THE EFFECT OF COUNTERIONS ON THE CHAIN CONFORMATION OF POLYELECTROLYTES, AS ASSESSED BY EXTENSIBILITY IN ELONGATIONAL FLOW - THE INFLUENCE OF MULTIPLE VALENCY

An elongational flow technique was used to determine the effect of cou nterions on the chain conformation of polyelectrolyte molecules in sol ution, by means of the extensibility of the chains in the flow field. It is demonstrated that adding excess cations of seven low molecular w eight salts, NaCl, CaCl2, BaCl2, SrCl2, MgCl2, AlCl3, and SnCl4, to a very dilute solution of fully sulphonated polystyrene (NaPSS) reduces the extensibility of the chains, that is, the facility by which a chai n can be extended to varying degrees, an effect associated with chain contractions. In the case of multivalent counterions, these contractio ns, which with monovalent counterions are primarily due to screening o f charges by excess counterions, are greatly enhanced, which we attrib ute to the formation of intramolecular ionic bridges. When, in the cas e of multivalent counterions, the polymer concentration is increased, an inversion of the effect, namely increase in chain extensibility on addition of ions, is observed. We attribute this latter effect to the ionic bridges becoming increasingly intermolecular, leading to effecti vely large molecules, and eventually to a gel. All these effects were accentuated with increase in valency. They could also be accompanied b y precipitation which were of two kinds: one due to formation of insol uble ionic associations and a second attributable to enhanced hydropho bic interaction within the contracted chain itself. (C) 1994 John Wile y & Sons, Inc.