Nonporous Polymer Membranes

Peppas and Reinhart have also proposed a model to describe the transport of solutes through highly swollen nonporous polymer membranes [155], In highly swollen networks, one may assume that the diffusional jump length of a solute molecule in the membrane is approximately the same as that in pure solvent. Their model relates the diffusion coefficient in the membrane to solute size as well as to structural parameters such as the degree of swelling and the molecular weight between crosslinks. The final form of the equation by Peppas and Reinhart is... 

The method of preparation also influences the properties of the film. Cast films of varying properties can be prepared by variation inter alia of the solvent power of the casting solution containing the polymer, although the complex processes involved in film formation are not yet fully understood. It is clear, however, that the conformation of the polymer chains in concentrated solution just prior to solvent evaporation will determine the density of the film, and the number and size of pores and voids. Dmg flux through dense (nonporous) polymer membranes is by diffusion flux through porous membranes will be by diffusion and by transport in solvent through pores in the film. With... 

The use of nonporous polymer membranes for liquid-liquid extraction suffers from very slow permeation of solute through the membrane, although this approach has been developed for a special case... 

POLYMER MEMBRANES. The transport of gases through dense (nonporous) polymer membranes occurs by a solution-diffusion mechanism. The gas dissolves in the polymer at the high-pressure side of the membranes, diffuses through the polymer phase, and desorbs or evaporates at the low-pressure side. The rate of mass transfer depends on the concentration gradient in the membrane, which is proportional to the pressure gradient across the membrane if the solubility is proportional to the pressure. Typical gradients for a binary mixture are shown in Fig. 26.2. Henry s law is assumed to apply for each gas, and equilibrium is assumed... 

However, all the gases used and the temperatures and pressures at which the referenced permeability measurements were made are listed. The references to gas permeabilities reported in this chapter are not comprehensive, but are representative of selected classes of homopolymers. References to some earlier gas permeabihty studies are included. This chapter also includes a brief review of the permeation mechanisms as well as a mention of the use of computer simulations of gas permeation in and through nonporous polymer membranes. 

It has been shown in a previous section that, in most cases of practical interest, the rate of gas permeation through nonporous polymer membranes is cOTitrolled by the diffusion of the penetrant gas in the polymer matrix. Many theoretical models have been proposed in the literature to describe the mechanisms of gas diffusion in polymers on a molecular level. Such models provide expressions for gas diffusion coefficients, and sometimes also for permeability coefficients, derived from free volume, statistical-mechanical, energetic, structural, or other considerations. The formulation of these coefficients is complicated by the fact that gas transport occurs by markedly different mechanisms in rubbery and glassy polymers. 

Gas transport in nonporous polymer membranes typically proceeds by a solution-diffusion mechanism in which the permeability (P) is given by. xD, where S and D denote the solubility and diffusivity of the permeating species, respectively. The solubility provides a measure of interaction between the polymer matrix and penetrant molecules, whereas the diffusivity describes molecule mobility, which is normally governed by the size of the penetrant molecule as it winds its way through the permanent and transient voids afforded by the free volume of the membrane [42], Therefore gas transport has to be strongly dependent on the amount of free volume in the polymer matrix. 

Yasuda, H. and Lamaze, C. E. 1972. Transfer of gas to dissolved oxygen in water via porous and nonporous polymer membranes. Journal of Applied Polymer Science, 16,595-601. 

Fiedler U, Ruzicka J (1973) Selectrode—the universal ion-selective electrode Part VII. A valinomycin-based potassium electrode with nonporous polymer membrane and solid-state inner reference system. Anal Chim Acta 67 179-193... 

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

Nonplant cost, 9 527 Nonpoint contamination source, 13 310 Nonpolar adsorbents, 1 674 for gas adsorption, 1 632 Nonpolar solvents, VDC polymer degradation in, 25 717-718 Nonporous dense membranes, 15 799 Nonporous silicone tubing, flow through, 15 722, 723... 

