(466b) Stabilization Of Complex Bicontinuous Phases By Reduction Of Packing Frustration In Diblock Copolymer Melts

Authors: 
Martínez-Veracoechea, F., Cornell University
Escobedo, F., Cornell University


Particle-based molecular simulations, together with a coarse-grained model for the diblock copolymer chains, are used to study the stabilization of different complex bicontinuous phases, in diblock copolymer melts. The stabilization approach entails attempting to reduce the packing frustration inside the bicontinuous phases nodes (formed by the minority "A" component of the diblock) by the addition of a "filler" with affinity for the A component. Two different strategies are considered: 1) the filler consists of small selective nanoparticles, and 2) the filler consists of homopolymer of a length equal to 80 % that of the diblock copolymer chains. Approximate phase boundaries were found via free-energy calculations and great care was taken to enact the commensurability of system size with the unit-cell dimensions of distinct candidate phases. A very dissimilar phase behavior is observed upon increasing the amount of the A component filler in the two different strategies. While with the first strategy (i.e., addition of small nanoparticles) we observed the progression Gyroid (G) → Perforated Lamella (PL) → Lamella (L) → Reversed-Gyroid (RG), including a long-lived metastable orthorhombic co-continuous phase known as O52. With the second strategy (i.e., addition of homopolymer) we observed the progression of morphologies G → Cylinders (C) → Double Diamond (DD) → Plumber's Nightmare (P). In both the DD and the P phases, the homopolymer concentrates preferentially in the nodes, suggesting the reduction of the nodes' packing frustration. In the same region where the P phase was found, a novel morphology was observed, wherein cylinders of two different diameters alternate in a tetragonal (square) packing; however, this "Alternating Diameter Cylinder" (ADC) phase seems to be metastable at the conditions examined. The contrasting difference in the resulting phase behavior for the two strategies considered is rationalized in terms of the difference in translational entropy exhibited by the homopolymer and by the small nanoparticles, which determines the way the "additive" molecules distribute in the A-component domain. Finally, transitions between different complex phases are achieved by rationally controlling the additive molecule's translational entropy, through changes in the additive's chain-length and architecture (but at constant additive volume fraction). This is the first particle-based study to successfully simulate the DD and the P phases in melt systems containing diblock copolymers.