Membrane properties, polymer transport

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... 

Nasr J, Barboiu M, Ono T, Fujii S, Lehn J-M (2008) Dynamic polymer membranes displaying tunable transport properties on constitutional exchange. J Membr Sci 321 8-14... 

The polymer materials mainly used for the membranes are glassy polymers, the first and foremost polyimides. The use of glassy polymers having a rigid ensemble of macromolecules results in high separation effectiveness. Separation effectiveness in pervaporation processes is characterized by the separation factor, /3p, which is determined by the diffusion component, /3d, and the sorption component, /3s [8,55]. Let us consider the effect of chemical composition of polymer membranes on their transport properties with respect to aromatic, alicyclic, aliphatic hydrocarbons and analyze ways to improve these properties. 

The transport and separation properties of the polysulfone membranes depend on the polymer concentration in the casting solution, on the temperature of the casting solution, and on the time of solvent evaporation from the film surface. The best membrane properties were obtained, using a casting solution which consisted of 12.5 wt. % polysulfone (dissolved in dimethylformamide) and had a temparature of 298 K, with no evaporation of the solvent. 

Figure 2 sketches different length scales on which water of hydration affects membrane structure and transport properties. The bottom picture represents the smallest unit of a membrane that still contains the essential ingredients of the membrane interior clusters of acidic functional groups and water molecules. The electronic structure of acidic groups and polymer side chains determines the kinetics... 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

Invention of several types of membranes—those for mediating redox catalyst systems for chemical synthesis, chemically stable anion exchange membranes, and polymer membranes with gradually changing structural properties along the direction of transport... 

Membrane Properties of p-222I. Copolyoxamide p-222I was selected for thorough study on the basis of encouraging results of measurements of transport parameters for this and related polymers. These results, obtained on ultrathin films prepared from trlfluoroacetlc acid solutions, are shown in Table II. 

T. Sata, R. Yamane and Y. Mizutani, Modification of properties of ion exchange membranes. VII. Relative transport number between various cations of cation exchange membrane having cationic polyelectrolyte layer and mechanism of selective permeation of particular cations, J. Polym. Sci., Polym. Chem. Ed., 1979, 17, 2071. 

An example of an enantiomerically pure polymer is also shown [11]. Aoki et al. showed that films of a polyacetylene substituted with a (-)-p-pinene derivative formed an effective membrane for chromatographic resolutions of racemic mixtures. ( )-2-Butanol was resolved to 29.8% eje. and unsaturated polymers for both liquid-phase and gas-phase separation applications (8, 9, 79]. It has been suggested that the rigidity and irregularity of the highly substituted polyacetylene chain, combined with the presence of aliphatic substituents which reduce interchain interactions, are important for the polymers transport properties [10]. 

Section I (Novel Membrane Materials and Transport in Them) focuses on the most recent advances in development of new membrane materials and considers the transport parameters and free volume of polymeric and even inorganic membranes. Kanehashi et al. (Chapter 1) present a detailed review of hyperbranched polyimides, which are compared with more common cross-linked polyimides. These polymers with unusual architecture were studied in the hope that they would show weaker tendency to plasticization than conventional linear polymers. However, many representatives of this new class of polymers reveal relatively poor film forming properties due to absence of chain entanglement. Nonetheless, some promising results obtained can show directions of further studies. 

The problem arises for those LBP versions which are proposed for the electronic consumer market, where a high temperature of operation is clearly not acceptable. Since this is considered a market where LPBs may find an exclusive and important role in replacing the present, low-energy and environmentally unfriendly Ni-Cd batteries, research has been devoted over recent years to the development of polymer ionic membranes with enhanced transport properties at ambient and subambient temperatures. 

J.H. Kim, W.J. Koros, D.R. Paul, Physical aging of thin 6EDA-based polyimide membranes containing carboxyl acid groups. Part I. Transport properties. Polymer 47 (2006) 3094-3103. 

Roy A, Hickner MA, Yu X, Li Y, Glass TE, McGrath JE (2006) Influence of chemical composition and sequence length on the transport properties of proton exchange membranes. J Polym Sci Part B Polym Phy s 44 2226... 

The type, concentration, and mobility of ions in polymers define the membrane properties of these polymers. Due to the importance of ion transport processes associated with the oxidation/reduction of the considered electroactive polymers, membrane properties are a subject of central importance for a quantitative understanding of their electrochemical behavior. The discussion of this subject constitutes the essential part of Chapter 3. 

Efforts of polymer scientists and fuel cell developers alike are driven by one question What specific properties of the polymeric host material determine the transport properties of a PEM, especially proton conductivity The answer depends on the evaluated regime of the water content. At water content above kc, relevant structural properties are related to the porous PEM morphology, described by volumetric composition, pore size distribution and pore network connectivity. As seen in previous sections, effective parameters of interest are lEC, pKa, and the tensile modulus of polymer walls. In this regime, approaches familiar from the theory of porous media or composites (Kirkpatrick, 1973 Stauffer and Aharony, 1994), can be applied to relate the water distribution in membranes to its transport properties. Random network models and simpler models of the porous structure were employed in Eikerling et al. (1997, 2001) to study correlations between pore size distributions, pore space connectivity, pore space evolution upon water uptake, and proton conductivity, as will be discussed in the section Random Network Model of Membrane Conductivity. ... 

Oya, S., and Aras, L., Measurement of transport properties of polyfmethylmeth-acrylate-co-methacryUc acid) ion-containing membranes, Br. Polymer J., 22, 155, 1990. 

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