Ionic Viscoelastic Materials

This paper has reviewed several studies, particularly those performed on the phosphate polymers, in which ionic forces were imjx>rtant in determining the properties or behavior of the material. Only very few properties were investigate among these were 1) the glass transitions as a function of the molecular we ht, the nature of the terminal group, and the nature of the counterion, 2) the viscoelastic properties as a function of the counterion and 3) the viscoelastic relaxation mechanism, with specific emphasis on the separation of the a and mechanisms. In this treatment, a deliberate attempt was made to present pertinent theories, insofar as they exist, but it seems evident that here as much work remains to be done as on the experimental level, if not more. 

The ionic groups, although present in small amoimts, dominate the viscoelastic behavior of ionomers, their transport properties and their ability to sorb a variety of solvents moreover, the ion effect is specific. In terms of morphology, the presence of ions leads to microphase separation into ionic and nonpolar domains. Increasing interest in structural aspects of ionomers is closely related to their numerous applications as bulk materials, in various devices, as catalysts, in controlled release systems, and as proton exchange membranes (PEM) in fuel cells (71). 

The introduction of small amounts of bonded ionic groups into a hydrophobic polymer normally produces an increase of the glass transition temperature (Tg), melt viscosity and the characteristic relaxation times of the polymer. The microstructure of these materials, i.e., ionomers, is generally agreed to be characterized by nanophase-separated aggregates, rich in the ionic species, and which behave as physical crosslinks between chains [1]. These ionic crosslinks can significantly modify the viscoelastic behavior of an ionomer [2]. 

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