Solvent inversion method

For the solvent inversion method the whole block copolymer has to be completely dissolved in a solvent before polymersome formation is initiated. Once the solvent containing the dissolved polymer is poured into an excess of water, the hydrophobic block becomes insoluble and polymersome formation is induced. Here, the created vesicles are typically between 100 and 200nm in diameter. Besides solvent inversion, film rehydration also relies on dissolving the amphiphilic block copolymer in a solvent other than water, hi contrast to solvent inversion, the solvent is slowly evaporated during this method to produce a thin film of precipitated polymer at the wall of the jar used. Once the film is created, the jar is filled with water and the self-assembly starts from the precipitated polymer film. Eventually, polymersomes are formed and the film is totally removed. If the jar surface is chemically altered, vesicles of up to 20 pm can be achieved. Otherwise, film rehydration yields the same vesicle sizes as solvent inversioa... 

Separation processes such as ultrafiltration and micro filtration use porous membranes which allow the passage of molecules smaller than the membrane pore size. Ultrafiltration membranes have pore sizes from 0.001 to 0.1 )im while micro filtration membranes have pore sizes in the range of 0.02 to 10 im. The production of these membranes is almost exclusively based on non-solvent inversion method which has two essential steps the polymer is dissolved in a solvent, cast to form a film then the film is exposed to a non-solvent. Two factors determine the quality of the membrane pore size and selectivity. Selectivity is determined by how narrow the distribution of pore size is. In order to obtain membranes with good selectivity, one must control the non-solvent inversion process so that it inverts slowly. If it occurs too fast, it causes the formation of pores of different sizes which will be non-uniformly distributed. This can be prevented either by an introduction of a large number of nuclei, which are uniformly distributed in the polymer membrane or by the use of a solvent combination which regulates the rate of solvent replacement. 

Separation processes such as ultrafiltration and microfiltration use porous membranes which allow the passage of molecules smaller than the membrane pore size. Ultrafiltration membranes have pore sizes from 0.001 to 0.1 pm while microfiltration membranes have pore sizes in the range of 0.02 to 10 pm. The production of these membranes is almost exclusively based on non-solvent inversion method which has two essen-... 

Starch<—The inversion method described for flour (p. 63) is employed. From 5 to 10 grams of the substance are freed from fat by extraction with petroleum ether or other suitable solvent and from sugars (and dextrins) by treatment with 25% alcohol. The glucose found, multiplied by 0-9, gives the quantity of starch. 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

An integrally skinned asymmetric membrane with a porous skin layer (hereafter called substrate membrane) is prepared from a polymer solution by applying the dry-wet phase inversion method and dried according to the method described later, before being dipped into a bath containing a dilute solution of another polymer. When the membrane is taken out of the bath, a thin layer of coating solution is deposited on top of the substrate membrane. The solvent is then removed by evaporation, leaving a thin layer of the latter polymer on top of the substrate membrane. 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

The normal method of operation is to drop the oxo compound that is to be reduced into a solution of LiAlH4 in one of the above-mentioned solvents. When the compound is difficult to reduce or when there is a danger of overreduction, the inverse method has become customary, i.e.9 the LiAlH4 solution or suspension is dropped into the solution of the oxo compound. 

A solvent-free method was introduced by chemical vapor deposition (CVD) of isoprene in a nitrogen atmosphere at 800 °C. This method yielded, after dissolution of the templating silica colloids, a conducting inverse carbon opal [77]. A semiconductor replica made of silicon could also be produced by CVD [78]. 

In addition, PEEK-WC membranes have been prepared by using a phase inversion process with supercritical fluids. The supercritical fluid acts as a non-solvent. In comparison to the dry/wet phase inversion method, the supercritical fluid allows the cell size and the membrane morphology to modulate by changing the experimental conditions, such as polymer concentration, temperature, and pressure. A dry membrane can be obtained rapidly and without additional post-treatments. ... 

New cellulose manbranes were recently prepared by the phase inversion method using a green solvent, the ionic liquid [BMIM][C1] [32], After functionalization with a synthetic ligand 2-(3-aminophenol)-6-(4-amino-l-naphthol)-4-chloro-5-triazine, these adsorptive membranes were evaluated for human immunoglobulin G (IgG) adsorption. The authors envisage that a change in the conditions and chemistry for membrane activation with the biomimetic ligand may improve the performance of the affinity cellulose membranes. 

All methods discussed earher at some point require the exchange of solvent. However, if a pH-sensitive material is used, polymersome formation can be initiated by only changing the solvent conditions with regard to the pH of the solvent. A great advantage of this method over the previously mentioned ones is that the solution does not need to be cleaned afterward and no solvent exchange is necessary. While for solvent inversion, vesicles of about 100-200nm can be obtained by the pH switch method. 

Kawakami et al. prepared dense and asymmetric membranes from 2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and bis[4-(4-aminophen-oxy)phenyl]sulfone (APPS) by solvent evaporation (dense) and by the dry-wet phase inversion technique [47]. The surface morphology was studied by AFM. They reported that the solvent evaporation method adopted for the preparation of the dense membrane influenced the formation of nodules, while the dry-wet process in which solvent/nonsolvent exchange was involved determined the roughness of the skin layer. 

Dual layer hollow fiber membranes have become increasingly attractive due to which they can be fabricated in a single step using the non-solvent induced phase inversion method. A good lamination between the two layers as well as a regular morphology are critical to get a functional hollow fiber membrane [80]. 

IP is one of the techniques used for preparing composite membranes, normally for the production of NF and reverse osmosis (RO) membrane. Historically, thin-film composite membrane prepared by IP was developed in order to overcome the limitations and the problems encountered by an asymmetric membrane constructed by the phase inversion method (Rao et al. 1997). IP was a breakthrough in the history of membrane technology and was developed by Cadotte at the North Star Research Institute for RO applications (Pinnau and Freeman 2000). It is based on the polymerization that occurs between two reactive monomers at the interface of two immiscible solvents an aqueous phase and an organic solvent such as hexane, as shown in Figure 5.1. 

Khayet and Matsuura (2003a) and Khayet et al. (2005a,b) proposed a new type of composite hydrophobic/hydrophiUc manbrane for MD. They modified the top surface of the PEI flat-sheet membranes using fluorinated SMMs (Cheng and Wiersma 1982, 1983). The SMM-modified and the SMM-unmodified membranes were prepared by the phase inversion method from casting solutions containing the solvent DMAc and the nonsolvent y-butyrolactone (GBL). They concluded that the SMM-modified PEI membranes have the potential to be used in MD. The performance of the new membranes was compared with two commercial PI PE manbranes. The authors results confirmed that the new membranes are promising for MD. 

Khayet and Matsuura (2004) studied the separation of chloroform-water mixture via PVDF flat membrane by using both PV and VMD techniques. Both PV and VMD membranes were prepared using the phase inversion method and the same polymer material. VMD membranes with different pore sizes were prepared using pure water as a pore-forming additive in the PVDF/dimethylacetamide (DMAC) casting solution, whereas PV membranes were obtained with higher polymer concentration, without nonsolvent additives (water) and with solvent evaporation before gelation. A comparative study was made between both. 

When using supercritical fluids in place of liquids (non-solvent),an advanced, and greener, manufacturing approach to induce precipitation of the polymer can be achieved. This approach is commonly named the supercritical assisted phase inversion method (SAPIM) (Cardea et al, 2010). 

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