Properties of ionomers

The mechanical properties of ionomers, such as their modulus or stiffness, tensile strength and energy-... 

In nonrigid ionomers, such as elastomers in which the Tg is situated below ambient temperature, even greater changes can be produced in tensile properties by increase of ion content. As one example, it has been found that in K-salts of a block copolymer, based on butyl acrylate and sulfonated polystyrene, both the tensile strength and the toughness show a dramatic increase as the ion content is raised to about 6 mol% [10]. Also, in Zn-salts of a butyl acrylate/acrylic acid polymer, the tensile strength as a function of the acrylic acid content was observed to rise from a low value of about 3 MPa for the acid copolymer to a maximum value of about 15 MPa for the ionomer having acrylic acid content of 5 wt% [II]. Other examples of the influence of ion content on mechanical properties of ionomers are cited in a recent review article [7],... 

The mechanical properties of ionomers can be appreciably altered by the manner in which the ionomer is prepared and treated prior to testing. Some of the factors that are influential are the degree of conversion (neutralization) from the acid form to the salt form, the nature of the thermal treatment or aging, the type of counterion that is introduced, the solvent that is used for preparation of thin films, and the presence and nature of any plasticizers or additives that may be present. In the scope of this chapter, it is not possible to provide a complete description of the influence of each of these variables on the wide variety of ionomers that are now commercially available or produced in the laboratory. Instead, one or more examples of the changes in properties that may be induced by each of the processing variables is presented and discussed. 

In the preparation and processing of ionomers, plasticizers may be added to reduce viscosity at elevated temperatures and to permit easier processing. These plasticizers have an effect, as well, on the mechanical properties, both in the rubbery state and in the glassy state these effects depend on the composition of the ionomer, the polar or nonpolar nature of the plasticizer and on the concentration. Many studies have been carried out on plasticized ionomers and on the influence of plasticizer on viscoelastic and relaxation behavior and a review of this subject has been given 119]. However, there is still relatively little information on effects of plasticizer type and concentration on specific mechanical properties of ionomers in the glassy state or solid state. 

The mechanical properties of ionomers are generally superior to those of the homopolymer or copolymer from which the ionomer has been synthesized. This is particularly so when the ion content is near to or above the critical value at which the ionic cluster phase becomes dominant over the multiplet-containing matrix phase. The greater strength and stability of such ionomers is a result of efficient ionic-type crosslinking and an enhanced entanglement strand density. 

Structural Organization and Dynamic Properties of Ionomer Membranes... 

In contrast, the micrographs of the ionomer pseudo-IPN coatings with opposite charge groups. Figure U- (A-2 to C-2), did not reveal any phase separation. No white particles of the VMCC phase were visible in the dark matrix of the PU phase. Presumably the ionic bonds between the carboxyl and tertiary amine groups provided the best opportunity for interpenetration between the linear chains of VMCC and the networks of PU to prevent any possible phase separation from the ionomer pseudo-IPN microphase. The physical properties of ionomer... 

M. Pineri and A. Eisenberg, eds., Structure and Properties of Ionomers, Vol. 198 of NATO Science Series C, Springer, Berlin, 1987. 

P. Dimitrova, K.A. Eriedrich, B. Vogt, and U. Stimming. Transport properties of ionomer composite membranes for direct methanol fuel cells. Journal of Electroanalytical Chemistry 532, 75-83 2002. 

The properties of ionomer solutions are sensitive to not only the degree of the ionic functionality and the polymer concentration, hut perhaps even to a greater extent, the ability of the solvent to ionize the ion-pairs (64). Thus, non-ionizing solvents, usually those with relatively low dielectric constant, favor association of the ionic groups even in dilute solutions. In contrast, ionomer solutions may exhibit polyelectrolyte behavior in polar solvents due to solvation of the ion-pair that leaves the hound ions unshielded. 

Although there is little doubt that the unique mechanical and transport properties of ionomers are due to a microphase separation between organic monomers and ionizable groups, their exact morphology is not yet understood. 

Various techniques have been used to study the solution properties of ionomers. These include viscosity (4, 1 ), static and dynamic light scattering (12.13.15-18), small-angle neutron scattering (11.14). and spectroscopy (10). Here, we use (static and... 

Surprisingly few studies have focused on the effect of solvents or diluents on the structure and properties of ionomers. Solution results are scarce due to the limited solubility of ionomers in conventional solvents, because of the strong intermolecular associations of the ionic groups(6,7). 

Of the microphase-structure dependent physical properties of ionomers, perhaps the most widely studied are glass transition temperatures, (Tg), and dynamic mechanical response. The contribution of the Coulombic forces acting at the ionic sites to the cohesive forces of a number of ionomeric materials has been treated by Eisenberg and coworkers (7). In cases in which the interionic cohesive force must be overcome in order for the cooperative relaxation to occur at Tg, this temperature varies with the magnitude of the force. For materials in which other relaxations are forced to occur at Tg, the correlation is less direct. 

