(540b) On the Morphology of Bicontinuous Phases in Blends of Diblock Copolymer and Homopolymer
We characterize the spontaneous development of morphologically complex phases in binary blends composed of an asymmetric diblock copolymer and a homopolymer with affinity to the minority block. The addition of homopolymer to a well-segregated copolymer melt has a drastic effect in the formation of microstructured bicontinuous phases, i.e., having periodic dividing interfaces without self-intersections. Such phases are of technological relevance for applications in which nanoporous materials with high internal surface area and periodic connectivity are desirable, e.g., templates for high conductivity composites, photonic materials and photovoltaic systems (solar cells). We use particle-based simulations to explore regions of the phase diagram, as function of the homopolymer volume fraction and the homopolymer/copolymer size ratio, where mainly triply periodic phases are predicted to be stable using self-consistent field theory calculations. The stability regions of the phases spontaneously formed in the simulations are in qualitative agreement with those predicted by the theory, and are primarily dominated by interfaces having symmetries corresponding to the diamond (D) and primitive (P) Schwarz surfaces; although other metastable bicontinuous and cocontinuous phases are also observed, some not previously seen before for diblock copolymer melts. At higher volume fractions of homopolymer, the conditions for phase equilibria between the stable bicontinuous phases and a homopolymer-rich phase are investigated with the expanded ensemble formalism. The thermodynamic conditions leading to the stabilization of the D and P phases are correlated to the topological characteristics of the interfaces, and their fluctuations, by following the evolution of geometrical measures during the spinodal decomposition in the particle-based simulations. Some preliminary results are also presented for the formation of bicontinuous phases in composites of diblock copolymers and nanoparticles of different sizes; i.e., for systems where nanoparticles replace the homopolymer in the blends discussed above.