Macrovoid

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. 

The presence of macrovoids in hoUow-fiber membranes is a serious drawback since it increases the fragUity of the fiber and limits its abUity to withstand hydrauhc pressures. Such fibers have lower elongation and tensile strength. 

Hollow fiber membrane(s), 70 766 76 1-31 additional types of, 76 24 advantages of, 76 3 categories of, 76 2-3 in desalination, 76 22 development of, 76 1 extractors, 70 787 fiber treatment for, 76 12-18 future prospects for, 76 26-28 glass and inorganic, 76 23-24 handling and unit assembly of, 76 15-18 interpenetrated wall matrix in, 76 15 low pressure, 76 24-26 macrovoids in, 76 12 materials associated with, 76 18-24 melt spinning of, 76 9-10... 

When the cell wall is fully swollen, the micropores are open and the interior of the cell wall can be accessed by entities that are smaller than the diameter of the micropores. When wood is dried from a water-saturated condition, as water is removed the lumen and other macrovoids, and then subsequently the ceU wall, lose moisture. As the water is removed, the micropores begin to collapse, and this process continues until the wood is dry. 

The development of the Loeb-Sourlrajan asymmetric cellulose acetate membrane (1) has been followed by numerous attempts to obtain a similar membrane configuration from virtually any available polymer. The presumably simplistic structure of this cellulose acetate membrane - a dense, ultrathln skin resting on a porous structure - has been investigated by transmission and scanning electron microscopy since the 1960s (2,3). The discovery of macrovoids ( ), a nodular intermediate layer, and a bottom skin have contributed to the question of the mechanism by which a polymer solution is coagulated to yield an asymmetric membrane. 

At this pressure range ( u 13.6 atm), the compaction and denslfication of the skin s nodular zone notably surpasses that of the adjacent macrovoids shown in the photomicrograph, the reason being that the polymer density around such macrovoids is very high. 

Their collapse pressure is several orders of magnitude greater than that of macrovoids which are located farther below the nodular zone, as will be discussed below. 

Figure 16. Reverse osmosis membrane exhibiting three types of macrovoids large (a), medium (b), and small (c). (A) Before testing and (B) after exposure to 13, atm hydraulic pressure. A longitudinal crack in the skin is designated by the per-...

Figure 17. Cross-section of a reverse osmosis membrane with dense-walled macrovoids...

Thorough analysis and evaluation of membrane morphology is mandatory for the understanding of transport phenomena in membranes, and es pecially for those with rather complex structures, as described in the present manuscript. Each single membrane can be viewed perhaps as a "black box" when operating in a certain well-defined system. Yet, any deduction on transport mechanism that is based solely on transport data is highly speculative. For example, the presence of a double skin, macrovoids, the densifica-tion of the nodular layer, and other items described herein cannot be predicted by the analysis of transport data. But they can be identified, and can be very supportive to "whoever dares to look into the black box."... 

Numerous asymmetric membranes were prepared under various conditions and their cross-section was examined by SEM. Typical of the results are those shown in Figure 5 for the membrane cast from 70 30 THF-formamide and gelled in IPA. Close inspection of Figure 5 reveals a thin, relatively dense skin supported by a microporous layer. The support layer contains macrovoids, the cause of which is presently under investigation. 

Figure 2.4 SEM micrograph of a cross-section of a hollow-fiber dialysis membrane (Polyflux, Cambro) with an anisotropic structure and macrovoids in the support layer (left), and details of the inner porous separation layer in two different magnifications (right reprinted from [12], with permission from Wiley-VCH, 2003).

Microfiltration and UF membranes can be asymmetric, with a denser side and a more open side, or uniform without macrovoids (See Figure 16.3). The open area behind the denser surface in an asymmetric design means there is less resistance to water permeating the membrane. Operating pressure can be lower and the membrane systems can be more productive. The limitation of the asymmetric design is that the material, predominately used in the hollow fiber configuration, is not as strong as the uniform cross section. 

A partial list of commonly encountered structural irregularities in phase Inversion membranes includes irregular gelation, wavemarks, macrovoids and blushing. 

It was even suggested that flnger-llke cavities were desirable In that they represented volume elements of low resistance which contributed to overall permeability. In fact, such cavities are never advantageous and should be avoided whenever possible. There are two basic reasons for the appearance of macrovoids. An... 

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. 

Figure 15.8 SEM image of a PANI membrane prepared by the phase inversion technique with the appearance of macrovoids. (Reprinted with permission from Advanced Functional Materials, High-performance, monolithic polyaniline electrochemical actuators by j.-M. Sansinena, J. Gao and H.-L. Wang, H.-L., 13, 9, 703-709. Copyright (2003) Wiley-VCH)...

In addition, the high affinity between solvent and nonsolvent (low /12) causes the instantaneous demixing. In the case of an instantaneous process, the governing factor is the concentration gradient, which diverges in the direction in which diffusion occurs and causing asymmetric structures and macrovoids. Macrovoids consist in poms that could have sizes similar to the thickness of the membrane. 

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. 

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