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The three possible states of polyelectrolyte complex coacervates : light and SANS signatures of the soluble, insoluble and macroscopic phase complexes.

Jean-Paul Chapel Centre de Recherche Paul Pascal (CRPP), Pessac

The complexation of oppositely charged polyelectrolytes (PEs) through electrostatic interaction is a ubiquitous associative process found in both natural and synthetic systems. Depending on the strength of the interaction, at charge stoichiometry the complexation proceeds either through a liquid-solid or a liquid-liquid phase separation leading to the formation of a solid precipitate/aggregate or a liquid complex coacervate, respectively. The aggregates are out-of-equilibrium structures very sensitive to the formulation pathway whereas the coacervate phase is in equilibrium with the polymer-poor phase (supernatant). Beyond the mixing method, the resulting morphologies are very sensitive to the individual PE features (weak/strong, charge density…) and external parameters like the nature and concentration of salt, temperature and pH of solution. We present in this work some experimental evidences of the local structures of the three different association states found in the PAA/PDADMAC polylectolyte coacervating system, i.e., soluble and dispersed PECs, together with the macroscopic coacervate phase. The distinctive SANS signatures of the various phases will help to disambiguate various morphologies found in a PE complex coacervate system. In particular, we show the unambiguous presence of the very controversial soluble complexes between PEs with a large chain length asymmetry. Indeed, with just few short guest chains, the long host chain holds the characteristic of a charged PE, that is, its water solubility in a thermodynamic sense. With more short chains, the hydrophobic segments start to associate and microphase separate generating dispersed PECs. The core-shell structure evolves into compact sphere as the mixing charge ratio Z approach 1. Soluble PECs are absent for more symmetric systems or in the presence of salt where only dispersed PECs are obtained. At stoichiometry (Z=1) complex coacervation occurs. This dense phase can be regarded as a network of random mixed polyion chains with a mesh size much smaller than the Rg of PE chains. An additional scattering maximum is found in our system at high q arising from the relatively stiff PDADMAC cylinders (non-electrostatic persistence length 3 nm), randomly distributed in the concentrated network as anticipated by the “jammed state” proposed by the Rawiso’s group.