Hello,
A change in allocation has been requested:
Requester: abelardo (Abelardo Ramirez-Hernandez)
Project: nano-soft-assembly
Title: Modeling Self Assembly in Nanostructured Soft Materials
Description: Understanding the fundamental principles of the collective phenomena leading to controlled assembly is central to molecular design of self-assembled materials with new and enhanced properties. The description of these materials requires to span several length and time scales, thus our work utilizes many different levels of description —atomistic detail, coarse-grained simulation, and continuum modeling— in order to probe assembly at those scales. The Blues cluster at Argonne’s Laboratory Computing Resource Center is ideal for performing mid-size simulations of self-assembly. We will use these simulations both to supplement work we have proposed as part of the ALCF INCITE program, and to produce independent results and publications. We request 1,000,000 hours to be used in fiscal year 2015.
The self-assembly of soft materials provides a means to create well-defined morphologies whose characteristic length scale range from nanometers to microns, for applications in nanoscience and technology. Self-assembly is an emergent property that arises as a consequence of collective phenomena occurring at a molecular level. It is also common to create ordering in a hierarchical way, where molecular assemblies are organized into a higher level and longer length scales. Therefore, a complete description of these systems requires an understanding of the interplay of all different length and time scales involved. For this reason, our work utilizes many different levels of description going from atomistic detail to coarse-grained and continuum models. With these, we can probe assembly at length scales from nanometers to several microns. Computational experiments are often able to examine relevant parameters more rapidly, cost-efficiently, and more thoroughly than that is possibl
e in traditional laboratory-based approaches. Experiment and simulation can thus coexist in a symbiosis where predictions are made, tested, and verified. In combination with experiments, simulations therefore provide a powerful tool for probing and explaining material behavior. Our efforts in this project will be focused on the self-assembly of two important classes of systems: Block copolymers (BCP) and Liquid crystals (LC).
Block copolymer molecules are chains composed of two or more chemically different polymers covalently bonded together. These materials self-assemble into structures whose dimensions range from 5 to 100 nm. The dimensions and symmetry of the structures depend on a few molecular parameters. In thin films, boundary conditions can play an important role on the block copolymer morphology. Lithographic applications of block copolymer films in the microelectronics and memory device industries require formation of morphologies with perpendicularly oriented domains and long-range order. By using molecular simulations we will predict conditions and material combinations to achieve long-range assembly of BCP. Long-range order implies that defects should occur with a very low probability. Thus, a prerequisite to achieve such order is to understand the origin and annihilation (relaxation) of such non-equilibrium states. In recent years, significant progress has been made in the directed s
elf-assembly of block copolymers for lithographic patterning applications, particularly regarding the equilibrium properties. However, important challenges remain with regards to development of materials and processes leading to perfect, defect-free assembly, both at equilibrium and far from equilibrium. A part of this project will be focused into explore the importance of kinetics on the elimination of defects, where an extraordinarily large thermodynamic driving force is not necessarily sufficient for defect removal. We will perform calculations of the minimum free energy pathway for defect annihilation in block copolymer thin films using the string method in combination with the theoretically-informed coarse-grained approach. We will explore the existence of kinetic energy barriers and their dependence on the defect type and on processing conditions such as the type of chemical pattern or the strength of the segregation force between blocks.
Most of the work on directed self-assembly of block copolymers has been focused, so far, on very thin films, which are appropriate for lithographic applications. However, recently the relevance of BCP materials to create photovoltaic devices, batteries and filtration membranes has been highlighted. For these kinds of applications, nanostructures with high aspect ratio are necessary. This means that the directed self-assembly should be achieved in thick films (several hundreds of nanometers thick or more). This is a new challenge because order should propagate not only along lateral directions but also over large distances from the guiding patterned substrate. For thick films, order can propagate both from “grains” nucleated at the free surface and from information encoded on the patterned surface at the bottom. To achieve long-range order through the whole film will require the reorganization of different grain boundaries. So far, very little is known about the mechanisms
involved on this reorganization and how they depend on film thickness. We will perform simulations to address these issues on thick films. Our predictions will be compared with experiments by Paul Nealey’s group.
Another interesting topic is the directed assembly of 3D structures; for most of the applications mentioned above lamellae- and cylinder-forming BCP, one- and two-dimensional objects respectively, are used. However, 3D structures, like spheres or bicontinuous morphologies, can offer new avenues to control and design the properties of BCP films. We will perform molecular simulations to predict conditions and material combinations to achieve long-range assembly for these materials, as well as to explore the effect of patterned-substrate properties on the equilibrium morphologies. For this and previous cases, our simulation predictions will also serve as an input to make predictions about the X-ray scattering patterns, which will then be compared to those obtained from Gisaxs experiments by the Nealey and Ferrier group. Thus, the research will be enhanced by the synergy between experimental work, simulations and theory.
