Phase macrovoid formation

Barzin, J. and B. Sadatnia. Correlation between macrovoid formation and the ternary phase diagram for polyethersulfone membranes prepared from two nearly similar solvents. Journal of Membrane Science 325(1) (2008) 92-97. 

The growth of a polymer-poor phase by SD or NG is an isotropical process, which takes place as soon as the solvent-nonsolvent contents supply the thermodynamic condition for dembdng. To understand the macrovoid formation, a quite interesting explanation was provided by McKelvey and Koros [28] as de-... 

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. 

Influence of solvent on macrovoid formation was studied by researchers. For instance, PVDF and polyethersulfone (PES) membranes prepared using DMAc as the solvent exhibit smaller macrovoids than that using NMP as the solvent [23,25,26]. Barzin and Sadatnia proposed that in a PES/DMAc/water system, the polymer-rich phase vitrifled earlier than that in PES/NMP/water system. As a result, macrovoid growth in the former system was suppressed [26],... 

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. 

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. 

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. 

---