Facilitated polymer transport, mechanisms

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

Clearly, a better understanding of the gas transport mechanisms in polymers would greatly facilitate the development of polymer membranes that exhibit both a higher selectivity and a higher (or lower) permeability to specified gases. It is beyond the scope of the present chapter to review this area of research, particularly since a number of extensive reviews are available [1,3-9,11-15,17,18]. 

Another approach that has been proven to be successful is the incorporation of reactive moieties in polymers to selectively increase the affinity of one penetrant over the others, the so-called facilitated transport mechanisms. In this case a chemical contribution is added to the physical flux in view of a chemical reaction of the target penetrant with the reactive moieties present in the membrane matrix, whereas aU of the other components are able to permeate only because of the physical mechanism. This approach allows the simultaneous achievement of high flux for the target compound and large selectivity, moving perpendicularly to the membrane upper bound. A typical example is the use of amine-based polymers as a membrane matrix under humid conditions because water is needed to catalyze the chemical reaction [37,38]. The carrier can be chemically bonded to the membrane matrix [39] or low-molecular-weight molecules able to freely move in the swollen matrix [40]. Nevertheless, this approach has also been used for the recovery of pharmaceutical chemicals, such as Cephalexin [41], as well as metal ions from waste water [42]. 

Sulfonated polymer membranes are highly hydrophilic and thus exhibit the feature of sorption of considerable amount of water molecules. The water uptake is mainly determined by the lEC value of a membrane, and the higher the lEC, the larger the water uptake. The water uptake of sulfonated polymer membranes is known to have a profound effect on membrane conductivity and mechanical properties [98]. Water molecules dissociate acid functionality and facilitate proton transport. However, excessively high levels of water uptake can result in membrane fragility and large dimensional change, which leads to the loss of mechanical properties. 

However, despite this lack of a basic understanding of the electrochemistry of these materials, much progress has been made in characterizing polymerization mechanisms, degradation processes, transport properties, and the mediation of the electrochemistry of species in solution. These advances have facilitated the development of numerous applications of conducting polymers, and so it can be anticipated that interest in their electrochemistry will remain high. 

Marosi, G., Keszei, S., Marton, A., Szep, A., Le Bras, M., Delobel, R., and Hornsby, P. Flame retardant mechanisms facilitating safety in transportation. In Fire Retardancy of Polymers New Applications of Mineral Fillers, M. Le Bras, C.A. Wilkie, S. Bourbigot, S. Duquesne, and C. Jama (Eds.), pp. 347-360. Cambridge, U.K. The Royal Society of Chemistry. 

Another technique to expedite the transport of the volatile components from the molten polymer is to increase the number and rate of bubbles formed [14], Techniques that have been used to increase the number of bubbles and their rate of formation (nucleation) are the addition of chemical nucleating agents [15] and ultrasound [16]. Nucleation of bubbles in the molten polymer can help expedite the achievement of equilibrium in conventional falling strand devolatilizers. However, this facilitation mechanism cannot get below equilibrium and thus has minimal value. 

During homogenization of a solution of polymer (Z3), solvent (Z,l, and water, droplets of different sizes are formed. Because of the interfacial energy difference, a transport of Z from the smaller to the larger droplets will take place. If droplets smaller than the critical size are formed, Aese droplets, especially, will rapidly lose Z to the surrounding larger droplets. The viscosity within these small droplets will increase and a further degradation may be mechanically hindered. In the presence of Zj, two effects that win facilitate subdivision of the emulsion may be encountered. 

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