Gas transport and Sorption

Monitoring fast pressure changes in gas transport and sorption analysis... 

Reichenauer G, Fella H J, Fricke J (2002) Monitoring fast pressure changes in gas transport and sorption analysis. Characterization of Porous Solids VI 144 443 49... 

G. Reichenauer, H.J. Fella, J. Fricke, Monitoring Fast Pressure Changes in Gas Transport and Sorption Analysis, Proceedings of COPS IV, Studies in Surface and Catalysis 144, F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Eds.), Elsevier Science B.V. (2002), p. 443. 

This review does not aim for an encyclopedic coverage of the rapidly expanding field of membrane science. The topics treated do, however, present a reasonable sampling of the current literature on membrane science. The subfects treated reflect directly the interests of the authors and many of the topics are fundamentally related to the fact that the membrane is a high polymer. Indeed, the transport and sorption theories presented provide parameters whose physical interpretation often is a useful complement to other methods of physical analysis of the polymeric state. The study of gas sorption and transport in rubbery polymers is petbaps the simplest example of such a case and will be treated first. 

An example in which it is possible to make reliable estimates of performance is aplastic carbonated beverage package. Since carbon dioxide is the primary gas of interest it is possible to apply a straightforward one-dimensional model for a soda can (25). Further, the transport and sorption models for carbon dioxide and most plastics have received considerable attention in the literature. 

The fit of these expressions to experimental results is very good. At low pressure regimes, the fit was shown to be even better than that of dual sorption expressions. Except for these regimes, the two models seem to do equally well in describing sorption and permeability data. Concentration dependent diffusivity and permeability have been considered before mainly for vapors. The new aspect of the matrix model is that it broadens these effects to fixed gases. The important difference between the matrix and dual sorption models is in the physical picture they convey of gas transport and interaction with the polymer. Additional experimental evidence will be needed to determine the preference of these different physical representations. 

The objects of the investigation by Jansen et al. (Chapter 4) were perfiuorinated copolymers of Hyflon AD. The authors reported novel data on free volume, presented the results of computer modelling and the gas permeation parameters. It should be stressed that such comprehensive smdy of a polymer becomes more and more popular today if one wants to understand transport and sorption parameters of a membrane material. This chapter will give much food for future comparisons with other perfiuorinated polymers as well as conventional glassy polymers. 

In Section IB we presented experimental evidence that diffusion coefficients correlate with PVC main-chain polymer motions. This relationship has also been justified theoretically (12). In the previous section we demonstrated that the presence of CO2 effects the cooperative main-chain motions of the polymer. The increase in with increasing gas concentration means that the real diffusion coefficient [D in eq. (11)] must also increase with concentration. The nmr results reflect the real diffusion coefficients, since the gas concentration is uniform throughout the polymer sample under the static gas pressures and equilibrium conditions of the nmr measurements. Unfortunately, the real diffusion coefficient, the diffusion coefficient in the absence of a concentration gradient, cannot be determined from classical sorption and transport data without the aid of a transport model. Without prejustice to any particular model, we can only use the relative change in the real diffusion coefficient to indicate the relative change in the apparent diffusion coefficient. 

These membranes are similar to the simple sorption-diffusion membranes, but involve some additional phenomena as well as simple penetrant dissolution and diffusion. Two types can be identified (i) facilitated transport for various gas types, and (ii) palladium and related alloys for hydrogen. 

Due to the limited response time of suitable sensors fast sorption or gas transport processes on a time scale below a second are hard to monitor. To significantly improve the resolution in time an interferometric pressure sensor can be applied. The central part of the interferometric pressure sensor presented is a Michelson-interferometer this set-up is sensitive to changes in gas pressure as the index of refraction, and thus the optical path length for a laser beam within the interferometer, is a function of the gas density. 

Every sorption set-up also allows for the extraction of information about sorption dynamics and gas transport provided that the resolution in time of the sensor applied is adjusted to the process under investigation. In an earlier publication we studied the transport of gases into the mesopores of monolithic materials by analyzing the pressure relaxation after the dosing of a defined amount of gas onto the sample [3]. An improvement of the resolution in time allows to investigate also small monoliths or samples with larger pores. 

Fundamental studies of gas transport in polymers other than rubbers began with the classical work of Meares in 1954 He was the first to demonstrate and theorize about the now well-known inflection in the Arrhenius plots of D near the ass transition temperature. He also speculated abcut two modes of sorption in glassy polymers. Later studies were initiated with many polymers by Barrer, Michaels and their coworkers together with important contributions by Brandt, Stern, Stannett and many others ... 

The permeation of a gas is strongly affected by the sorption properties of the combination of gas and membrane and by the ratio of the molecular diameter of the gas molecule and the pore diameter. Mixtures must be classified on the basis of these two properties and the transport properties of these classes differ considerably. 

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. 

Consequently, it appears that the L/D ratio of PLA influences the gas permeability but no conclusion can be reached regarding the water vapour transport in this pol)uner. Moreover, generally speaking, the crystallization of the PLA matrix makes it possible to decrease the gas permeability and the organic compound sorption in PLA. However, no agreement has been formed regarding the influence of crystallinity degree on water vapour transport in PLA due to the variety of PLA composition (L-lactic acid content) and measurement systems. 

Measurements of gas transport in AF 2400 filled by non-porous hydrophobic fumed silica (FS) nanoparticles showed that the inorganic filler enhances gas permeability [2] such an increase is more pronounced for larger penetrants and leads to a lower size selectivity of the mixed matrix membrane with respect to the unloaded polymer. The sorption... 

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