Transient network topology of interconnected polyelectrolyte complex micelles

Abstract

In this paper we study transient networks formed by co-assembly of an ABA triblock copolymer with charged A blocks and neutral water-soluble B blocks, and an oppositely charged homopolymer. Above the CMC, the polymers associate into flowerlike micelles consisting of a polyelectrolyte complex core and a neutral corona of loops formed by the middle blocks. At higher concentrations, the micelles become connected to each other by bridging middle-blocks, leading to a physical network with visco-elastic properties. We use a combination of (dynamic) light scattering, small-angle X-ray scattering and rheometry, to characterize how the network structure changes with polymer concentration and ionic strength, and how these changes affect the macroscopic elastic properties of the network. We find that an increase in ionic strength leads to a decrease in aggregation number and size of the individual micelles, which simultaneously leads to an increase in the number density of micelles. These two effects compensate each other such that the probability of bridge formation is equal at all salt concentrations, meaning that the elastic modulus of the gels is independent of salt concentration.