Macrovoids, phase Inversion

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

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)...

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. 

Widjojo, N., and Chung, T. S. (2006). The thickness and air-gap dependence of macrovoid evolution in phase-inversion asymmetric hollow fiber membranes. Ind. Eng. Chem. Res. 45, 7618. 

One specific advantage of this process is that the dense polymer layer firmly adheres to a porous support offering the ability to withstand high pressure. Hollow fiber membranes utilized in gas separation need to withstand continuous pressures up to and exceeding 1000 psi. For gas separation, it is desirable to have a continuous porous stmrture as the support without macrovoids, which can often be produced if the phase inversion variables (such as polymer solution concentration) are not optimized. For ultra- and microfiltration membranes, macrovoids often exist but do not critically restrain the... 

Weaker nonsolvents with a lower solubility parameter other than water, such as ethanol, are sometimes used. These weaker nonsolvents lead to the formation of a denser membrane (Albrecht et al. 2001 Young and Chen 1995). Systems with a rapid phase inversion rate (strong nonsolvent) tend to form macrovoids with finger-like structures, whereas systems with a slow phase inversion rate yield sponge-like structures (Young and Chen 1995). In addition, polymer crystallinity can be affected by the choice of nonsolvent, as reported by Buonomenna et al. (2007b) for the (DMA/ water and DMA/C1-C8 alcohols) system. 

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