The characteristics discussed above are mainly related to clustering in the ionic phase, but the role of the hydrophobic phase also is quite important. In some cases it controls the gas transport properties of the material (e.g. 02 through PFSA) (4). And, it makes it possible to keep hydrophobic reactions in the neighborhood of the ionic domain species (5). Moreover, metal complexes with bulky hydrophobic ligands can be supported in the ionomers because of synergystic interaction of both polymer phases (6). Interesting electrocatalytic or photocatalytic systems take advantage of these unique properties of ionomers (7-8). Moreover, support of the reactants in ionomers may be useful for reactant/product separations. 

The studies described in this paper were designed to use the special properties of ionomers, but they are not the only type of chemistry that does so. Other chemical systems that employ ionomer properties include their use as sources of protons of high effective acidity (superacidity) in the catalysis of organic reactions (9-11) and then-use as integral components of chemically active membranes (e.g. cell dividers or electrode coatings) (12). 

Ionomers are certainly not the only useful support for transition metals and ions. Indeed, inorganic oxides, such as silica, zeolites and aluminas, are the most widely used at present (13). Among the organic polymeric supports now used, the most closely related to the ionomers are the well known ion-exchange resins. While they are polyelectrolytic, as are the PFSA and PSSA ionomers, they are not thought to possess the potentially useful morphological properties of ionomers. 

In the discussion of the matrix we noted that the dry clusters have physical contact when c is large. By solvation we expect that this will happen for a smaller value of c. One sees here the strong tendency of the clusters, favored by solvation, to collapse into a network of channels. Similar structures have been already proposed (13) they are of fundamental importance, for practical use, in explaining the striking transport properties of ionomers. 

The composite nature and Its Influence on physical properties of Ionomers and relevant theories are reviewed. Using perflourinated Ionomers as examples,... 

Transport properties of ionomer blends, characterized by a given type of spheroids and the aspect ratio, e/a, can now be analyzed by the effective medium theory discussed in the previous section. In this theory, the two phases are assumed randomly mixed and the probability of finding each phase is equal to its volume fraction f.. The effective conductivity, o, of the composite for either Na+ of OH ions is given by (15) ... 

So far, the classical work on polyelectrolyte nonaqueous solutions conducted during the 1950s has been described. Here we will describe more recent work on polyelectrolyte nonaqueous solutions, conducted during the 1980s and 1990s, in particular the work on the solution properties of ionomers in a polar solvent, which may be considered as weakly charged polyelectrolytes. 

Hara M, Wu J, Lee AH. Solution properties of ionomers. 2. Simple salt effect. Macromolecules 1989 22 754-757. 

Tonic interactions in macromolecules represent simultaneously one of the oldest and one of the most recent areas of activity in modem polymer science. While on the one hand polyelectrolyte solution studies have been pursued in a host of laboratories for decades, the study of the bulk properties of ionomers dates back hardly more than ten years ago— as a matter of fact the word ionomer came into common use only about a decade ago with the advent of the first commercial material of this type based on ethylene. Since that time, that aspect of the field has grown profoundly, both in regard to bulk investigations as well as studies in nonaqueous solutions, as is evidenced by the periodic symposia in the field, the recently initiated Gordon Conference, and the appearance of two books dealing primarily with bulk properties. 

The composition and structure of the base EPDM and the sulfonate content of the metal sulfonate-containing EPDM exert substantial effects on the mechanical and flow properties of ionomers. The systems discussed to this point have all contained zinc sulfonate groups. In terms of melt flow the zinc cation is the cation of choice. The metal cation borne by... 

This chapter shows that NMR spectroscopy is a powerful tool for the structural analysis of ionomers. Ionomers are already widely used in various fields of application. In the near future, it is expected that research on the physical, mechanical, electrical, and transport properties of ionomers using NMR techniques will increase. 

In contrast to homogeneous polymer systems, the pendant ionic groups in ionomers interact or associate, forming ion-rich aggregates immersed in the nonpolar matrix of polymer backbone (Figure 15.10). The extent of ionic interactions and, hence, the properties of ionomers, are dictated by the ionic content, degree of neutralization, type of polymer backbone, and cation. 

FIGURE 21.44 Methanol permeation rate of the membranes as a function of thickness and at different temperatures (a, a, o, ) commercial samples N112, N115, and N117 (b, bj, , ) composite membranes. Temperature at 25°C, plots a and b (open symbols), and at 65°C, plots a and bl (full symbols). (Reprinted from J. Electroanal. Chem., 532(1-2), Dimitrova, R, Friedrich, K.A., Vogt, B., and Stimming, U., Transport properties of ionomer composite membranes for direct methanol fuel cells, 75-83, Copyright 2002, with permission from Elsevier.)... 

---