Ionomers, cluster formation

IR spectroscopy can be used to characterise the formation of ionomers by studying the environment of the anions [85, 86]. Risen and co-workers [87, 88] used far-IR spectra (150 to 400 cm"1) to demonstrate the sensitivity of low frequency vibrations to the anions and cations and the degree of cluster formation in ionomers. For example, styrene sulfonic acid ionomers with Na+ cation shows absorption bands at 220 cm 1, whereas the Cs+ cation shows bands at 100 cm 1. 

Interestingly, ionic cluster formation was observed in the case of another aromatic ionomer, the Na salt of sulfonated polyether-ketone47,67. 

A recent investigation gives some insights into one aspect of the kinetics of cluster formation, both above and below Tg. In a study of the expansion coefficients of the styrene ionomers as a function of ion contentit was.found that the equilibrium values were independent of ion content. However, whenever the sample had been allowed to remain at room temperature for a few hours after cooling from above Tg, and subsequently heated again above the glass transition temperature, the first run yielded a value of the liquid expansion coefficient ca 25% lower than that... 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Note 1 The mobility of the polymer segments surrounding the multiplets is reduced relative to that of bulk material. With increasing ion content, the number density of the ionomer multiplets increases, leading to overlapping of the restricted mobility regions around the multiplets and the formation of clusters. 

Recent proton NMR relaxation studies have shown that the method can be used to study the formation of ionic clusters and the structure of the physical network in ionomers [153]. 

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]. 

As the polarity difference between the two blocks increases, (xAB)cr the Krause (1969,1970) theory (see Section 4.7.1) will become smaller, or alternatively, phase separation will occur for shorter block lengths. A limiting case in this argument involves the carboxylic rubbers and ionomers, in which one block consists of isolated single salt (ionic) mers, incorporated in an otherwise hydrocarbon-type backbone. It should be emphasized that the ionic block length is one monomer unit long. Phase separation and formation of ionic clusters occurs because of the extreme difference in... 

Plasticizers may slow down the formation of crystalline structures because they interfere in sphemlites growth since they are present in fold surfaces of crystalline lamel-lae. Sphemlites grown in the presence of tricresyl phosphate are coarser and less bire-fringent. In ionomers, the plasticizer helps to form aggregates (multiplets) and clusters in which polymer chains have restricted mobility. 

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