Membrane chemical valve

Size-selective separation Controlled permeation of membranes—chemical valve 14, 29, 56... 

Many polymer-polymer complexes can be obtained by template polymerization. Applications of polyelectrolyte complexes are in membranes, battery separators, biomedical materials, etc. It can be predicted that the potential application of template polymerization products is in obtaining membranes with a better ordered structure than it is possible to obtain by mixing the components. The examples of such membranes from crosslinked polyCethylene glycol) and polyCacrylic acid) were described by Nishi and Kotaka. The membranes can be used as so-called chemical valves for medical applications. The membranes are permeable or impermeable for bioactive substances, depending on pH. 

Osada et al. [52] have also grafted PMAA to a porous substrate in order to achieve an environmentally sensitive membrane. This sensitivity was termed a chemical valve function because mechanochemical forces caused the pores to enlarge and contract. PMAA was grafted onto poly(vinyl alcohol) (PVA) films which had a mean pore radius of 4 pm. The water permeation of the membrane was strongly affected by the conformational state of the PMAA grafts. At low pH the chains were contracted and the water permeability was... 

When supramolecular polymers are treated with bulky stopper groups, they may form poly[2]rotaxane daisy chains [32,60-68]. Cyclic tri[2]rotaxanes (daisy chain necklace) containing cyclodextrins have been prepared from the mixture of 6-(4-aminocinnamoyl)-Q -CD and 2,4,6-trinitrobenzene sulfonic acid sodium salt [50,59] in an agueous solution (Fig. 21). If the molecule changes its conformation (or co-conformation), the ring may expand or shrink by external conditions (temperature, solvents, photochemically, elec-trochemically). These compounds are important because the cycle can be used as a chemical valve as seen in ion channels in biological membranes. 

FIGURE 25.27 Schematic representation of the Oj profile generated in a tubular reactor for the partial oxidation of alkanes (a) by a classical inert uniform porous membrane, and (b) by the suggested chemical valve membrane. (From Julbe, A., Farrusseng, D., and Guizard, C /. Membr. ScL, 181, 3, 2001.)... 

The results obtained with single gases confirmed that the membrane behaves as a chemical valve its permeance is higher when it is reduced and lower when it is oxidized. The ratio between the permeances of pure n-C4Hio and pure O2 is about 70 at 500°C (Figure 25.28). 

Figure 5 Schematic representation of a chemical valve, (a) swollen poly(acrylic acid) blocks insulin transport, (b) glucose oxidation causes a decrease in pH that results in deswelling and insulin transport across the membrane (reprinted from ref. 61, with permission from Elsevier Science)...

Julbe A, Farmsseng D, Cot D, and Guizard C. The chemical valve membrane a new concept for an auto-regulation of O2 distribution in membrane reactors. Catal. Today 2001 67 139-149. 

It must be recognized that these channels are not empty but have poly(acrylic acid) (PAA) chains tethered to their inner walls. However, since the relative volume occupied by these chains is significantly lower than the parent PtBA, these channels are rendered substantially porous. A very interesting feature of these porous membranes is the chemical valving effect , wherein the relative permeability of the membranes was shown to vary by almost two orders of magnitude as a function of pH, with the lowest permeability being witnessed at a pH of 3 [41]. The explana-... 

In another study poly(acrylic acid-g-styrene) copolymers were also shown to have good emulsifying ability and high water absorbency [105]. Membranes produced from intermolecular complexes of the above materials with poly(ethylacry-late-g-ethylene oxide) copolymers behaved like chemical valves, whose perme-ativity could be controlled reversibly by changing the pH of the surrounding medium, since both graft copolymers behave as polyelectrolytes in aqueous solution. 

Figure 2.16. Top, O2 profiles generated in a membrane reactor for the partial oxidation of alkanes, (a) conventional MR (b) a chemical valve MR. Bottom, Evolution of permeance of the chemical valve membrane as a function of the ratio C3H8/O2 without any catalyst.

Figure 16 is a schematic representation of an electrically activated chemical valve membrane whose pore size expands and contracts reversibly in response to an electrical stimulus. When the chemomechanical contraction is developed iso-metrically, i.e., keeping the membrane dimensions constant, the contractile stress... 

FIGURE 16 Apparatus for electrodriven chemical valve membrane and the change in permeability of a polyacrylic acid-polyvinyl alcohol composite membrane as the DC source is switched on and off. (From Ref. 46.)... 

Figure 16 also shows the effect of a chemomechanical contraction of the PVA-PPA membrane on water permeation when 6.5 V DC was applied in alternate on and off cycles [46]. It can be seen that the chemical valve membrane can increase and decrease the water permeability many times on electrical stimulation. Water permeability increased in proportion to the DC current. This makes it possible to use the membrane as a permeation-selective membrane continuously separating solute mixtures with different molecular sizes. This type of electrically activated chemical valve membrane exhibited long-term stability. 

Controlled Porosity, Chemical Valve . Environmentally controlled change in macromolecular size from a compact hydrophobic globule to an expanded hydrophilic coil is exploited when smart pol5nners are used in systems of environmentally controlled porosity, so-called chemical valves. When a smart polymer is grafted to the surface of the pores in a porous membrane or chromatographic matrix, the transition in the macromolecule affects the total free volume of the pores available for the solvent and hence presents a means to regulate the porosity of the system (see Membrane Technology). 

Fig. 6. Schematic of a chemical valve. Glucose oxidase is immobilized on a pH-responsi ve polyacrylic acid grafted onto a porous polycarbonate membrane (a) poly(acrylic acid) is in an expanded conformation that blocks insulin transport (b) the oxidation of glucose is accompanied by a decrease in pH and the transition of poly(acrylic cid) into a compact conformation that results in opening of the pores and transport of insulin. Redrawn from Ref. 82.

Figure 4.28. Schematic illustration of a "chemical valve" membrane prepared with graft polymer on a porous support [ 121 ].

Thus, the chemomechanical PEG treated PMAA membrane can behave as a chemical valve , expanding and contracting the pore size. Flow tests with hemoglobin and albumin solutions were conducted on this chemical membrane. Figure 4.34 shows that both... 

PMAA in water It is possible that the contraction of the PMAA membranes is not only related to the stability constant of the complexes, and to the extent of viscosity drop in solution, but also to a certain structural complementarity between the dissolved polymer and the membrane. If this is true, the contraction of the PMAA membranes should be discussed in terms of size and geometry of the polymers in solution and the chemical structure, both of which should influence the rate of penetration (penetration mechanism). The modest contraction observed with PVPdn, PVA, and PMVE seems to support this assumption. The penetration mechanism of the membrane contraction was proved experimentally and will be described in more detail in the paragraph of chemical valves (Section 3.4.1). 

Fig. 20 a and b. Schematic representation of chemo-mechanical contraction of a PMAA membrane by polymer-membrane complexation ( chemical valve ) a) isotonic contraction, b) isometric contraction... 

Another ty of chemomechanical membrane having chemical valve function has a composite structure. It consists of a porous substrate onto which methacrylic acid (MAA) has been graft-polymerized by the method of plasma-initiated polymerization The principle of the chemomechanical expansion and contraction of... 

Fig. 24a-d. Schematic illustration of a chemical valve membrane prepared with graft copolymer (a-d see text)... 

Fig. 30. Apparatus for the electro-activated chemical valve membrane and change of the water permeability by alternative on and off of an electric field Electric field 2.6 V/cm. The membrane was prepared by polymerization of AMPS in the presence of a porous (average pore size 8 pm) polyfvinyl alcohol) sheet...

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