Diffusion-induced phase separation

Bulte AMW, Mulder MHV, Smolders CA, and Strathmann H. Diffusion induced phase separation with crystallizable nylons. Mass transfer processes for nylon 4,6. J. Membr. Sci. 1996 121 37 9. 

Bulte AMW, FoUters B, Mulder MHV, and Smolders CA. Membranes of semicrystalUne aliphatic polyamide nylon 4,6 Formation by diffusion-induced phase separation. J. Appl. Polym. Sci. 1993 50(l) 13-26. 

The Loeb-Sourirajan process often is referred to as diffusion induced phase separation (DIPS) to reflect the role of diffusion in forming the asymmetric structure. Liquid-liquid phase separation and the resulting asymmetric structure arise from diffusion of a solvent (acetone) out of the film and diffusion of a nonsolvent (water) into the film. This physical interpretation provided the basis for the development of asymmetric membrane manufacturing processes for other polymer - solvent - non-solvent systems. 

Asymmetric membrane structures have been created from these materials using the diffusion induced phase separation process (DIPS) as well as a thermally induced phase separation process (TIPS) [23] that relies on temperature gradients to produce a gradient in phase separated domain size. Moreover, membranes formed by either process can be further modified by stretching or drawing to alter pore size and porosity. 

Ion 4,6 Formation by diffusion-induced phase separation. Journal of Applied Polymer Science 50 (1993) 13. 

CHE2 Cheng, L.-P., Lin, D.-J., Shih, C.-H., Dwan, A.-H., and Gryte, C.C., PVDF membrane formation by diffusion-induced phase separation, J. Polym. Sci. Part B Polym. Phys., 37, 2079, 1999. 

Non-solvent induced phase separation (NIPS), or diffusion-induced phase separation, involves dissolution of the polymer in a good solvent in order to obtain a homogeneous solution, followed by the addition of a non-solvent miscible with the first solvent. This will cause precipitation of the polymer when the non-solvent concentration becomes significant. 

Since increasing temperature leads to increased fluidity and thus to a faster probe diffusion, pyrene Hpids have been frequently used to study phase transition in membranes [161,162]. Phospholipid phase separation increases the local concentration of dye labeled Hpids and can, therefore, be investigated via the characterization of exdmer formation. The binding of proteins or ions, however, may induce phase separation as well as decreasing lateral lipid diffusion. Since these two effects are opposing in terms of excimer formation, the binding of such proteins or ions cannot be studied by the (Ex/Mo)-ratio. The time-resolved analysis of the monomer fluorescence of the labeled lipid, however, allows for the separation of... 

In the bulk phase-separation approach, an organic solution of a polymer dissolved in a water-miscible solvent is injected into the tissue defect. After injection, the solvent diffuses away from the injection site, resulting in precipitation of the water-insoluble polymer. Selection of an appropriate solvent, which must be non-cytotoxic and not harmful to host tissue, is a key factor for success of the bulk phase-separation system. Two solvents that meet these criteria are N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). In recent years, improved strategies for removal of the solvent and release of growth factors have been active areas of investigation. However, the requirement of a solvent to induce phase separation of the polymer limits the scale at which this approach can be applied in vivo. Even for relatively biocompatible solvents such as NMP and DMSO, injection of large volumes is anticipated to adversely affect host tissue, as well as the ability to eliminate the solvent from the body. 

Phase inversion is known to be an effective way to create porous structures in membranes, where a competitive mutual diffusion between solvent and nonsolvent occurs to yield the porous structure. Phase inversion can be described as a demixing process whereby the initially homogeneous polymer solution is transformed in a controlled manner from a liquid to a solid state [24]. Apart from immersion in a nonsolvent bath, or immersion precipitation (IP), a variety of related techniques, such as precipitation by solvent evaporation, precipitation by absorption of water Irom the vapor phase, and precipitation by air cooling, corresponding to thermally induced phase separation (TIPS), vapor-induced phase separation (VIPS), and air-casting phase separation... 

The coupling between the stress field and the diffusion was first noticed and studied by Brochard and de Gennes [5] for polymer solutions far from the critical point. Relating to this, shear effects on complex fluids have recently attracted much attention because of its unusual nature known as Reynolds effect for example, a shear flow that intuitively helps the mixing of components induces phase separation in polymer solutions [6]. There are many examples of such effects in complex fluids such as surfactant systems, block copolymers, and charged colloidal systems. This is caused by the coupling between shear velocity fields and the elastic internal degrees of freedom of complex fluids. To explain this unique feature of polymer solution, there have been considerable theoretical efforts [5, 7-12]. 

One method used to modify the stracmre of chitosan to yield a porous scaffold architecture is thermally induced phase separation, which precipitates the polymer and ice crystals [79]. The ice crystals are subsequently removed under vacuum, which leaves a macroporous stracmre suitable as a scaffold for electrode construction. The porous stracmre is amenable to designs for MET and DET processes soluble mediators can freely diffuse within the macroporous matrix and the open stracmre can accommodate additional conductive nanomaterials. 

The previous section presented a model study of the coagulation of a polymer solution brought upon by diffusion of a nonsolvent. The present section focuses on the faster process of phase separation inside a binary polymer-solvent system induced by an abrupt change in external temperature and/or pressure. This problem is of obvious importance in polymer processing techniques based on thermally induced phase separation (31) or on rapid expansion of supercritical solutions (32). A typical example of the latter is the flash spinning process. Here, a... 

Bulk addition of nanoparticles refers to the use of nanoparticles as additives during the membrane synthesis process by phase inversion (Kim and Van der Braggen 2010). Nanoparticles are dispersed in the polymer solution, which is then cast on a support layer and contacted with a nonsolvent, in the case of diffusion- or nonsolvent-induced phase separation (DIPS or NIPS). In the eventual membrane, the nanoparticles are present in the inner structure of the membrane, and not exclusively on the membrane surface. Therefore, the functionalities of the nanoparticles that were used can only be partly exploited. For example, catalytic activities would not be efficient, as the nanoparticles that should act as catalyst are shielded by the polymer material. This is a concern particularly for photocatalytic materials such as Ti02, which are often employed for mixed matrix membranes. 

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