Mark A. Horsch

Hydrodynamics and microphase ordering in block copolymers: Are hydrodynamics required for ordered phases with periodicity in more than one dimension?

We use Brownian dynamics (BD), molecular dynamics, and dissipative particle dynamics to study the phase behavior of diblock copolymer melts and to determine if hydrodynamics is required in the formation of phases with greater than one-dimensional periodicity. We present a phase diagram for diblock copolymers predicted by BD and provide a relationship between the inverse dimensionless temperature epsilon/k(B)T and the Flory-Huggins chi parameter, allowing for a quantitative comparison between methods and to mean field predictions. Our results concerning phase behavior are in good qualitative agreement with the theoretical predictions of Matsen and Bates [M. W. Matsen and F. S. Bates, Macromolecules 29, 1091 (1996)]; however, fluctuation effects arising from finite polymer lengths substantially alter the phase boundaries. Our results pertaining to the hydrodynamics are in contrast to earlier work by Groot et al. [R. D. Groot, T. J. Madden, and D. J. Tildesley, J. Chem. Phys. 110, 9739 (1999); D. Frenkel and B. Smit, Understanding Molecular Simulation, 2nd ed. (Academic, New York, 2001)]. In particular, we obtain the hexagonal ordered cylinder phase with BD, a method that does not include hydrodynamics.

Local ordering of polymer-tethered nanospheres and nanorods and the stabilization of the double gyroid phase

We present results of Brownian dynamics simulations of tethered nanospheres and tethered nanorods. Immiscibility between tether and nanoparticle facilitates microphase separation into the bicontinuous, double gyroid structure (first reported by Iacovella et al. [Phys. Rev. E 75, 040801(R) (2007)] and Horsch et al. [J. Chem. Phys. 125, 184903 (2006)], respectively). We demonstrate the ability of these nanoparticles to adopt distinct, minimal energy local packings, in which nanospheres form icosahedral-like clusters and nanorods form splayed hexagonal bundles. These local structures reduce packing frustration within the nodes of the double gyroid. We argue that the ability to locally order into stable structures is key to the formation of the double gyroid phase in these systems.

Self-assembly of end-tethered nanorods in a neat system and role of block fractions and aspect ratio

We report a computational study of the self-assembly of end-tethered nanorods in a neat system (no solvent). We present morphological phase diagrams for low and moderate aspect ratio rods as a function of inverse temperature vs. relative tether fraction. Our simulations predict that the end-tethered rods self-assemble into hexagonally arranged chiral cylinders, hexagonally perforated lamellae, monolayer and bilayer arrowhead structures and wavy lamellae. For high aspect ratio tethered nanorods and small tether fractions, we observe that the tethered nanorods self-assemble into smectic and zig-zag lamellar morphologies.

Self-Organization of Nanoscopic Building Blocks into Ordered Assemblies

We studied the self-assembly of nanoscopic building blocks comprised of polymer-tethered nanoparticles using computer simulation and predict that these building blocks can assemble into mono- and multi-layer sheets and shells. The simulations further demonstrate that for some nanoparticle geometries and tethered nanoparticle topologies, ideas from block copolymers, surfactants and liquid crystals can be used to predict the ordered morphologies attained via self- assembly and that for specific cases the morphologies are consistent with Israelachvili packing rules.

Teaching Computational Materials Science for Nanoscale Science and Engineering

We describe the development of a graduate level course designed to teach computational materials science and its application to nanoscale science and engineering. We discuss the use of MatDL, a web-based digital library and materials science resource, as a collaborative learning tool within the context of the course.