Ionic interactions in ionomers

Spectroscopic and Thermal Studies of Ionic Interactions in Ionomers... 

Ionic interactions in ionomers are reflected in their spectra both directly and indirectly. They indirectly influence the spectral features associated with the polymeric backbone and the pendant sites as well as some of the spectral characteristics of polyatomic cations. Studies of these spectral properties are extensive. Ionic interactions are probed more directly by observing the vibrations of the cations at their anionic sites. Ion-motion vibrational bands, which occur in the far-infrared spectrum, have been studied in PFSA (Nation) (1), PEMA (2-3), PSMA (4-5), and PSSA (6) ionomers with a range of cations, ionic site concentrations, and other conditions. The force field elements that can be derived from them reflect how the interionic forces vary with the nature of the ionomer. 

The work described in the present paper concerns the Influence of water and organic solvents on the ionic interactions in lightly sulfonated polystyrene (SFS) ionomers. The focus will be specifically directed towards the Influence of the solvent environment on the cation-anion and cation-cation interactions. Fourier transform Infrared spectroscopy (FTIR) was used to probe the former while electron spin resonance spectroscopy (ESR) was used to study the latter. Experiments were carried out with dissolved, swollen, and bulk ionomers. 

Chemical modification of polyolefins is a broad and rapidly growing field of science. Such modification, often times, is done to introduce either subtle or gross changes that enhance the attributes of the original polymer. For example, introduction of ionic interactions in polymers provides a means of controlling polymer structure and properties. As would be expected, ion-containing polymers, otherwise known as ionomers , display properties which are dramatically different from those of the parent polymer. Therefore, a broad spectrum of material properties may be created by varying the ion content, type of counter ion, and extent of neutralization. 

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. 

In the case of the p-sulfonated polystyrene ionomer, the ion pairs are at the same distance from the backbone as in thep-carboxylated polystyrene ionomer. As was mentioned above, however, the ionic interaction in the sulfonated ionomer is stronger than that in the carboxylated ionomer. Thns, at the identical ion content, the modulus-temperature curves show longer ionic platean, and the volume fraction of reduced mobility regions for the p-sulfonated ionomer is smaller than for the p-carboxylated ionomer (103), leading to less clnstering for the p-snlfonated ionomers. 

In nonpolar solvents the counterions are not dissociated from the ionomer, instead forming ion pairs, due to the strong ionic interactions in the medium with low dielectric constant. These ion pairs attract each other, leading to aggregation. On the other hand, in polar solvents many of the counterions are dissociated from the ionomer chain, due to weaker ionic interactions in the medium with high electric constant. The electrostatic interactions between ionic... 

The best combination of properties of polyethylene-based ionomers, such as stiffness, strength, transparency, and toughness, are realized at partial degrees of conversion of about 40-50% [13]. The initial increase in properties is a result of the presence of ionic interactions, which strengthen and stiffen the polymer. There is, however, some loss of crystallinity as a result of the presence of the ionic groups. When the loss of crystallin-... 

A route to compatibility involving ionomers has been described recently by Eisenberg and coworkers [250-252]. The use of ionic interactions between different polymer chains to produce new materials has gained tremendous importance. Choudhury et al. [60] reported compatibilization of NR-polyolefin blends with the use of ionomers (S-EPDM). Blending with thermoplastics and elastomers could enhance the properties of MPR. The compatibility of copolyester TPE, TPU, flexible PVC, with MPR in aU proportions, enables one to blend any combination of these plastics with MPR to cost performance balance. Myrick has reported on the effect of blending MPR with various combinations and proportions of these plastics and provided a general guideline for property enhancement [253]. 

Ionic aggregate, in a polymer matrix of low polarity, formed through interactions of ionomer multiplets. 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

In this work we used polystyrene-based ionomers.-Since there is no crystallinity in this type of ionomer, only the effect of ionic interactions has been observed. Eisenberg et al. reported that for styrene-methacrylic acid ionomers, the position of the high inflection point in the stress relaxation master curve could be approximately predicted from the classical theory of rubber elasticity, assuming that each ion pah-acts as a crosslink up to ca. 6 mol %. Above 6 mol %, the deviation of data points from the calculated curve is very large. For sulfonated polystyrene ionomers, the inflection point in stress relaxation master curves and the rubbery plateau region in dynamic mechanical data seemed to follow the classical rubber theory at low ion content. Therefore, it is generally concluded that polystyrene-based ionomers with low ion content show a crosslinking effect due to multiplet formation. More... 

By using Equation 4, the effective ionic diameters, D, can be estimated. The initial slope of each curve in Figure 1 may be obtained by either a simple graphical method or a curve fitting method. The effective diameter is a measure of the distance of closest approach of the centers of the macrolons and reflects the range of interaction of ionomer molecules with other ionomer molecules. 

Even less is known about ionomer/plasticizer interactions on a molecular level. A variety of scattering and spectroscopic techniques that can probe this level have been mentioned, but they have been applied primarily to the specific case of water in ionomers, and in particular to hjdrated perfluorinated ionomers. At the least, these studies demonstrate the powerful potential of the techniques to contribute to a more complete understanding of structure-property relationships in plasticizer/ionomer systems. For e.xample, to return to the question of the effect of nonpolar plasticizers on the ionic domains how can the decrease in the ionic transition temperature be reconciled with the apparently minimal effect on the SAXS ionomer peaks and with the ESR studies that indicate (not surprisingly) tiiat these plasticizers have essentially no influence on the local structure of the ions Is it due to their association with the hydrocai bon component of the large aggregates or clusters Or if these entities do not exist, as some researchers postulate, what is the interaction between the nonpolar plasticizer, the hydrocarbon component and the ionic domains These questions are, of course, intimately related to the understanding of ionomer microstructure even in the absence of plasticizer. The interpretation of SAXS data in particular is subject to the choice of model used. 

Undervacuum Stress Relaxation Studies. The stress relaxation behavior of the Nafion system presents some unusual characteristics. The relaxation master curves of the precursor, as well as of Nafion in its acid and salt forms, are very broad and are characterized by a wide distribution of relaxation times. The individual stress relaxation curves and the master curves for the precursor (45), Nafion acid and Nafion-K (46), are shown in Figures 14, 15 and 16 with the reference temperatures Indicated in the captions. Time-temperature superposition of stress relaxation data appears to be valid in the precursor and in the dry Nafion acid, at least over the time scale of the experiments. In the case of Nafion-K, time-temperature superposition is not valid, because it leads to a breakdown at low temperatures, which is reestablished at high temperatures (above ISO C). Similar behavior was also observed for a low molecular weight (5x10 ) styrene ionomer. The addition of small amounts of water to the Nafion acid can lead to a breakdown in the time-temperature superposition. The Influence of crystallinity and of strong ionic interaction will be discussed in the section on underwater stress relaxation studies. 

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