Hello,
A yearly allocation for the LCRC cluster has been requested with the
following updated information:
Submitter/PI: Abelardo Ramirez-Hernandez
Project Name: nano-soft-assembly
Division: IME
Project title: Modeling Self Assembly in Nanostructured Soft Materials
Associated funding: PSE Strategic LDRD (FY2013-15)
Other Systems: ALCF Discretionary Project
Science: Polymeric and liquid crystal self-assembly have benefited considerably from theory and simulation; however, metastable, non-equilibrium morphologies, which are particularly relevant for applications in energy research, present new challenges. For example, the morphologies that arise on patterned substrates often represent non-equilibrium states, where the structure reflects the process of assembly. Available theoretical and computational approaches for description of dynamics and eventual end states in directed assembly processes with entangled polymer blends, block polymers, and liquid crystalline systems are simply inadequate. We are developing a fundamental understanding of structure and dynamics during directed self-assembly in nanostructured, macromolecular and liquid crystalline materials through the concerted use of theory and simulations, tightly coupled to experimental validation.
Project description: The self-assembly of soft materials provides a means to create well-defined structures 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 possible 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 and dynamical behavior of two important classes of systems: Polymers and Liquid crystals.
In the case of polymer systems, our interests lies on the equilibrium and non-equilibrium behavior of polymer brushes and block copolymers. Polymer brushes are particularly interesting because their responsiveness to environmental changes, such as temperature, stress, etc. Thus, by decorating surfaces with these brushes, macroscopic responses to specific environmental clues can be designed, for example, controlled wettability in space and time. One of our goals is to understand the physics of polyelectrolyte brushes in the presence of multivalent salt solutions, both at- and out-of-equilibrium. Experimental data suggest a complex scenario where polymer brushes’ response to the salt solution depends on the charge-state of the ions, grafting density, solvent quality and other factors. Experiments suggest inhomogeneous polymer brushes collapse at high grafting density with multivalent ions, however there is not theory or simulations explaining such inhomogeneities. To address
this problem, we will perform large-scale simulations of polyelectrolyte brushes in multivalent salt solutions, these calculations will provide with structural data but also with information about consequences on the dynamical level of the molecular details of brushes and salts. These simulations will require large system sizes (10^5 − 10^6 particles) for 10^6 − 10^7 time steps with explicit long-range electrostatic interactions, over a vast variety of salt concentrations, thus necessitating large computational resources. Preliminary results using LAMMPS show
good scaling on Blues, but we will also made use of the GPU-nodes available on Blues to accelerate the calculations, which at least doubles the speed of the explicit electrostatics k-space calculation, making this study possible.
Another polyelectrolyte system of our interests are charged block copolymers. These materials are a promising alternative for the next generation of batteries and photovoltaic devices, as they combine phase segregation and charge transport within domains sizes on the order of nanometers. As interesting as these materials are, there are very few works addressing the microscopic mechanisms of ion transport in these polyelectrolyte systems. We will perform atomistic simulations of two important diblock copolymer systems: PS-P2VP and PS-PMMA. We will study the transport mechanisms of ions both in amorphous and phase-segregate systems while varying functionalization percentage and relative humidity. Note that these simulations are very expensive as the atomistic description will also require explicit long-range electrostatic interactions to be computed. Thus, Blues is ideal for performing such simulations. We have found that Gromacs shows very good scaling for the atomistic model
used to represent these systems, and it will be used to perform such calculations.
On the technological application side, block copolymers are promising materials for the large-scale fabrication of devices with nanoscale features such as high-density memory devices and computer chips. Although there has been significant progress in the directed self-assembly of block polymers for lithographic patterning applications, important challenges remain with regards to development of materials and processes leading to perfect, defect-free assembly. Our minimum free-energy pathway calculations have demonstrated the importance of kinetics on the elimination of defects. Kinetic energy barrier heights are shown to strongly depend on the defect type and on processing conditions such as the type of chemical pattern or the strength of the segregation force between blocks. Thus, kinetics plays a very important role defining the structure that finally will appear under specific conditions. We have developed a coarse grained simulation approach that faithfully represents both
entangled and unentangled dynamics of homopolymers, our TIEPOS approach. We have demonstrated the ability of the TIEPOS approach to quantitatively predict the rheological behavior of polymer melts, both in linear as non-linear regimes. We will perform simulations using TIEPOS approach to explore how confinement and entanglements affect the kinetics of assembly in the presence of chemical patterns directing the structure formation. We will also study the effect of nematic interactions, between functional units distributed along the polymer backbones, on the microphase segregation kinetics and the directed assembly in the presence of chemical patterns.
Nematic liquid crystals (LCs) are anisotropic fluids that are aligned along the so-called nematic director. The equilibrium nematic director can be determined by minimizing the Frank free energy, leading to a uniform nematic director field in the bulk. The interaction between LC molecules and a surface, which is referred to as “anchoring”, can distort the overall nematic director. The long-range orientational order of nematic LCs amplifies the distortion of the nematic director over multiple length scale. The optical responses of the LCs are particularly sensitive to such distortions. The perturbations that disrupt the native orientation of the LCs at an interface, such as the adsorption of an amphiphile molecule at aqueous interfaces, lead to macroscopic reorganization of the LC which can be detected optically. Such triggerable reorientations allow label-free screening of biological events and enable a new generation of LC-based biological sensors.
