Membrane and Desalination Technologies - TCE Moodle Website

82 J. Ren and R. Wang
the spinneret. For some membranes with the inner skin layer due to the strong coagulant in the
bore side, the shear rate of the dope solution on the inner edge of the annular dope flow in the
spinneret was representative.
The shear rate of the dope solution within the spinneret strongly influenced the membrane
performance, which was shown in Fig. 2.29. (98). With an increase in shear rate of the dope
solution, the molecular weight cut-off (MWCO) changed from 110,000 to 19,000 Da, and the
flux decreased greatly from 159 to 38 L/m 2 h atm. The shear rate of the dope solution within
the spinneret also strongly influenced the membrane morphology (98). With an increase in
shear rate, the defect on the surface of hollow fiber membranes could be suppressed and a low
flux and a good selectivity were obtained (98). With an increase in shear rate, the inner
surface of hollow fiber membranes became much tighter, and the outer surface of hollow fiber
membranes became smoother and denser.
5.1.2. Nascent Hollow Fiber Membranes in the Air Gap
When the hollow fibers are spun using the dry-jet wet spinning process, the nascent hollow
fiber membranes are strong influenced by the viscoelastic properties of the polymer dope. A
viscoelastic fluid exhibits both energy dissipation and energy storage in its mechanical
behavior, which can be reflected according to creep recovery test and dynamic oscillation test.
VISCOELASTICITY AND RELAXATION TIME
Usually the dope solution prepared for spinning hollow fiber membranes is a viscoelastic
fluid. During the dry-jet wet spinning process, the rheology characteristics of the dope
solution are very important to the nascent fibers. Figure 2.30 shows the dynamic frequency
Flux ( L/m 2 .h.atm)
300
250
200
150
100
50
0
1000000
100000
10000
0 2000 4000 6000 8000 10000
Shear rate (s –1 1000
)
Fig. 2.29 Influence of shear rate of dope solution within the spinneret on the performance of hollow
fiber membranes (polymer solution: 21 wt% P84; H2O/NMP: 3.6/96.4 (wt/wt); a = 0.45. Spinneret:
OD = 1 mm; ID = 0.44 mm) [adapted from ref. (98)].
MWCO