Ionic phase relaxation

The dynamic mechanical results of this study demonstrate that backbone crystallinity plays an important role in the properties of these materials. Moreover, it is observed that thermal history affects the properties of the materials investigated in a more complex manner than can be explained by simple changes in the degree of crystallinity. At low levels of sulfonation the materials generally behave very much like linear polyethylene, but this behavior is modified significantly as the level of sulfonation is increased. Evidence is clearly present for the existence of an ionic-phase relaxation that supports the proposed model for micro-phase-separated domains (5,6,7). However, owing to the effects of crystallinity the concentration at which the ionic-phase relaxation first... 

There are several reasons why quantitative analysis of the dielectric-relaxation behavior of ionomers is difficult. As discussed above, there is a significant effect of even the smallest amounts of water on the position and magnitude of the a ionic phase relaxation. The removal of the last traces of water is very difficult due to the possibility of the coordination of water to the ionic species. 

We had good evidence from the results on Fe2+ as a function of water content that the water is associated with the ionic phase. Given the volume fraction of water in the wet polymer (35 %), the aver ge distance between ions at 33 Fe/100 SO3 is calculated as 12.5 A. It is precisely at this distance that paramagnetic hyperfine splitting (slow relaxation) is found to disappear in frozen solutions (15). The idea that the iron is present in an aqueous phase is therefore confirmed. 

The dynamic mechanical and DSC results for sulfonated PP s combine to give strong evidence for the existence of ionic clusters in these materials. In addition they suggest that these clusters are only present above a critical sulfonate concentration of 10 mol %. These conclusions are based on the existence of an ionic-phase a relaxation for materials sulfonated above 10% and the deviation of the Tg vs. % sulfonation plot from typical copolymer-type behavior at this same concentration. 

From Figure 10 it appears that a dipolar relaxation labeled a is superimposed on the phenomenon we have just discussed. The behavior of this a peak correlates well with the behavior of the dynamic mechanical a relaxation since it increases in magnitude and decreases in temperature with increasing sulfonation. The presence of this peak in the dielectric spectra of these materials and its behavior as a function of sulfonate concentration are consistent with the assignment of the mechanical a relaxation to an ionic-phase mechanism. However, it is not possible to cite this dielectric peak as proof of the mechanical assignment the known presence of ionic impurities in these systems and the unknown origin of the large increases in tan 8 and c dictate that the dielectric results be interpreted with caution. 

The a relaxation observed by dynamic mechanical techniques is attributable in part to an ionic-phase mechanism. The existence of this relaxation is suggested as evidence for the presence of phase-separated clusters in these materials. The temperature at which the fi relaxation occurs is found to result from the complex interaction of crystallinity and ionic group concentration. 

The adsorption of water has a tremendous effect on dielectric properties. In the case of ionomer salts, the a peak shifts to lower temperatures due to a plasticization effect on the ionic phase and an additional peak appears near — 43°C whose magnitude is approximately proportional to the content of water but whose position is only slightly affected by increasing water content. Model calculations of the relaxation strength using the dipole moment of water show that motions of the water molecule itself can account for this low-temperature peak. 

Thus for undiluted polymers the relaxation behaviour can be examined over a wider range of apparent frequencies. Similar functions can be constructed for other regions of the phase diagram and other rheological experiments. The method of reduced variables has not been widely tested for aqueous crosslinked polymers. Typically these are polyelectrolytes crosslinked by ionic species. Some of these give rise to very simple relaxation behaviour. For example 98% hydrolysed poly(vinyl acetate) can be crosslinked by sodium tetraborate. The crosslink that forms is shown in Figure 5.31. 

Following the introduction of basic kinetic concepts, some common kinetic situations will be discussed. These will be referred to repeatedly in later chapters and include 1) diffusion, particularly chemical diffusion in different solids (metals, semiconductors, mixed conductors, ionic crystals), 2) electrical conduction in solids (giving special attention to inhomogeneous systems), 3) matter transport across phase boundaries, in particular in electrochemical systems (solid electrode/solicl electrolyte), and 4) relaxation of structure elements. 

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