Non-solvent induced phase separation

Removal of the solvent through freeze-drying or extraction can produce porous polymer scaffolds. Phase separation can be induced by changing the temperature or by adding non-solvent to the polymer solution. These are known as thermally induced and non-solvent-induced phase separation, respectively. The scaffolds obtained by... 

TCA removal was also studied using SBS asymmetric membranes prepared by non-solvent induced phase separation (NIPS), as reported in the US patent by Sikdar et al. (2008). In this work, the temperature and vacuum pressure were varied while the feed concentration ( 120 ppm) was kept constant. TCA, water and total flux increased with increasing the temperature (from 10°C to 35°C) at the constant pressure of 0.05 bar. At a permeate pressure of 0.05 bar and temperature of 34°C, TCA flux was about 0.018 kg m i. Separation factor ((Ztca/Hjo) increased from 900 to 4600 at higher temperature (from 10°C to 35°C) and from 640 to 2300 changing the permeate pressure from 0.01 to 0.05 bar. 

S. BonyadL T.S. Chung, W.B. Krantz, Investigation of corrugation phenomenon in the inner contour of hoUow fibers during the non-solvent induced phase-separation process. Journal of Membrane Science, 299 (2007) 200-210. 

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. 

We have demonstrated miscible blends of PET-PHB/PEI can be formed by rapid solvent casting from the mixed solvent of phenol and tetrachloroethane. The miscibility was confirmed by the systematic movement of Tg in the DSC studies. However, the blend is unstable and undergoes thermally induced phase separation with a miscibility window reminiscent of LCST. The dynamics of spinodal decomposition is non-linear in character and obeys the power law with kinetic exponents of -1/3 and 1 in accordance with the cluster dynamics of Binder and Stauffer as well as of Furukawa. In the temporal scaling analysis, the structure function exhibits universality with time, suggesting temporal self-similarity of the system. 

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. 

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. 

A more convenient experimental method is to induce phase separation by the addition of a non-solvent to a dilute solution of the polymer. The temperatures of the solution and non-solvent are maintained at the same value. Non-solvent is added to the solution until precipitation is just observed. After allowing a period for equilibrium to be achieved, the precipitate is... 

Figures 12.1.22 and 12.1.23 explain technical principles behind formation of efficient and selective membrane. Figure 12.1.22 shows a micrograph of hollow PEI fiber produced from N-methyl-2-pyrrolidone, NMP, which has thin surface layer and uniform pores and Figure 12.1.23 shows the same fiber obtained from a solution in dimethylformamide, DMF, which has a thick surface layer and less uniform pores. The effect depends on the interaction of polar and non-polar components. The compatibility of components was estimated based on their Hansen s solubility parameter difference. The compatibility increases as the solubility parameter difference decreases. Adjusting temperature is another method of control because the Hansen s solubility parameter decreases as the temperature increases. A procedure was developed to determine precipitation values by titration with non-solvent to a cloud point. Use of this procedure aids in selecting a suitable non-solvent for a given polymer/solvent system. Figure 12.1.24 shows the results from this method. Successfid in membrane production by either non-solvent inversion or thermally-induced phase separation requires careful analysis of the compatibilities between polymer and solvent, polymer and non-solvent, and solvent and non-solvent. Also the processing regime, which includes temperature control, removal of volatile components, uniformity of solvent replacement must be carefully controlled.

Vapour induced phase separation (VIPS) - in this process, the polymer solution is exposed to an atmosphere containing a non-solvent, which is absorbed and causes precipitation of the polymer. 

For simple coacervation induced by non-solvent addition in aqueous systems, ethanol, acetone, dioxane, isopropanol, and propanol are the most preferred to cause polymer desolvation and phase separation. In organic systems, mainly non-polar solvents. 

Genf, L. Demirel, M. Giiler, E. Hegazy, N. Micro-encapsulation of ketorolac tromethamine by means of a coacervation-phase separation technique induced by addition of non-solvent. J. Microencapsulation 1998, 15 (1), 45-53. 

This process involves only one polymer (e.g., gelatin, polyvinyl alcohol, carboxymethyl cellulose), and the phase separation can be induced by conditions that result in desolvation (or dehydration) of the polymer phase. These conditions include addition of a water-miscible non-solvent, such as ethanol, acetone, dioxane, isopropanol, or propanol,addition of inorganic salts, such as sodium sulfate,and temperature change. ... 

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