In an effort to optimize the solvent-containing passive sampler design, Zabik (1988) and Huckins (1988) evaluated the organic contaminant permeability and solvent compatibility of several candidate nonporous polymeric membranes (Huckins et al., 2002a). The membranes included LDPE, polypropylene (PP), polyvinyl chloride, polyacetate, and silicone, specifically medical grade silicone (silastic). Solvents used were hexane, ethyl acetate, dichloromethane, isooctane, etc. With the exception of silastic, membranes were <120- um thick. Because silicone has the greatest free volume of all the nonporous polymers, thicker membranes were used. Although there are a number of definitions of polymer free volume based on various mathematical treatments of the diffusion process, free volume can be viewed as the free space within the polymer matrix available for solute diffusion. 

Nonporous gel membranes - these membranes do not contain a porous structure and thus diffusion occurs through the space between the polymer chains (the mesh). Obviously in this case, molecular diffusion rather than convective transport is the dominant mechanism of diffusion in these membranes. 

Various theories have been proposed to describe the transport in all of these types of polymer membranes. Theories for macroporous and microporous membranes have been based on hydrodynamic and frictional considerations while those for nonporous gels have been based on Eyring s theory and use a free volume approach to describe the movement of solute through the mesh of the polymer. 

The theories developed for transport in microporous membranes cannot be applied to nonporous gel membranes. The pore structure in microporous membranes is not analogous to the mesh of the nonporous gels. Thus a different set of theories had to be developed for the treatment of nonporous polymer gel membranes. These theories are based on the idea of the existence of free volume in the macromolecular mesh. As a result, diifusion through nonporous membranes is said to occur through the space in the polymer gel not occupied by polymer chains. 

The first theory of transport through nonporous gels was presented by Yasuda et al. [150] and was proposed as a result of previous experimental results [151, 152]. This theory relates the ratio of diffusion coefficient in the polymer membrane and diffusion coefficient in the pure solvent to the volume fraction of solvent in the gel membrane or in Yasuda s terminology, the degree of hydration of the membrane, H (g water/g swollen polymer). Yasuda et al. use the... 

As seen, diffusion in nonporous gel membranes differs from that in macro-porous or microporous membranes. Various theories based on solute diffusion through the macromolecula r free volume in the membrane have been proposed. It is clear from these theories that structural parameters of the polymer network such as degree of swelling, molecular weight between crosslinks, and crystallinity in addition to factors such as solute size and solvent free volume play important roles in this type of transport. 

A polymer membrane (nonporous polypropylene) to separate an aqueous sample from an organic extractant... 

Ion exchange membranes (lEMs) have recently been studied as a means for overcoming the above problems (A). The carrier is the counter ion in the lEM. The carrier is bound in the membrane by ionic charge forces. The lEM is a nonporous polymer which is swelled by the solvent. Because the lEM is nonporous, no "short circuiting" occurs if the membrane loses solvent. The carrier also remains bound in the membrane. The membrane can be resolvated and continue performing without a loss in capacity... 

Immobilized Liquid Membranes. Facilitated transport liquid membranes for gas separations can be prepared In several configurations. The complexatlon agent solution can be held between two nonporous polymer films (2j1), Impregnated Into the pore structure of a micro-porous polymer film (25), or the carrier can be exchanged for the counterion In an Ion exchange membrane (it). 

Abe A, Aibertsson AC, Dusek K, Jeu WH, Kausch HH, Kobayashi S, Lee KS, Leibler L, Long TE, Manners I, Mdlier M, Nuyken O, Terentjev EM, Voit B, Wegner G, Wiesner U, Vicent MI (eds) (1985) Polymer membranes (Advances in Polymer Science). SpiingCT Verlag Baker RW (2004) Membrane technology and applications. Wiley, New York Sammells AF et al (eds) (2006) Nonporous inorganic membranes for chemical processing. Wiley VCH, Weinheim... 

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