Synthesis and viscometric properties of low charge density ampholytic ionomers

Abstract

The addition of ionic groups onto the molecular structure of a macromolecule produces marked changes in its physical properties. A large volume of literature exists pertaining to the solution and solid phase properties of these charged species spanning a broad range of charge densities. Until recently, however, little work has been published on ionomers containing opposite chargess (i.e., polyampholytes) except with regard to highly charged species. In this study, we focus on the synthesis and solution properties in fresh and high ionic strength solutions of low charge density acrylamide-based polyampholytes not necessarily possessing an equimolar ratio of anionic and cationic units. The results show that the separation of these oppositely-charged monomer units via neutral acrylamide moieties establishes a control over physical properties not readily attainable in the highly-charged polyampholytes (generally insoluble in fresh water) or conventional polyelectrolytes. At low charge densities, typically < 10 mole per cent, the solution properties of these polyampholytes are controlled primarily through intermolecular interactions. That is, gel-like properties are observed above the chain overlap concentration in fresh water due to the anionic-cationic interactions. As a result the viscosity decreases as the ionic strength of the solution is increased. Interestingly, at moderate charge densities, generally 10 mole per cent or higher, intramolecular interactions dominate behaviour in fresh water. As a result, the dilute solution viscosity in fresh water is relatively low, but is markedly enhanced as the ionic strength of the solution is increased. That is, gel-like properties are approached as the chains expand and begin to overlap. Moreover, these results are influenced by the nature of the chain backbone, molecular weight, the chemistry of the dissolved salt, and the level of mobile charge situated on the chain backbone.