Membrane and Desalination Technologies - TCE Moodle Website

Preparation of Polymeric Membranes 73
polymer properties for gas separation, pervaporation, reverse osmosis, et al. For the second
path (path 2), the decomposition path of the membrane surface will go to the binodal curve.
And the polymer concentration at the membrane surface will be always on the binodal
position, which is in equilibrium with solvent concentration of the coagulant bath at the
interface between the membrane and coagulant bath (38, 65, 67). With the time going on, the
polymer concentration will change along the binodal curve, but no liquid–liquid demixing
occurs on the surface layer because no polymer-poor phase is formed. At this period the skin
layer cannot be solidified. The liquid–liquid demixing occurring at the sublayer underneath
the membrane surface will influence the skin layer, and cause different pore size distributions
and different skin thicknesses. The surface structure with different pore size distributions will
be solidified until it reaches point B. The resulting membranes may be ultrafiltration,
microfiltration and some gas separation membranes et al. Actually, for a dry-phase inversion
process induced by forced-convective evaporation, the polymer concentration on the membrane
surface sometimes crosses the binodal curve and goes into the spinodal decomposition
(path 3) (68). Because of relative long gelation time, the coarsening of the bicontinuous
structure occurs at a later stage of the phase separation process (before solidification) to form
spherical nodules. Because of an additional physical process such as capillary pressure arising
from the curvature of the interface of polymer-poor phase contacting with an external gas
phase, an essentially homogeneous nascent skin layer will be formed by the coalescence of
highly plasticized nodule network (43, 68, 69), shown in Fig. 2.21.
4.2.6. Macrovoid Formation
Macrovoids are very large elongated pores which can extend over the membrane thickness.
It may have different shapes such as tear-like, pin-like, finger-like and so on. Macrovoids are
undesirable because they cause poor mechanical strength in the membrane. The formation of
macrovoids in the phase inversion process has been studied extensively in the literature (1, 4,
70–77). And a variety of mechanisms have been proposed and used to describe the formation
of macrovoids such as the shrinkage of polymer matrix (71, 72), surface tension gradientinduced
convective flow (75), osmotic pressure (73), instantaneous phase demixing (70), and
concentration gradient-induced instability (76). However, a lot of controversy exists
concerning the original of the macrovoids.
Fig. 2.21 Contracting
forces of polymer
particles resulting from
capillary pressure in
interstitial spaces of film
[adapted from ref. (68)].