Fundamentals of Sorption and Transport Processes in Polymers

4 FUNDAMENTALS OF SORPTION AND TRANSPORT PROCESSES IN POLYMERS 20.4-1 General Discussion 

The following two subsections provide physical interpretations of the forms of sorption isotherms and concentration-dependent diffusion coefficients observed for robbery and glassy polymers, respectively. These sections are not required for simulation of module operations if a complrae set of emfurical pressure, temperature, and composition-dependent permeation data are available. A simple polynomial fit of permeability data as a function of all operatit variables would suffice for design arid simulation. 

There is some irtteraction boween componetts of the binary mixture during permeation which 

The fast gas was slowed down by die slow gas and the slow gas was speeded up by the fast gas.  

Silicone robber Silkone-polycatbonale block copolymer Silicone-niliite copolymer Natural robber SBR 

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This article focuses on transport that proceeds by the solution-diffusion mechanism. Transport by this mechanism requires that the penetrant sorb into the polymer at a high activity interface, diffuse through the poljuner, and then desorb at a low activity interface. In contrast, the pore-flow mechanism transports penetrants hy convective flow through porous pol5uners and will not be described in this article. Detailed models exist for the solution and diffusion processes of the solution-diffusion mechanism. The differences in the sorption and transport properties of rubbery and glassy pol5uners are reviewed and discussed in terms of the fundamental differences between the intrinsic characteristics of these two types of polymers. 

The basic transport mechanism through a polymeric membrane is the solution diffusion as explained in Section 4.2.1. As noted, there is a fundamental difference in the sorption process of a rubbery polymer and a glassy polymer. Whereas sorption in a mbbery polymer follows Henry s law and is similar to penetrant sorption in low molecular weight liquids, the sorption in glassy polymers may be described by complex sorption isotherms related to unrelaxed volume locked into these materials when they are quenched below the glass transition temperature, Tg. The various sorption isotherms are illustrated in Figure 4.6 [47]. 

A substantial number and variety of models of gas transport in polymers have been proposed during the last 20-30 years, in view of the great practical and scientific importance of this process. Molecular-type models are potentially most useful, since they relate diffusion coefficients to fundamental physicochemical properties of the polymers and penetrant molecules, in conjunction with the pertinent molecular interactions. However, the molecular models proposed up to now are overly simplified and contain one or more adjustable parameters. Phenomenological models, such as the dual-mode sorption model and some free-volume models, are very useful for the correlation and comparison of experimental data. 

The transport of liquids, vapours, and gases though polymer blends and IPN s is of fundamental importance to a polymer scientist. The driving force behind the transport process is the concentration difference between the two polymer phases or the chemical potential of the penetrant in the phases separated by the membrane. The transport process involves the sorption, diffusion, and permeation of the penetrant into the polymer system. 

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