Dynamic Mechanical Properties of Networks of Wormlike Micelles Formed by Self-Assembled Comblike Amphiphilic Copolyelectrolytes

Abstract

The rheological properties of viscoelastic aqueous solutions of wormlike micelles formed by the self-assembly of comblike copolyelectrolytes have been investigated by flow and dynamic measurements. The comblike polymers consisted of a polystyrene backbone grafted with a fixed amount of pendant <i>N</i>,<i>N</i>-dimethyl quaternary ammonium alkyl groups of various lengths ranging from C12 up to C18. Upon increasing concentration, the increase in size of the wormlike micelles and their branching results in the formation of a system spanning network through a percolation process at a critical concentration that decreases when salt is added or when the temperature is decreased. In this manner transient gels are formed with a viscoelastic relaxation time that does not depend on the polymer concentration or on the ionic strength, but their elastic modulus increases with increasing polymer or salt concentration. When the size of the alkyl groups is increased from C12 to C16, the relaxation time increases very strongly, but the temperature dependence remains characterized by the same activation energy. For C18, the systems are frozen at least up to 80 °C