Clustering in ion-containing polymers

Clustering in Ion-Containing Polymers Preliminary Small-Angle Neutron Scattering Experiments... 

Summary. We have shown that ion transport in "Nafion" per-fluorinated membrane is controlled by percolation, which means that the connectivity of ion clusters is critical. This basically reflects the heterogeneous nature of a wet membrane. Although transport across a membrane is usually perceived as a one-dimensional process, our analysis suggests that it is distinctly three-dimensional in "Nafion". (Compare the experimental values of c and n with those listed in Table 7.) This is not totally unexpected since ion clusters are typically 5.0 nm, whereas a membrane is normally several mils thick. We have also uncovered an ionic insulator-to-conductor transition at 10 volume % of electrolyte uptake. Similar transitions are expected in other ion-containing polymers, and the Cluster-Network model may find useful application to ion transport in other ion containing polymers. Finally, our transport and current efficiency data are consistent with the Cluster-Network model, but not the conventional Donnan equilibrium. 

An ion-containing polymer with low mol% of ionic groups along the polymer backbone chains or as pendant groups (defined as ionomers) shows clustering of ionic units. This is in contrast to the ionic-ionic repulsions in polyelectrolytes. 

In aqueous solution, the mixture of solutions containing cadmium and sulfide ions induces a precipitation of CdS semiconductor. When adding a protecting polymer such as sodium hexainetaphosphate (HMP) in the solution, no precipitation is observed and a yellow solution remains optically clear, indicating the formation of CdS clusters. In reverse micelles, similar behavior of the latter is observed, as shown later. 

There is a considerable body of experimental and theoretical evidence for two types of ionic aggregates termed multiplets and clusters (95). The multiplets are considered to consist of small numbers of associated contact ion-pairs that are dispersed in the matrix of low dielectric constant, but do not themselves constitute a second phase. The number of ion-pairs in a multiplet is sterically limited by the fact that the salt groups are bound to the polymer chain. On the other hand, clusters are considered to be small microphase separated regions (<5 nm) of aggregated multiplets. Thus, the clusters are rich in ion-pairs, but they also contain an amount of the organic polymer. 

Tphe phenomenon of ion clustering in polymers containing ionic co-monomers has received considerable attention recently, primarily in materials based on styrene, ethylene, or butadiene backbones containing a small concentration of pendant carboxylic acid groups randomly distributed along the chain. These materials have been reviewed in two... 

Ionic bonds, in which electrons are donated to, or received from other atoms, occur in a few polymers. Du Pont Surlyn ionomers are copolymers of ethylene and methacrylic acid. Part of the methacrylic acid is neutralised with zinc or sodium ions. Ionic bonds are relatively strong and ionomers contain clusters of ions, which act rather like crosslinks. Some ionomers are blended with other polymers to improve toughness. 

Joo and co-workers [22] have discussed a new type of composite membrane, consisting of functionalised carbon nanotubes (CNT) and sulfonated polyarylene sulfone (sPAS) for direct methanol fuel cell applications. The CNT modified with sulfonic acid or platinum-rubidium (PtRu) nanoparticles were dispersed within the sPAS matrix by a solution casting method to give SOs-CNT-sPAS or PtRu-CNT-sPAS composite membranes, respectively. Characterisation of the composite membranes revealed that the functionalised CNT were homogeneously distributed within the sPAS matrix and the composite membranes contained smaller ion clusters than the neat sPAS. The composite membranes exhibited enhanced mechanical properties in terms of tensile strength, strain and toughness, which leads to improvements in ion conductivity and methanol permeability compared with the neat sPAS membrane, which demonstrates that the improved properties of the composite membranes induce an increase in power density. The strategy for CNT-sulfonated composite membranes in this work can potentially be extended to other CNT-polymer composite systems. 

---