Phase Transitions and Spatial Organization in Nanoparticle-Block Copolymer Mixtures

Abstract

Introducing nanoparticles into nanostructured block copolymer phases can dramatically influence the polymer host. Computer simulations [Balazs et al., PRL, 89, 155503 (2002)] suggest inclusions can actually trigger transitions from one polymer phase to another. Simultaneously, the nanoparticles can be organized into complex superstructures, giving composite materials with novel mechanical, electrical and optical properties. Potential applications include catalysts, selective membranes and optical filters. We have developed a first principles theory predicting polymer phases and nanoparticle distributions. We find modification by nanoinclusions of the free energy of stretched polymer domains in lamellar, cylindrical or spherical geometries triggers structural changes and determines particle distributions. Our framework builds on Semenov's description of AB copolymers in the strongly stretched limit by incorporating nanoinclusions. Energy favors segregation into particle-rich regions, while entropy favors particle mixing into the energetically preferred block, say A. When entropy wins (small particles), an A-core cylindrical-to-lamellar phase transition is induced. Interestingly, large particles by contrast microphase separate into copolymer domains (analogous to Semenov's conclusions for homopolymer-copolymer mixtures) triggering reverse phase transitions (e.g. lamellar to A-core cylindrical). We present our results as a complete phase diagram.