Formation of macrovoids

The presence of macrovoids is not generally favourable, because they may lead to a weak spot in the membrane which is to be avoided especially when high pressures are applied, such as in gas separation. For this reason it is necessary to avoid macrovoid formation as much as possible, which can be achieved when the mechanism of macrovoid formation is understood. 

In w hat membrane-forming systems do macrovoids actually appear The examination of many systems indicates that systems those exhibiting instantaneous demixing often show macrovoids, whereas when a delayed onset of demixing occurs macrovoids are absent. Hence, it would seem that the mechanism which determines the type of membrane formed, i.e. the onset of liquid-liquid demixing, also determines whether or not macrovoids are present. This means that the parameters that favour the formation of porous membranes may also favour the formation of macrovoids. 

The main parameter involved is the choice of sol vent/nonsolvent pair. Ahigh affinitj between the solvent and the nonsolvent is a very strong factor in the formation of an ultrafiluation/microfiltrationtype of membrane. Solvent/water pairs with DMSO. DMF, NMP, DM.Ac, triethylphosphate and dioxan as the solvent exhibit very high mutual affinities (see also figure III - 42) and macrovoids can be found in membranes prepared from these systems irrespective of the polymer chosen for their preparation. 

Besides the miscibOity of the solvent and the nonsolvent, other parameters which affect the instant of onset of liquid-liquid demixing also have an influence on the presence of macrovoids. However, before discussing these parameters, it is first necessary to describe the mechanism of macrovoid formation. In this respect, two phases in the formation process have to be considered i) initiation and ii) propagation or growth. 

In the case of a delayed onset of liquid-liquid demixing, nucication is not possible until a cenain period of time has elapsed. In the meantime the polymer concentration has increased in the top layer. After a finite lime has elapsed nucleation starts in the layer beneath the top layer. However, the composition now in front of these first-formed nuclei is such that the formation of new nuclei has been initiated. 

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Spinbath concentration can be adjusted to obtain the desired microstmcture. Low spinbath concentration promotes rapid solvent extraction but this also produces a thick skin on each filament which ultimately reduces the rate of solvent extraction and may lead to the formation of macrovoids. High spinbath concentrations give a denser microstmcture, but solvent extraction is slow and filament fusion can occur. Other spinbath conditions that affect coagulation and microstmcture are dope soHds, spinbath temperature, jet stretch, and immersion time. 

In preparing membranes via the phase inversion process for applications in pressure-driven processes, the formation of macrovoids should be avoided completely. These finger-like pores of the type present in the substructure of membranes (b) and (c) of Fig. 3.6-1, severely Hmit the compaction resistance of the membrane. Membranes with a sponge-Hke structure (Fig. 3.6-la) are to be preferred. 

In addition, many nuclei are growing at the same time so there will be less solvent available for their growth, as all of them are consuming it. Competition between nuclei for the solvent and the limitations of physical space are factors that restrain the growth of the pores and do not allow the formation of macrovoids in these systems. Figure 19.4 compares these two phenomena. 

C.A. Smolders, A. J. Reuvers, R.M. Boom, I.M. Wienk, Microstructures in phase-inversion membranes. Part 1. Formation of macrovoids. Joumo of Membrane Science 73 (1992) 259. 

Figure 2.12 shows the cross sections and the microporosity from one of the representative as-spun fibers collected from different fiber-spin trials. Macrovoids are observed for every fiber sample regardless of dope solution concentration. This result is consistent with previous studies in which the polyaniline fibers were spun from concentrated EB solutions dissolved in NMP [39] and DMPU [36]. Due to the formation of macrovoids in the as-spun fibers, the diameter of the fiber (250-300 p,m) was always larger... 

The formation of macrovoids in these asymmetric polyaniline hollow fibers is undesirable as they weaken the mechanical strength and may lead to defects in the selective layer, which make them nonviable for many separations applications. The formation of macrovoids in hollow fiber membranes is generally caused by fast precipitation kinetics of the polymer solution in the coagulation bath. The formation of macrovoids when spinning polyaniline hollow fibers was foimd to be highly dependent on... 

The influence of shear rate on membrane structure is illustrated in Figures 31.4 (Ren et al., 2002) and 31.5 (Chung et al., 2002). Figure 31.4 exhibits the cross-section morphology of hollow-fiber membranes spun at shear rates of 812 and 2436 s. Some macrovoids can be observed near the inner skin of fibers spun at a low shear rate (812 s ), while these macrovoids are apparently eliminated or suppressed when the shear rate increases to 2436 s Clearly, high shear rates modify the precipitation path and retard the formation of macrovoids. In addition, with an increase in shear rate, the membrane structure becomes... 

Formation of macrovoids in a membrane is always observed in a system that facilitates instantaneous demixing. Presence of macrovoids is beneficial to the manbrane permeability as the tortuosity is reduced. However, mechanical properties of manbianes... 

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