Lastly, we will also explore the rheological behavior of BCPs, both in bulk and thin films. For this purpose we have developed a coarse grained simulation approach that faithfully represents both entangled and unentangled dynamics of homopolymers. We will perform a systematic comparison with experimental rheological data reported for a well-defined series of homopolymers, to establish the validity of our approach to describe and predict linear and non-linear rheology of polymer systems. Subsequently, we will perform simulations involving the rheological response of nanostructured BCPs, both neat as well as filled with nanoparticles.
Liquid crystals are fascinating materials in that molecular-level events can reorganize the ordering at mesoscopic length scales. For this reason, these materials are being used as novel detectors for toxic biomolecules, an example being bacterial endotoxin. Recent experimental studies have shown that when LCs are confined to spherical geometries (droplets), their behavior may be drastically changed. Consequently, a wide range of new morphologies are presented, opening the possibility of new promising technological applications. Experiments have shown that LC droplets are sensitive to endotoxin at concentrations as low as one picogram per milliliter, provoking a reorganization of the internal structure of the droplets, which allows their use as biosensors. The addition of nanoparticles inside, or on the surface, of the LC droplet could serve for hierarchical assembly of functional structures, as the interplay of elastic distortions, anchoring and confinement could result in n
ew complex morphologies. The effect of chirality, external fields, or confinement remains terra incognita to be explored, we will address some of these parameters by using both microscopic models (p.e. Gay-Berne-type model) and continuum descriptions (Landau–de Gennes free energy expansion).
Our recent work utilizing LCRC resources has contributed to the following publications. Several more are currently under review.
• Measuring liquid crystal elastic constants with free energy perturbations
A. A. Joshi, J. K. Whitmer, O. Guzm ́an, N. L. Abbott and J. J. de Pablo
Soft Matter 10, 882 (2014).
• Block-Copolymer Assembly on Nanoscale Patterns of Polymer Brushes Formed by Electrohydrodynamic Jet Printing
M. S. Onses, A. Ramirez-Hernandez, S. M. Hur, E. Sutanto, A. G. Alleyne, P. F. Nealey, J. J. de Pablo and J. A. Rogers
ACS Nano 8, 6606 (2014).
• Coarse-Grained Simulations of Defects Annihilation in Block Copolymer Thin Film via Solvent Annealing
S. M. Hur, G. S. Khaira, A. Ramirez-Hernandez, M. Muller, P. F. Nealey and J. J. de Pablo
Phys. Rev. Lett. submitted (2014).
• Directed-Assembly of Block Copolymer Films Using Topcoats
H. S. Su, A. Ramirez-Hernandez, J. J. de Pablo, P. F. Nealey et al.
ACS Nano, to be submitted (2014).
• Block Copolymers Droplets: The Interplay of Surface Energy and Ordering
S.M. Hur, A. Ramirez-Hernandez, M. S. Onses, P. F. Nealey, J. A. Rogers and J. J. de Pablo
ACS Macro Lett., to be submitted (2014).
• Nematic-Field-Driven Positioning of Particles in Liquid Crystal Droplets
J. K. Whitmer, X. Wang, F. Mondiot, D. S. Miller, N. L. Abbott and J. J. de Pablo
Phys. Rev. Lett. 111, 227801 (2013).
• Modeling the polydomain-monodomain transition of liquid crystal elastomers
J. K. Whitmer, R. Shekhar, T. F. Roberts, N. L. Abbott and J. J. de Pablo
Phys. Rev. E 87, 020502(R) (2013).
• Liquid-Crystal Mediated Nanoparticle Interactions and Gel Formation
J. K. Whitmer, A. A. Joshi, T. F. Roberts and J. J. de Pablo
J. Chem. Phys. 138, 194903 (2013).
Current: undetermined amount
Justification: Over the past year of LCRC supported research we have optimized the efficiency of our codes to near perfection. We achieve excellent scaling, and make near-optimal use of LCRC resources. We also note that much of the work proposed here will be used in the preparation of a Software Center Proposal to be submitted to DOE in the Spring.
Requested: 1000000
A specific reason has been given:
We are currently performing large-scale simulations of unexplored phenomena in liquid crytals and block copolymers using NAMD and LAMMPS. This simulation packages run efficiently on Blues.
This needs to be approved and the final allocation amount decided upon.
Thank You,
The LCRC Accounts System
