Membrane and Desalination Technologies - TCE Moodle Website

78 J. Ren and R. Wang
Thickness of the sponge-like portion(µm)
15
10
5
I II
III
0
0 10 LC 20 30 40 50 60
Thickness of the entire membrane (µm)
Fig. 2.26 Influence of membrane thickness on the membrane structure (20 wt% P84; solvent: NMP;
coagulant bath: water; casting temperature: 25 C) [adapted from ref. (83)].
thickness of 500 mm, but there were no macrovoids at thicknesses of 150 and 300 mm. Li et al.
(83) investigated the thickness of sponge-like portions at different entire membranes with
PES and copolyimide P84. When the membranes were prepared with 20% P84 in NMP
(casting temperature, 25 C; coagulant, water), three regions could be identified along the
abscissa corresponding to different membrane structures shown in Fig. 2.26. In region I, the
cross sections of the membranes have a fully sponge-like structure. The membrane morphology
transits from a sponge-like to a finger-like structure with some degrees of fluctuation in
sponge thickness in region II. In region III, the membranes are of mainly finger-like structure
with an almost constant thickness of sponge-like portion, which is independent of overall
membrane thickness. There exists a critical structure-transition thickness, Lc, that reflects the
transition of membrane morphology from a sponge-like to a finger-like structure during the
formation of asymmetric flat membranes.
5. PREPARATION OF ASYMMETRIC MEMBRANES BY PHASE
INVERSION TECHNIQUE
5.1. Preparation of Hollow Fiber Membranes
Polymeric hollow membranes were first mentioned in 1966 (84). They were prepared by
extruding a polymer solution through an annular spinneret and a bore fluid flowed in the
annular centre. The spinning of hollow fiber membranes is a very complicated process, which
involves many spinning parameters as shown in Fig. 2.27. Basically, the fabrication of hollow
fiber membranes involves the thermodynamics of the polymer solution and the phase
inversion process, the rheologies of the polymer solution inside the spinneret and at the air
gap, and other spinning conditions as well. The thermodynamics of the polymer solution and