Immersion precipitation technique

In this experimental section, the production of polysulfone microcapsules containing vanillin is described. Our method is based on the phase inversion by immersion precipitation technique. The herein described process has been successfully employed for the encapsulation of vanillin into polysulfone microcapsules and, in addition, vanillin release from those capsules has been characterized. Polysulfone/vanillin microcapsules could have an application in laundry industry and also in medical applications. ... 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

There are a number of different techniques belonging to the category of phase inversion solvent evaporation, precipitation by controlled evaporation, precipitation from the vapor phase, thermal precipitation, and immersion precipitation (13,34—36). The most commercially available membranes are prepared by the last method. 

In the precipitation technique, the prepol3nner was stirred for 0.5 hr. in cyclohexane. MMA was added to this solution or highly swollen slurry, and immersed in a preheated bath. The product was isolated as given above. 

By far the most used technique for membrane preparation is the immersion precipitation method (nonsolvent-induced phase separation). A homogenous polymer solution is cast as thin him and subsequently immersed into a nonsolvent bath, typically water or mixtures of water and solvent. The dif-fusional exchange of solvent and nonsolvent brings the him solution into an instable state resulting in phase separation, either by liquid-liquid (l-l) and/or solid-liquid (s-l) demixing, depending on the type of polymer and the precipitation conditions employed [92,93]. 

Most of the membranes in use today are phase inversion membranes obtained by immersion precipitation. Phase inversion membranes can be prepared ftom a wide variety of polymers. The only requirement is that the polymer must be soluble in a solvent or a solvent mixture. In general the choice of polymer does not limit the preparation technique. 

Membrane formalion by phase inversion techniques, e.g. immersion precipitation, is a non-equilibrium process which cannot be described by thermodynamics alone since kinetics have also to be considered. The composition of any point in the ca.st film is a function of place and time. In order to know what type of demixing process occurs and how it occurs, it is necessary to know the exact local composition at a given instant. However, this composition cannot be determined very accurately experimentally because the change in composition occurs extremely quickly (in often less than 1 second) and the film is very tto (less than 200 itm). However it can b e described theoretically. 

Immersion precipitation is one of the most important techniques to prepare phase inversion membranes. During this process demixing can occur instantaneously or delayed. 

Asymmetric membranes are mainly prepared by immersion precipitation whereas this technique is also used for the sublayer in composite membrane upon which a very thin selective layer is deposited by one of the following techniques ... 

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... 

All kinds of synthetic materials can be used for preparing membranes. The basic principle involved is to modify the material by means of an appropriate technique in such a way so as to obtain a membrane structure with morphology suitable for a specific (class) of separation or application. The choice of the material limits the preparation techniques employed, the membrane morphology obtained and the separation principle allowed. Several techniques are frequently applied to produce tissue engineering scaffolds, e.g., liquid-induced phase separation (LIPS), immersion precipitation. 

Among the four types of phase inversion processes, immersion precipitation remains by far the most important membrane preparation technique, both for dense and porous membranes. This technique is also mostly used to prepare PVDF membranes for MD application. Due to the importance of this technique, this section covers it in detail. 

Phase separation membranes. This category includes membranes made by the Loeb-Sourirajan technique involving precipitation of a casting solution by immersion in a nonsolvent (water) bath. Also covered are a variety of related techniques such as precipitation by solvent evaporation, precipitation by absorption of water from the vapor phase, and precipitation by cooling. 

Precipitation of the cast liquid polymer solution to form the anisotropic membrane can be achieved in several ways, as summarized in Table 3.1. Precipitation by immersion in a bath of water was the technique discovered by Loeb and Souri-rajan, but precipitation can also be caused by absorption of water from a humid atmosphere. A third method is to cast the film as a hot solution. As the cast film cools, a point is reached at which precipitation occurs to form a microporous structure this method is called thermal gelation. Finally, evaporation of one of the solvents in the casting solution can be used to cause precipitation. In this technique the casting solution consists of a polymer dissolved in a mixture of a volatile good solvent and a less volatile nonsolvent (typically water or alcohol). When a film of the solution is cast and allowed to evaporate, the volatile good solvent evaporates first, the film then becomes enriched in the nonvolatile nonsolvent, and finally precipitates. Many combinations of these processes have also been developed. For example, a cast film placed in a humid atmosphere can precipitate partly because of water vapor absorption but also because of evaporation of one of the more volatile components. 

The first phase separation membrane was developed at UCLA from 1958 to 1960 by Sidney Loeb, then working on his Master s degree, and Srinivasa Sourirajan, then a post-doctoral researcher. In their process, now called the Loeb-Sourirajan technique, precipitation is induced by immersing the cast film of polymer solution... 

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