Rubbery polymers transport properties

At such extraordinarily low penetrant concentrations, plasticization of the overall matrix is certainly not anticipated. Motions involving relatively few repeat units are believed to give rise to most short term glassy state properties. In rubbery polymers, on the other hand, longer chain concerted motions occur over relatively short time scales, and one expects plasticization to be easier to induce in these materials. Interestingly, no known transport studies in rubbers have indicated plasticization at the low sorption levels noted above for PVC and PET. 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

Several detailed analyses of the diffusion process in both rubbery polymers and in hindered glasses are offered in Chapter 2 (28). Approximate molecular interpretations have been offered for the parameters in these models (25). Nevertheless, more work is needed to verify any molecular scale connection between such parameters and the structures and motions of the polymer backbone. Spectroscopy and molecular modeling of the differences in segmental motions in a systematically varied family of polymers, e.g, the polyesters, or polyamides, can offer insight in some cases. Unfortunately, the exact segmental motions involved in the diffusive process are only partially understood, so one must be cautious about drawing conclusions based on such studies unless they are supported by actual complementary transport data. Hopefully the structure-property results presented in this book will further stimulate thinking to improve the connection between spectroscopically sensed motions, and diffusion to complement the correlations based on specific free volume in Chapters 5 S 7 (50,51). ... 

Although metallic and ceramic materials are used as membranes, polymeric materials account for the vast majority of commercial products. Polymer selection depends on a number of factors including intrinsic transport properties, mechanical properties, thermal stability, chemical stability (e.g., chemical resistance and biocompatibility), membrane manufacturability, cost, and patentability. The two most common types of polymers are glassy engineering thermoplastics and rubbery polysiloxanes. 

White [25] investigated the transport properties of a series of asymmetric poly-imide OSN membranes with normal and branched alkanes, and aromatic compounds. His experimental results were consistent with the solution-diffusion model presented in [35]. Since polyimides are reported to swell by less than 15%, and usually considerably less, in common solvents this simple solution-diffusion model is appropriate. However, the solution-diffusion model assumes a discontinuity in pressure profile at the downstream side of the separating layer. When the separating layer is not a rubbery polymer coated onto a support material, but is a dense top layer formed by phase inversion, as in the polyi-mide membranes reported by White, it is not clear where this discontinuity is located, or whether it wiU actually exist The fact that the model is based on an abstract representation of the membrane that may not correspond well to the physical reality should be borne in mind when using either modelling approach. 

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 transport properties of glassy and rubbery polymers are related to their microstructural morphology. For a penetrant to diffuse, a minimum characteristic packet of imoccupied volume is required. The penetrant diffuses by jumping through transient gaps between packets of unoccupied volume. The lifetime, size, and shape of these volume packets and the transient gaps that connect them are dependent upon the micromotions of the polymeric media. New techniques such as... 

For cases involving a random copolymer or a miscible blend of two amorphous rubbery polymers, the behavior is generally a volume fraction weighted average of the permeabilities of the two homopolymers. On the other hand, the transport properties of immiscible blend systems depend significantly on the relative permeabilities and the morphology of the immiscible blend. 

Oriontation-Induced Effects. Orientation and combined heat and orientation processing affect the transport properties of glassy polymers. Especially when crystallites are present, the effects can become surprisingly large. As noted for rubbery semicrystalline materials, the obvious improvements in barrier properties associated with organization of lamellar crystalline domains with their platelets perpendicular to the direction of penetrant flow can produce significant... 

The experimental results are briefly discussed in terms of thermodynamic and mass transport properties in the glassy polymer mixture. The aim of the discussion is to highlight peculiarities of solubility and difiusivity in polymeric systems below the glass transition temperature and to consider possible interpretations. The focus is on the effect of swelling on the thermodynamic and transport properties in glasses. Indeed, it is well known that, contrary to the case of rubbery systems, the solute partial specific... 

In the past 25 years, relatively few attempts to increase gas separation membrane performance with dense film mixed matrices of zeolite and rubbery or glassy polymer have been reported. Table I summarizes practically all of the reported O2/N2 mixed matrix membranes. Permeabilities and permselectivities are specified as a range to encompass the various zeolite volume fractions studied. In general, an increase in permeability is observed with zeolite addition coupled with a slight increase in permselectivity. Despite the wide variety of combinations of zeolites with rubbery and glassy polymers, reported mixed matrix membranes fail to exhibit the desired O2/N2 performance increases. These failures have generally been attributed to defects between the matrix and molecular sieve domains. While this is certainly a possible practical source of failure, our work earlier 8) has addressed a more fundamental source caused by inattention to matching the transport properties of the molecular sieve and polymer matrix domains. This topic is discussed briefly prior to consideration of the practical defect issue noted above. 

PolydimethylsUoxane (PDMS) embedded with zeolite particles displayed permeation improvements compared to the original polymer, but only when zeolite loadings of 40 wt% or larger are implemented.Certain target separations, such as n-pentane/ i-pentane, did not improve relative to neat PDMS permeation properties. Fundamental transport of gas molecules through solid-rubbery polymer systems has been studied using zeolite 5A-silicone mbber membranes. ... 

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