Mechanical 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. 

The self-organized structure and mechanical properties of ionomer aggregates determine the water sorption properties and the stability in PEMs. These properties in turn are the key to PEM operation in PEFCs they govern water distribution and transport, proton density and conductivity, and the membrane response to mechanical stressors. 

Kara M, Sauer JA (1994) Mechanical properties of ionomers. J Macromol Sci Polym Rev C34 325-373... 

The effect of ionomer concentration on the mechanical properties of PP-EPDM blends is given Table 9. It is seen that the tensile strength and modulus show a maximum at 5 wt% of both ionomer A and B, thereafter, it decreases at higher ionomer loading. The properties are higher for ternary blends containing ionomer B than these containing ionomer A. On the other hand, addition... 

Very recently, Williams, Billington Pearson (1992) have examined the effect of reinforcement by silver or silver-tin alloy on the mechanical properties of three glass-ionomer cements. Measurements of compressive, flexural, tensile (measured by the diametral compressive procedure) and shell strength are given in Table 5.17. These results show that the effect of reinforcement varies from cement to cement but, in general, increases it. 

Elliott, J., Holliday, L. Hornsby, P. R. (1975). Physical and mechanical properties of glass-ionomer cement. British Polymer Journal, 1, 297-306. 

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. 

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. 

Table III. Mechanical Properties of Perfluoro Ionomer Membrane. (Ion Exchange Capacity 0.03 meq/g Dry Resin).

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... 

Metal sulfonate-containing ethylene-propylene-diolefin ter-polymers (EPDM) were plasticized with stearic acid and derivatives for the reduction of the melt viscosities of these ionomers through interaction with the very strong ionic associations. Substantial improvements in melt flow were achieved with stearic acid and the zinc, lead, and ammonium stearates, while other metal stearates were ineffective. Zinc stearate and lead stearate not only markedly improved melt flow but, remarkably, also enhanced the mechanical properties of the plasticized systems. These unique additives were fully compatible with the EPDM ionomers and provided thermoelastic systems with excellent physical properties and ready processability. 

Zinc stearate provides a rather remarkable result. It markedly enhances the melt flow of metal sulfonate ionomers while simultaneously improving the mechanical properties of the resultant system. 

Next, an attempt was made to clarify the effect of crystalline phase on the mechanical properties of ethylene ionomer. Dynamic mechanical measurements... 

The microphase structure and mechanical properties of the blends containing neat acrylonitrile-butadiene-styrene copolymer (ABS), styrene-acrylonitrile copolymer (SAN) and sodium sulfonated SAN ionomer have been investigated as a function of ion content of the ionomer in the blend by Park et a/.51 The interfacial adhesion was quantified by H NMR solid echo experiments. The amount of interphase for the blend containing the SAN ionomer with low ion content (3.1 mol%) was nearly the same as that of ABS, but it decreases with the ion content of the ionomer for the blend with an ion content greater than 3.1 mol%. Changing the ionomer content in the blends shows a positive deviation from the rule of mixtures in tensile properties of the blends containing the SAN ionomer with low ion content. This seems to result from the enhanced tensile properties of the SAN ionomer, interfacial adhesion between the rubber and matrix, and the stress concentration effect of the secondary particles. 

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. 

Su et al. (8) studied the mechanical properties and morphological structure relationship of blends based on sulfated EPDM ionomer and PP. They synthesized Zn neutralized low degree sulfated EPDM (Zn-SEPDM) ionomer and PP blends and studied their mechanical properties. They found that Zn + neutralized low degree sulfated EPDM ionomer and PP blends have better mechanical properties than those of PP/EPDM blend, as shown in Fig. 14.4. They explained the reason why mechanical properties are higher for Zn-SEPDM and PP than for PP and EPDM using scanning electron microscopy (SEM) (Fig. 14.5). Finer dispersed phase size and the shorter interparticle distances are the main reasons for the improved mechanical properties of the PP/EPDM blend. 

Mechanical Behavior. The ionomers, - poly[Isobutylene-co-(4-methyl styrenyl, triphenyl phosphonium bromide or tetraphenyl borate)] were found to be different in physical appearance(hard and strong) and tougher than the starting material, the Exxpro elastomer. The mechanical properties of these quaternary phosphonium... 

---