Polyionic Complexes

Polyionic complexes (also called coacervates) form between complementarily charged polymer chains, i.e. polycations (e.g. polyamines, biopolymers) and polyanions (e.g. polyacids, DNA). The chains come together in water due to electrostatic attraction to form loose complexes or gels with hydrophobic domains (but without membrane). The release of counterions is the entropic driving force for this complex formation. While polyionic complexes are frequent components in nature in form of soft tissue, they are also very attractive for materials science and nanotechnology.

Schacher et al., Micellar Interpolyelectrolyte Complexes, Chem. Soc. Rev., 2012, 6888-6901

If block copolymers are used instead of homopolymers, IPEC formation creates nanostructures (micelles, morphologies, capsules) whose shape and properties depend on the block copolymer composition and the mixing ratio of the polyions.

We use interpolyelectrolyte complexation as a novel tool to create surface patterns and nanotopographies on micro- and nanoparticles. The latter is of particular interest in drug delivery as the surface structure of a drug carrier is paramount for the interaction of carriers with the cell membrane and thus the internalization process.

  • IPECs ontop of Nanoparticles

    Tethering one of the polyions onto the surface of nanoparticles changes the morphology of the IPEC. The surface grafted polyionic chain is oriented perpendicular to the surface and directs the growth of the nanostructure during IPEC formation. We could demonstrate this growth of surface topographies (3D surface reliefs) on spherical block copolymer micelles, rod-like cellulose nanocrystals, and fibre-like polymer brushes. The example of IPECs on spherical micelles is depicted below.

    Cryo-TEM images of block copolymer micelles with nanostructured shell
    © ACS

    These block copolymer micelles feature a polyacrylic acid corona that is complexed with PEO-b-Poly(L-lysine) or PEO-b-Poly{[2-(methacryloyloxy)ethyl] trimethylammonium iodide} to form a brush-on-brush system. High chain crowding within the particle periphery causes microphase separation, where morphologies orient perpendicular to the particle core, i.e. IPEC cylinders and lamellae stand upright on the curved surface. The images here were recorded on a 300kV cryogenic transmission electron microscope (cryo-TEM) that visualizes the particles in the vitrified state. Using electron tomography and 3D reconstruction provided valuable structural details and insight into the geometric arrangement. From volumetric calculations, we found that the observed morphologies follow classical rules of microphase separation, i.e. at φIPEC = 20 vol.% we obtained cylinders and φIPEC = 45 vol.% we obtained lamellae.

    Since the same concept also worked on cellulose nanocrystals as well as polymer brushes, IPEC formation could become a universal tool to equip the surface of nanoobjects with specific topographies. The capabilities are currently researched in our group.