Membranes spinodal decomposition

Membranes (Continued) liquid, see Liquid membranes morphology p os treatments spinodal decomposition pemieab ility... 

Kim et al. discussed surface structure and the phase separation mechaiusm of polysulfone membranes by AFM [43]. A membrane formed by immersion in a pure water coagulation bath showed a nodular structure with a nodule size of about 25 nm, which was believed to be the result of spinodal decomposition. A membrane formed by immersion in a coagulation bath mixture (water/NMP 20/80 by weight) had the porous structure with a mean radius of 146 nm, which was the result of nucleation and growth of the polymer-poor phase. 

Broadhead and Tresco studied the effects of fabrication conditions on the structures and performances of membranes formed from poly(acrylonitrile-vinylchloride) (PAN-PVC) by using the phase inversion process [85]. They reported the relationship of the fine-surface structure of PAN-PVC membranes to the membrane performance and membrane fabrication method. The fine-surface structure of nodular elements and the size of these elements could be altered by changing the precipitation conditions. Membranes were prepared at 22 on 55 mm diameter polished silicon wafers by spinning at 1500 rpm for 20 s with a spin coater [86]. The film was immediately precipitated in one of the four different precipitation media. The first three media consisted of deionized water at 4,22, and 54 °C. These membranes were referred to as Type 1 , Type 2 , and Type 3 , respectively. The fourth medium was a 50/50 mixture of deionized water and N,iV-dimethylformamide (DMF) at 54 °C and coded as Type 4 . Figure 4.53 shows the histograms of the nodule size distributions observed at the skinned surface of the membranes made under four different precipitation conditions. The sizes of these nodular elements became smaller and more uniform with milder precipitation conditions, which supports the theory that nodules are formed through spinodal decomposition under these conditions. In addition, the size of these nodules could be related to water permeability. Hence, water transport occurred through the interstitial spaces where the pores could be situated. 

Figure 1.4.36-1.4.38 shows SEM micrographs of the PMAA membranes formed via thermal inversion process starting with a 7%/3%/90% solution of PMAA/MAA/water at room temperature and quenched at 80°C. The micrographs show fibrous, interconnected network, with open pores, indicative of spinodal decomposition. As phase separation time increases to about 2 min, a cellular... 

Nunes, S.P. and T. Inoue. 1996. Evidence for Spinodal Decomposition and Nncleation and Growth Mechanisms dnring Membrane Formation. Journal of Membrane Science 111(1) 93-103. doi 10.1016/0376-7388(95)00281-2. 

On the other hand, when the solution enters region B, which is the unstable region, phase separation takes place without the presence of any nucleus. The solution separates into an interpenetrating network of polymer-rich and polymer-lean phases, as shown in Figure 15.6 (N). This mechanism is called spinodal decomposition. The nascent membrane has both interconnected polymer matrix and pores. 

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