1:0, the membrane was mainly a sponge-like
structure. When the approaching ratio was below a * , it was suggested that the advancing rate
of viscous fingering was faster than that of the phase-inversion front, hence a finger-like
membrane structure was formed. When the approaching ratio was above a * , the composition
of casting solution moved closer to the binodal curve and the phase inversion process could
occur more easily in the water bath. As a result, the mechanism of phase inversion process
changed and the viscous fingering could not be initiated, hence only a sponge-like membrane
structure could be formed.
INFLUENCE OF MEMBRANE THICKNESS ON MEMBRANE MORPHOLOGY
The influence of membrane thickness on the membrane morphologies was first studied by
Vogrin et al. (82) with a ternary cellulose acetate/acetone/water system. In their study,
macrovoids appeared on the membranes prepared from a 12.5 wt % casting solution at the
β
100%
80%
60%
40%
20%
0%
Finger-like structure Transition structure Sponge-like structure
α∗
0 0.1 0.2 0.3 0.4 0.5
α
0.6 0.7 0.8 0.9 1
Fig. 2.25 Influence of approaching ratio a on the relative sponge-like thickness (15 wt% P84; mixed
solvent: NMP/H2O; coagulant bath: water; casting temperature: 25 C) [adapted from ref. (57)].