ABSTRACT
Acknowledgements............................................................................ 146
Glossary of Sym bols.......................................................................... 146
References........................................................................................... 148
1 Introduction
1.1 Ionomer Solutions
Ionic polymers that contain a relatively small number of ionic groups (up to 10-15 mol%) in nonionic backbone chains are referred to as ionomers. A typical example is a lightly sulfonated polystyrene (SPS) ionomer, in which several mol% of repeat units carry sulfonate groups (typically Na salt). Because of significant changes in physical properties caused by in corporation of ionic groups (and ionic bonds) into polymer chains, much
work has been devoted to elucidating the structure-property relationship of ionomers in the solid state. 1-5 Compared with the work conducted on solid state behaviour, relatively little has been done to study the structureproperty relationship of ionomer solutions. This is in marked contrast to the situation of another class of ionic polymers, poly electrolytes, where major interest has been concentrated on (aqueous) solution properties.6-11 During the past decade, however, ionomer solutions have begun to be actively studied because of the realization of their unique behaviour; and interesting results have been reported. 12-16 Since a definite distinction between polyelectrolytes and ionomers has not been established, here we follow the usual terminology that is based on a material: 17 ionic polymers are referred to as ionomers when the concentration of ionic groups is too low for water solubility, while they are called polyelectrolytes when they have sufficient number of ionic groups to be water soluble. Naturally, ionomer solutions deal with non-aqueous solutions. However, ionic polymers containing a small number of ionic groups, dissolved in a polar solvent, may also be called weakly-charged polyelectrolytes according to the terminology that is based on behaviour. 17
It is now well established that ionomers show two types of behaviour depending primarily on the polarity of the solvent: 14,18 one is polyelec trolyte behaviour arising from electrostatic interactions among ions (fixed ions and counterions) in a polar solvent, such as dimethylformamide (DMF) (e = 37), where many of the counterions are dissociated; another is aggregation behaviour arising from dipolar attractions of ion pairs in a non-polar or low-polarity solvent, such as toluene (e = 2.4) or tetrahydrofuran (THF) (s = 7.6), where many of the counterions are asso ciated with fixed ions to form ion pairs. Here, e is the dielectric constant of the solvent.
