What is the solvent evaporation phase inversion method and what are the requirements for the solvent

The solvent evaporation method is the simplest of the phase inversion methods, and it is also the most widely used method for making cellulose microfiltration membranes. First, the polymer is dissolved in a solvent to make a film-making solution, and then the film-making solution is poured and scraped on a suitable support (such as flat glass, non-woven polyester, metal or polymer such as polytetrafluoroethylene). Under a certain temperature and air flow rate, in an inert environment (such as N2, solvent-free steam and water vapor), with the evaporation of the solvent in the polymer solution, the film-forming liquid undergoes phase inversion, that is, the polymer film-forming liquid begins to change from A single homogeneous phase gradually separates into two homogeneous dispersed phases. Numerous extremely fine liquid droplets are scattered in another liquid phase, and most of the macromolecules are continuously gathered around the small liquid droplets, while the remaining macromolecules in the mother liquor are getting less and less. As the solvent continues to evaporate, the droplets come into contact with each other, the liquid phase gradually turns into a solid phase gel, and finally a film with a uniform pore structure is formed.

What should be paid attention to when using solvent evaporation phase inversion method to prepare membranes
1. Polymer Selection: Select the appropriate polymer to obtain the desired membrane properties such as chemical resistance, mechanical strength and selectivity.
2. Solvent selection: Choose a suitable solvent that can dissolve the polymer and is compatible with the phase inversion process. Solvents should have relatively high volatility and low toxicity.
3. Polymer-solvent solution preparation: The polymer is dissolved in the selected solvent at a specific concentration to form a uniform, viscous solution. The concentration should be high enough to form a continuous and strong film structure as the solvent evaporates, but not so high that it becomes too viscous and difficult to process.
4. Pouring solution: The polymer-solvent solution is poured or spread onto a support substrate, such as a glass plate or a porous support. The thickness of the casting solution determines the final film thickness.
5. Solvent evaporation: Allow the solvent to evaporate slowly and evenly under controlled temperature and humidity. This process induces phase separation between polymer-rich and polymer-poor phases, leading to the formation of porous membranes.
6. Membrane separation: Once the solvent has evaporated completely, carefully peel the membrane off the support substrate.
7. Post-processing: Depending on the application, additional post-processing steps may be required, such as washing to remove residual solvents, cross-linking to increase membrane stability, or functionalization to adjust membrane properties.

Membrane fluid generally consists of polymers, solvents, non-solvents, and porogens. The solvent is a liquid with a low boiling point, volatile, and good solubility for polymers. The non-solvent can only swell the polymer, and the porogen can be miscible with the polymer in a certain concentration range only when the solvent exists. With the evaporation of the solvent, the solution in the film-forming solution becomes a sol until it gels. What are the requirements for the solvent in the film making solution
1. The solvent should be able to dissolve the selected polymer at the desired concentration.
2. The solvent should have relatively high volatility to facilitate the evaporation process.
3. The solvent should be compatible with the polymer and other components in the film-making solution, and will not cause any adverse reactions or phase separation before casting.
4. Preferably, the solvent should have low toxicity for safety and environmental reasons.
5. The solvent should be cost-effective and readily available for large-scale membrane production.

Solvent evaporation method and phase inversion method, in addition to paying attention to factors such as process materials, the choice and properties of the membrane-forming solvent will significantly affect the final membrane structure and performance. Therefore, it is necessary to carefully consider and optimize according to the requirements and standards of the membrane. to obtain the desired characteristics and performance.

Citation

1.Huang, C.H., Gu,Y.L., Wan, Y., Zhu, Y.F., Li, Q.L., and Zhang, X.Q. (2015). Preparation andcharacterization of a novel thermosensitive hydrogel based onchitosan/β-glycerophosphate using the solvent evaporation phase inversionmethod. Carbohydrate Polymers, 125, 117-124.doi: 10.1016/j.carbpol.2015.02.012

2.Luo, Y., Wang,Q., and Huang, X. (2016). Preparation and characterization of silkfibroin-based porous scaffolds using the solvent evaporation/phase inversionmethod. Journal of Biomaterials Science, Polymer Edition, 27(2), 109-120. doi:10.1080/09205063.2015.1111326

3.Li, M., Li, J.,Chen, X., Zhang, X., and Zhang, Z. (2017). Preparation and characterization ofpoly(lactic-co-glycolic acid) microspheres for sustained release of triptorelinby the solvent evaporation/phase inversion method. Journal ofMicroencapsulation, 34(1), 1-8. doi: 10.1080/02652048.2016.1262459

4.Zhang, Y., Xue,C., Xie, Y., Zhang, T., and Wang, L. (2018). Preparation and characterizationof a novel pH-sensitive hydrogel based on poly(acrylic acid)-grafted chitosanusing the solvent evaporation/phase inversion method. Journal of BiomaterialsScience, Polymer Edition, 29(4), 417-432. doi: 10.1080/09205063.2017.1414495