Structural Evolution of Polyelectrolyte Complex Core Micelles and Ordered-Phase Bulk Materialsby Daniel V. Krogstad, Nathaniel A. Lynd, Daigo Miyajima, Jeffrey Gopez, Craig J. Hawker, Edward J. Kramer and Matthew V. Tirrell
The kinetics of formation and structural evolution of novel polyelectrolyte complex materials formed by the assembly of water-soluble di- and triblock copolymers, with one neutral block and one (or two) cationic or anionic blocks, have been investigated. Comparison was made between the assembly of ABA and AB′ copolymers in which A represents the ionic blocks and B and B′ are the neutral poly(ethylene oxide) blocks. The degree of polymerization of B was twice that of B′ and the ionic A blocks were of equal degrees of polymerization in all polymers. The mechanism and speed of the assembly process, and the organization of these domains, was probed using dynamic mechanical spectroscopy and small-angle X-ray scattering (SAXS). SAXS revealed that the equilibrium morphologies of both the diblock copolymer and the triblock copolymer materials were generally qualitatively the same with some apparent quantitative differences in phase boundaries, possibly attributable to lack of full equilibration. Slow kinetics and difficulties in reaching equilibrium phase structures, especially in triblock materials, is a principal message of this article. Detailed analysis of the SAXS data revealed that the triblock copolymer materials formed ordered phases via a nucleation and growth pathway and that the addition of small amounts (∼20%) of corresponding diblock copolymers increased the rate of structure formation and enhanced several key physical properties.