Influence of Nonstoichiometry on the Viscoelastic Properties of a Polyelectrolyte Complex

Abstract

Depending on conditions such as the mixing ratio, speed and order of addition, and ionic strength, when solutions of oppositely charged polyelectrolytes are mixed, the stoichiometry of the formed polyelectrolyte complexes (PECs) can vary. In fact, for most conditions, some degree of nonstoichiometry is inevitable because the ratios of positive and negative charges do not “lock” themselves to 1:1, as was believed in some early work. PEC morphologies and mechanical properties depend on the molar ratio between the polycation and polyanion. In this work, poly­(diallyldimethylammonium) and poly­(styrene sulfonate) were used to make PECs with varying stoichiometries. PECs that were overcompensated with one polyelectrolyte were shown to have lower glass transition temperatures and moduli due to the plasticizing effect caused by increased water content. In addition, time–temperature superposition revealed a correlation between segmental relaxation times and stoichiometry. Nonstoichiometric PECs were shown to have elevated fractional free volumes and decreased polymer volume fractions, which are responsible for changes in their mechanical properties