Backbone structure ionomers

Nafion variants consist of fluorocarbon polymer backbones, sulfonate ionomers as ionic groups, and cations (counter ions). Fig. 2.1 (b) illustrates a Nafion polymer chain that has hydrogen ions acting as cations. Ionic groups, speciflcally sulfonate ionomers for this analysis, are fixed to the polymer backbone whereas cations are free to move. This structure... 

Figure 8.7. Selected structural correlation functions (or site-site radiai distribution functions) between distinct components of self-organized CL inks (C carbon particies, B ionomer backbones, S ionomer sidechains, W water moiecuies, H hydronium ions) [7]. (Reprinted with permission from J Phys Chem C, 2007, ill, 13627-34. Copyright 2007 American Chemical Society.)...

Schematic depiction of the structural evolution of polymer electrolyte membranes. The primary chemical structure of the Nafion-type ionomer on the left with hydrophobic backbone, side chains, and acid head groups evolves into polymeric aggregates with complex interfacial structure (middle). Randomly interconnected phases of these aggregates and water-filled voids between them form the heterogeneous membrane morphology at the macroscopic scale (right).

The simple water charmel models can explain the ionomer peak and the small-angle upturn in the scattering data of fhe unoriented samples as well as of the oriented films. Interestingly, the helical structure of backbone segments is responsible for fhe sfabilify of fhe long cylindrical charmels. The self-diffusion behavior of wafer and protons in Nation is well described by the water channel model. The existence of parallel wide channels af high wafer uptake favors large hydrodynamic confributions to electro-osmotic water transport and hydraulic permeation. 

Cui et al. performed similar analyses to fhose of Dupuis and co-workers. The side chain-side chain radial disfribufion functions (RDFs) reported by Cui et al. show remarkable qualitative deviation from fhose in Zhou et al. i It is of note that the united atom approach used by Cui and co-workers ignored electrostatic interactions between CP2 groups of the polymeric backbone. This can lead to a poor description of fhe hydrated structure in the regions close to the polymeric backbones, unlike the all-atom force field used in Zhou et al. ° For the sake of limited computational resources, Cui et al. used a relatively short representation of Nation ionomer chains consisting of three monomers as compared to the ten monomers used by Vishnyakov and Neimark or Urata et al. It can be expected that structural correlations will strongly depend on this choice. 

Nation ionomers are produced by copolymerization of a perfluorinated vinyl ether comonomer with tetrafluoroethylene resulting in the chemical structure shown in Figure 8.25 [162,166], This polymer and other related polymers consist of perfluorinated, hydrophobic, backbones that give chemical stability to the material. The material also contains sulfonated, hydrophilic, side groups that make hydration possible in the acidic regions, and also allow the transport of protons at low temperatures, since the higher limit of temperature is determined by the humidification of the membrane, since water is a sine qua non for conduction [166], The material exhibits a proton conductivity of 0.1 S/cm at 80°C [162], The membrane performance is then based on the hydrophilic character of the sulfonic acid groups, which allow proton transport when hydrated while the hydrophobic... 

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

The proportion of the salt groups that reside in either of the two environments in a particular ionomer is determined by the polarity (i.e., dielectric constant) of the backbone, the total concentration of the salt groups, the nature of the anion and cation, and to some extent by non-equlllbrlum effects that arise from the fact that equilibration of the structure requires the diffusion of the ionic species through a viscous and low dielectric medium. 

Viscoelastic measurements of ionomers have been used to indirectly characterize the microstructure and to establish property structure relationships. Forming an ionomer results in three important changes in the viscoelastic properties of a polymer. First, T usually increases with increasing ionization. This is a conseqi nce of the reduced mobility of the polymer backbone as a result of the formation of physical, ionic crosslinks. Second, an extended rubber plateau is observed in the modulus above T, again as a result of the ionic network. Third, a high temperaturi mechanical loss is observed above T, which is due to motion in the ion-rich phase. The dynamic mechan cal curves for SPS ionomers shown in Fig. 9 clearly demonstrate these three characteristics. 

As shown in Scheme 11.Id, these polymers consist of the main backbone of (i) a nonconductive polymer (25-27), or a polymeric ionomer (29) or (ii) a backbone of an ECP (28) to which pendant, localized redox-centers, such as ferrocene (Fc), bipyridine-complexes of Ru, Os, and so forth, or even low-molecular-weight thiophene oligomers, are covalently attached (25, 27, and 26, respectively). Covalent attachment is characteristic of the structure 28, whereas 29 contains a typical electrostatic bond between the electroactive bipyridine-complex of Ru and the polymeric ionomer s backbone. 

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. 

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