Ionomers structures, theoretical model

Mauritz summarized a number of molecular models of ionomer structure, including those pertaining to Nafion, that had had been formulated up to 1996. Within the context of the title of this review, it should be appreciated that the results contribute to the state of understanding only if they are verifiable by careful experimentation. To be sure, theoretical predictions are welcome in the design of experiments and pointing the way toward useful applications. 

Theoretical Model for the Structures of Ionomers Application to Nafion Materials... 

A pseudo-quantitative application of the theoretical formalism has been made for Nafion. The values for the requisite molecular parameters were estimated from a combination of experimental bulk thermodynamic data and molecular structure calculations using both molecular and quantum mechanics (23,24). A constraint was imposed in the development of the structural formalism. The model was constructed so that the predicted structural information could be used in a computer simulation of ion transport through an ionomer, that is, modeling the ionomer as a permselective membrane. 

X-ray diffraction work (11,15) shows that there is an ionomer peak at 4°C which is absent in the acid precursor. This low, broad peak is not affected by annealing or ion type and persists up to 300°C. Since the 4°C peak corresponds to a spacing of about 2.5 nm, it is reasonable to propose a structural feature of this dimension in the ionomer. The concept of ionic clusters was initially suggested to explain the large effects on properties of relatively sparse ionic species (1). The exact size of the clusters has been the subject of much debate and has been discussed in a substantial body of literature (3,4,18—20). A theoretical treatment has shown that various models can give rise to supramolecular structures containing ionic multiplets which are about 10 nm in diameter (19). 

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