Membrane and Desalination Technologies - TCE Moodle Website

64 J. Ren and R. Wang
as nucleation and growth (NG). At the metastable region II, only the structure of the polymerrich
phase is interpenetrating and the polymer-poor phases is discrete. Closed pores can be
observed in the membrane structure, which is shown in Fig. 2.17. The similar situation takes
place in metastable region III, and the system is a suspension solution in which the polymer
particles are dispensed in a polymer-poor phase.
When the phase separation (point M) occurs inside the unstable region IV, the system is
unstable. Any infinitesimal concentration fluctuation can induce phase separation and the mechanism
of phase separation in this region is called spinodal decomposition (SD). It is a spontaneous
process without the need of a nucleus. For the spinodal decomposition process, an interpenetrating
(bicontinuous) three-dimensional network is formed, which is also shown in Fig. 2.17.
Since the polymer solution cannot be vitrified in the initial stage of the phase separation (M
point), the original phase separation structures based on NG and SD (shown in Fig. 2.17) are
not stable. They will grow and coarsen within a limited gelation time (shown in Fig. 2.17) and
two fully separated phases may be obtained with infinite gelation time (39–42). Due to the
coarsening phenomenon, it is almost impossible to attribute obtained membrane structures to
the demixing processes (4). For a SD system within a short gelation time, the coalescence
process is “frozen” early enough by vitrification and a morphology with high interconnectivity
is obtained. With the increase of the gelation time, the interpenetrating structure coarsens
and the interconnectivity declines (43). For a long gelation time, the phase coalescence may
lead to a closed cell structure (44). Thus, the coarsening processes are very important in the
phase separation process, which can determine the final size and interconnectivity of the
porous structure of the membranes. The coarsening processes can be controlled or adjusted
according to the gelation time. Different membrane morphologies can be obtained by
controlling the different gelation times.
4.1.2. Diffusion Induced Phase Separation
PHASE DIAGRAM
The thermodynamic behavior of different spinning systems in the process of DIPS can be
also described in a phase diagram, which was first used by Strathmann in 1971 (45) and then
advanced by Smolders et al. (46, 47). For an immersion precipitation process, at least three
components of polymer, solvent and nonsolvent are involved. Usually, other components
such as blended polymers, mixed solvents, and nonsolvent additives are used to get desired
membrane morphology with good performance. Nevertheless, a ternary combination of
polymer, solvent and nonsolvent is discussed here for the simplicity. Figure 2.18 is a
schematic illustration of the phase behavior in a polymer, a solvent and a nonsolvent system.
From the phase diagram, four different regions are found.
Region I: one phase solution
Region II: liquid–liquid two-phase solution
Region III: liquid–solid two-phase swelling
Region IV: one phase glass and swelling
In order to distinguish different regions mentioned above, Berghmans’ point B, the
spinodal curve, the binodal curve, the vitrification boundary, the swelling boundary, and