Hydrophilic ionic domains

The above example verifications of the physical model show that it could be used to aid in membrane development. For example, high-temperature membranes need to remain conductive at low relative humidities. According to the physical model, one approach would be to have very small, highly interconnected, and hydrophilic ionic domains in a hydrophobic matrix. Such a structure would allow for water to remain in the ionic domains at elevated temperatures and still have a relatively short conduction pathway across the cell. 

Perfluorosulfonated membranes have a microscopic phase-separated structure with hydrophobic regions and hydrophilic domain. Hydrophobic regions provide the mechanical support and hydrophilic ionic domains provide proton transport channel. Many morphological models for PFSA have been developed based on SAXS and wide-angle x-ray scattering (WAXS) experiments of the membranes. However, because of the random chemical structure of the PFSA copolymer, morphological variation with water content and complexity of coorganized crystalline and ionic domains, limited characteristic detail proved by the SAXS and WAXS experiments, the structure of the PFSl has been still subject of debate. Here, a brief description of seven membrane structure models is provided. 

The proton exchange membranes (PEMs) used in PEM fuel cells consist of two domains the hydrophobic polymer backbone domain and the hydrophilic ionic cluster domain. The most commonly used membrane is Nafion , produced by DuPont. 

Ionomers are polymers that are functionalized with ionic groups (usually anionic sites) attached at various points along polymeric backbones that are not extensively crosslinked (1-2). Such materials have a tendency to form ionic domains in which the anionic groups and their associated cations are microphase separated from the typically hydrophobic portions of the polymer. Thus, the ionic domains formed are isolated by a medium of low dielectric constant (i.e. the polymeric backbone) although, in some cases, hydrophilic channels have been reported to connect adjacent ionic domains (3). The size and structures of these domains vary with the nature of the cation, the stoichiometry of the polymer, the degree of solvation of the system and the method of preparation. They can be as small as ion-pairscor small multiplets, but in some cases they have been reported to be in the 20-100 A" diameter range. 

The resemblance of the under-water stress relaxation curves and the dissimilarity of the stress relaxation behavior of the Nafion acid and the salt in the dry state may be explained as follows. The phase separated hydrophilic regions are expected to contain a substantial fraction of the ether side chains which are anchored in the ionic domains by their polar end groups. In the dry state, the coulombic interactions within the ionic aggregates are so strong that these domains probably serve as effective crosslinks. This would not only reduce the mobility of molecules within the domains but would also control the mobility of the fluorocarbon matrix through the side chains this, in turn, leads to the rise in the primary relaxation temperature. 

When the samples are immersed in water, the ionic domains must swell due to their hydrophilic nature. According to Mauritz et al., (36,37) the direct interaction of the bound anion and the unbound... 

Nafion in the bulk phase separates into hydrophobic and hydrophilic SO3H regions. The ionic domains or clusters are inverted micelles surrounded by a fluorocarbon matrix, and the ionic domains are connected by short channels. Because of the inverted micelle within the fluorocarbon matrix, availability of the acid sites for catalysis is greatly diminished. Two approaches were initially used to increase the catalytic activity of bulk Nafion polymer blends (2) and coating a liquid conposition of Nafion on a hydrophobic support (3). 

In terms of the structure within the membrane, the idealized Hsu and Gierke cluster-network model is used as a picture where the pathways between the clusters are interfacial regions. These pathways are termed collapsed channels since they can be expanded by liquid water to form a liquid-filled channel. In essence, the collapsed channels are sulfonic acid sites surrounded by the polymer matrix having a low enough concentration such that the overall pathway between two clusters remains hydrophobic. In other words, they are composed of bridging ionic sites [31] and the electrostatic energy density is too low compared to the polymer elasticity to allow for a bulk-like water phase to form and expand the channels. In all, for a vapor-equilibrated membrane the structure is that of ionic domains that are hydrophilic and contain some bulk-like water. These clusters are connected by... 

Amphiphile strength (IV) > (II). Outside the non-ionic domain, an alcohol of medium chain length is a weaker amphiphile than if the hydrophilic head is ionisable (e.g., a sulfate or sulfonate group). 

Guiver et al. of National Research Council, Canada developed comb-shaped poly(arylene ether) electrolytes containing 2-A sulfonic acid groups on aromatic side chains (d) [76]. Their membranes showed relatively high proton conductivity and well-developed and continuous ionic domains. However, trade-off relationship between water uptake and proton conductivity of their membranes was not better than that of Nafion. In order to pronounce the hydrophilic/hydropho-bic differences, another series of comb-shaped aromatic ionomers with highly fluorinated main chains and flexible poly(a-methyl styrene sulfonic acid) side chains were developed [77]. The membranes seemed to have better properties than their previous version, however, chemical instability of the side chains needed to be improved. 

To date, many types of block copolypeptide amphiphiles that form stable vesicular assemblies have been developed. The first of these utilized diethylene glycol-modified lysine residues (i.e., K ) that impart both non-ionic water solubility as well as ordered a-helical conformations to the hydrophilic polypeptide domains... 

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