LOWRIE LECTURE I: Matthew Tirrell

Dean and Founding Pritzker Director of the Institute for Molecular Engineering, University of Chicago

Polyelectrolytes in multivalent ionic media: New physics and new materials

 

            Multi-valent interactions in systems of polyelectrolytes can exhibit dramatic, non-monotonic effects, for example, switching forces from repulsive to attractive, and back to repulsive again, in some cases. We have been studying these patterns of behavior with the surface forces apparatus (SFA) and with electrochemical methods, such as cyclic voltametry, which enables the quantitative determination of the number of multi-valent ions residing in thin layers of charged polymers. At fixed ionic strength, all cause strong shrinkage and condensation of poly(styrene sulfonate) brushes over a narrow range of ratio multi-valent to mono-valent ions. When the multi-valent ion is an oppositely charged polymer, new fluid phases can form. We have clarified the quantitative aspects of the phase diagram for a simplified polyelectrolyte complex system. See Figue 1. Charged blocks in copolymers leads to materials with new types of ordered phases. Effects of these multi-valent interactions on supermolecular and biomolecular assembly will be discussed. There are many possibilities for the creation of new materials based on electrostatic assembly involving multi-valent interactions.

Figure 1.  Complete polyelectrolyte complexation phase diagrams. (a) Phase diagram for solutions comprising PLK100 and PRE100. Two experimental approaches were employed to map the phase boundaries (blue symbols) and were in excellent agreement with the simulation results (red  symbols). The filled circles (complex phase) and open circles (supernatant phase) are the results from TGA;  are the results from salt resistance methods. Representative tie-lines are shown as dotted lines to demonstrate their negative slope, which are contrary to the predictions from the Voorn-Overbeek theory. (b) Phase diagram for PLK20+PRE20 (red), PLK50+PRE50 (yellow), and PLK400+PRE400 (green). The solid (complex phase) and open (supernatant phase) squares, diamonds and triangles are the results from TGA on the three respective systems;  are the results from salt resistance methods. The overall shape of the binodal phase boundary was preserved, while the two-phase region became larger with increasing polymer chain lengths.