Cluster perfluorinated membranes

Ion clusters are commonly observed in the ionized forms of the perfluorinated membranes. The size of the clusters appears to be larger for sulfonate than for carboxylate membranes." " The size increases in the order Na, and Cs" and decreases with increasing number of functional groups per chain and with increasing temperature.As in the case of ethylene ionomers, the perfluorinated carboxylic acid membranes do not form ion clusters, at least in the dry state." The electrostatic interaction may be too weak to form ionic clusters. These observations are expected according to the Eisenberg theory (see Section II.2). 

Studies of these perfluorinated membranes in dilute and in concentrated solution environments still leave many unanswered questions about the nature of membrane transport properties. However, the obvious importance of these polymers in membrane separation applications, coupled with the fundamental significance of their ion clustered morphology, makes the continued study of these materials a fruitful area of research for the future. 

A model for ionic clustering in "Nafion" (registered trademark of E. I. du Pont de Nemours and Co.) perfluorinated membranes is proposed. This "cluster-network" model suggests that the solvent and ion exchange sites phase separate from the fluorocarbon matrix into inverted micellar structures which are connected by short narrow channels. This model is used to describe ion transport and hydroxyl rejection in "Nafion" membrane products. We also demonstrate that transport processes occurring in "Nafion" are well described by percolation theory. 

In this work we propose a model for ionic clustering, which we have called the cluster-network model (2), to account for hydroxyl rejection in nNafionM perfluorinated membranes. In developing this model we have been guided by two requirements 1. the model should be consistent with the available data on the microscopic structure of the polymer (1-5) 2. the model should... 

In the next section we will present the data and arguments on which the cluster-network model is based. We will also discuss the effects of equivalent weight, ion form, and water content on the dimensions and composition of the clusters. In the third section we will present a formalism, which follows from the cluster-network model, based on absolute reaction rate theory (2) and hydroxyl rejection in "Nation perfluorinated membranes. Finally we will outline the concepts of percolation theory and demonstrate that ion transport trough "Nation" is well described by percolation. 

Figure 5. Cluster-network model for Nafion perfluorinated membranes. The polymeric ions and absorbed electrolyte phase separate from the fluorocarbon backbone into approximately spherical clusters connected by short, narrow channels. The polymeric charges are most likely embedded in the solution near the interface between the electrolyte and fluorocarbon backbone. This configuration minimizes both the hydrophobic interaction of water with the backbone and the electrostatic repulsion of proximate sulfonate groups. The dimensions shown were deduced from experiments. The shaded areas around the interface and inside a channel are the double layer regions from which the hydroxyl ions are excluded electrostatically.

Figure 6. Schematic potential seen by a hydroxyl ion as it moves across a Nafion perfluorinated membrane in a chlor-alkali cell. This potential consists of two parts a constant sloping portion that arises from the voltage drop across the membrane and an oscillating part that arises from electrostatic restriction of the hydroxyl ions. Physically, the hills and troughs correspond to the channel and cluster regions, respectively. For simplicity, a one-dimensional, periodic, model potential is used to evaluate the membrane current efficiency although the real potential is three-dimensional and aperiodic.

The reasons for the success of this model are elaborated in the next section, which also addresses the physicochemical and transport properties of Nafion . It may be noted that the existence of cluster networks in the perfluorinated membranes was demonstrated by Gierke et al. [40] by transmission electron microscopy. 

Based on the cluster network model [72], the perfluorinated membranes undergo phase separation on a molecular scale when swollen by contact of water. Clusters are formed when sodium ions (Na ) separated from the fixed ironic sites joints the aqueous water separated from the fluorocarbon matrix. The ions and the sorbed solutions are all in clusters. A cluster s diameter varies from 3 to 5 nm and contains approximately 70 ion-exchange sites. The clusters are connected by short narrow channels with diameter of 1 nm estimated from hydraulic permeability data. The channels are formed by fixed ionic sites hydrated and embedded with water phase. 

Figure 3.3.5 (A) Chemical structure of sulfonated perfluorinated polyethylene (Nafion ). (B) Schematic illustration of the microscopic structure of hydrated Nafion membrane perfluorinated polyethylene backbone chains form spherical hydrophobic clusters. Sulfonic end groups interface with water-filled channels and mediate the migration and diffusion of protons. The channels are filled with water and hydronium ions. Figure adapted from [4].

Figure 3.43. Structure of Nafion-115 at ambient humidity derived from small-angle X-ray spectroscopy. The lighter areas are cluster structures in the material. (Reprinted with permission from J. Elliott, S. Hanna, A. Elliott, G. Cooley (2000). Interpretation of the small-angle X-ray scattering from swollen and oriented perfluorinated ionomer membranes, Macromolecules 33, 4161-4171. Copyright American Chemical Society.)...

Gierke, T., Hsu, W. (1982). The cluster-network model of ion clustering in perfluoro-sulfonated membranes. In "Perfluorinated lonomer Membranes", American Chemical Society Symp. Series 180, Washington, DC. 

The preparation of the supported catalytic Fe-clusters have been recently reported by our laboratory for Fe/Nafion membranes [1,2,3] for Fe/Nafion/glass-mats [4,5] micro-encapsulated Fe-alginate beads [6] Fe-amorphous polycrystalline thin film fused copolymers [7] and finally Fe/silica woven fabrics [8]. Experiments were conducted with Nafion perfluorinated cation transfer membrane. Photolysis experiments were carried out by means of a Hanau Suntest Lamp with tunable light intensity equipped with an IR filter to remove infrared radiation. Light... 

Hopfinger and Mauritz and Hopfinger also presented a general formalism to describe the structural organization of Nafion membranes under different physicochemical conditions. It was assumed that ionic clustering does not exist in the dry polymer. This assumption is applicable to the perfluorinated carboxylic acid polymer" but not the perfluorosulfonate polymers." They consider the balance in energy between the elastic deformation of the matrix and the various molecular interactions that exist in the polymer. 

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