Physical gels based on charge-driven co-assembly

Gels are used in a variety of products ranging from personal care products and food products to explosives. An important area where aqueous physical gels are applied is the water-based coatings industry. Currently, classical associative thickeners are used to form transient networks based on hydrophobic interactions. Although this technology has greatly improved the properties of water-based coatings, there remain some problems related to the use of these classical associative thickeners. In this Thesis we investigate aqueous physical gels based on interconnected polyelectrolyte complex micelles, as a new two-component associative thickener. The physical gels are prepared from an ABA triblock copolymer, with charged A-blocks and a neutral hydrophilic B-block, mixed with either an oppositely charged homopolymer or nanoparticle. Electrostatic interaction is the driving force for association of the oppositely charged components, leading to transient networks of interconnected flowerlike micelles or particles, depending on the origin of the oppositely charged component. The fact that we deal with a two-component system, as well as a completely different driving force for association, could potentially solve some of the current problems related to the use of the classical associative thickeners. Throughout this Thesis we have tried to link the microstructure of the gels to the macroscopic properties. We do so by combining microscopic experimental techniques, such as (dynamic) light scattering, small-angle X-ray scattering and (cryo-) scanning electron microscopy with macroscopic experimental techniques such as rheometry. In chapter 2 we show that we successfully prepared an aqueous multi-responsive reversible gel based on the bridging of polyelectrolyte complex micelles. At low concentrations these two oppositely charged polymers co-assemble spontaneously to form flowerlike polyelectrolyte complex micelles. If two micelles come close enough to each other, the micelles can become connected to each other, because a triblock copolymer can stick both end-blocks in two different micellar cores. This bridging is reversible, meaning that the micelles are continuously connected and disconnected. At high concentrations enough micelles become interconnected to form a percolating path through the sample, hence the solution becomes a physical gel. Due to the electrostatic driving force for the co-assembly of micelles, these gels are truly multi-responsive. The influence of the charge ratio on the formation of polyelectrolyte complex micelles and their networks is studied in detail in chapter 3. Our measurements suggest an asymmetry, with respect to the charge stoichiometric point, in the shape and size of the co-assembled complexes. In chapter 4 we take a closer look at the network topology and how this is influenced by total polymer concentration and salt concentration. In chapter 5 we show that it is possible to use charged inorganic nanoparticles as nodes in a transient network bridged by triblock copolymers with charged end-blocks. Since gels of this kind have never been described before, we named this class of gels 'Complex Composite Gels'.