Recent studies have shown that ordering transitions in nitrile-containing mesogenic molecules 4-cyano-4'-pentylbiphenyl (5CB) can be triggered by the self-assembly of specific amphiphiles near a flat aqueous-LC interface. In the absence of adsorbed amphiphiles, the bright interference colors of optical microscopy images indicate parallel orientation of the LC at the aqueous interface. Self-assembly of amphiphile molecules at the LC-aqueous interface triggers spontaneous reorientation of LC from parallel to perpendicular. The spatial and temporal reorientation of LC is recognized when the bright areas in microscopy images develop into dark domains (transition from Figure 1a to Figure 1b). These dark regions correspond to areas of the LC-aqueous interface at which the LC has adopted a perpendicular orientation. A number of observations indicate that the surfactant hydrophilic headgroup has marginal effect on the orientation of 5CB whereas the aliphatic tail structure, length, a
nd conformation greatly affects the ordering of the LC. The penetration of amphiphile at the interface into the LC after ordering transition disturbs the nematic directors, leading to a phase transition from nematic to isotropic. The LC exhibits a bright optical appearance in the isotropic phase. While past experiments have revealed the ordering transitions and many experiments have been performed to understand the effect of amphiphile structure on ordering transition, the underlying mechanisms of ordering transition remains elusive. Here, we propose to study the nature of molecular interactions between amphiphiles, 5CB, and water to understand the mechanism of ordering transitions. We will adopt the force field parameters of amphiphile molecules, and employ the AMBER-like united-atom force field of 5CB along with TIP4Q model of water. Our ultimate goal is to establish molecular design rules for amphiphiles that exhibit highly cooperative organizations within monolayers and
control the orientations of LCs at aqueous-LC interfaces. Specifically, we seek to understand the optimal chemical structure and size of the tail and head groups which can lead to a highly non-linear response in the LC as a function of the density of amphiphiles at the interface.
Recently, we have performed large-scale molecular dynamics (MD) simulations of 5CB hybrid channel by confining 5CB films between air and an aqueous medium. The system consisted of approximately 250,000 atoms, consisting of a combination of 8,000 liquid crystal molecules (5CB: 152,000 atoms), and 24,000 water (96,000 atoms). We examined the influence of water and 5CB interaction in orientation of 5CB molecules at the interface. Consistent with experiments, our results indicate that 5CB exhibits planar alignment at aqueous interfaces. The interactions between polar head of 5CB molecules and water at the interface modulates mutual orientations of 5CB and water in a narrow interfacial region. To study the interaction of amphiphile with 5CB, we will use the equilibrium structure of hybrid channel and insert a monolayer of an organic amphiphile molecule at the water-LC interface to trigger an anchoring transition from planar to homeotropic. Similar to lipid molecules, amphiphilic m
olecules consist of a hydrophilic head and a hydrophobic tail which stabilizes a monolayer at the LC-water interface. Insertion of amphiphiles at the LC-water interface largely modifies the orientation of 5CB molecules relative to their native configuration in the absence of surfactant molecules.
Our recent work utilizing LCRC resources has contributed to the following publications. Several more are currently being prepared to be submitted.
1. Simulation of Defect Reduction in Block Copolymer Thin Films by Solvent Annealing
S. M. Hur, G. S. Khaira, A. Ramı́rez-Hernández, M. Müller, P. F. Nealey and J. J. de Pablo
ACS Macro Lett. 4, 11 (2015).
2. Characterizing the Three-Dimensional Structure of Block Copolymers via Sequential Infiltration Synthesis and Scanning Transmission Electron Tomography
T. Segal-Peretz, J. Winterstein, M. Doxastakis, A. Ramı́rez-Hernández, N. Zaluzec, M. Biswas, J.
Ren, A. J. Liddle, J. W. Elam, J. J. de Pablo and P. F. Nealey
ACS Nano 9, 5333 (2015).
3. Theoretically Informed Monte Carlo Simulation of Liquid Crystals by Sampling of Alignment-Tensor Fields
J. C. Armas-Perez, A. Londono-Hurtado, O. Guzman, J. P. Hernandez-Ortiz and J. J. de Pablo
J. Chem. Phys. 143, 044107 (2015).
4. Interplay of Surface Energy and Bulk Thermodynamic Forces in Ordered Block Copolymer Droplets
S.M. Hur, M. S. Onses, A. Ramı́rez-Hernández, P. F. Nealey, J. A. Rogers and J. J. de Pablo
Macromolecules 48, 4717 (2015).
5. Self-consistent description of electrokinetic phenomena in particle-based simulations
J. P. Hernandez-Ortiz and J. J. de Pablo
J. Chem. Phys. 143, 014108 (2015).
6. Sculpting bespoke mountains: Determining free energies with basis expansions
J. K. Whitmer, A. M. Fluitt, L. Antony, J. Qin, M. McGovern and J. J. de Pablo
J. Chem. Phys. 143, 044101 (2015).
7. Molecular Pathways for Defect Annihilation in Directed Self-Assembly
S. M. Hur, V. Thapar, A. Ramı́rez-Hernández, G. S. Khaira, T. Segal-Peretz, P. A. Rincon-
Delgadillo, W. Li, M. Müller, P. F. Nealey and J. J. de Pablo
Proc. Natl. Acad. Sci. USA 112, 14144 (2015).
8. A Multichain Polymer Slip-Spring Model with Fluctuating Number of Entanglements for Linear and Nonlinear Rheology
A. Ramı́rez-Hernández, B. L. Peters, M. Andreev, J. D. Schieber and J. J. de Pablo
J. Chem. Phys. 143, 243147 (2015).
9. Liquid crystal free energy relaxation by a theoretically informed Monte Carlo method using a finite element quadrature approach
J. C. Armas-Perez, J. P. Hernandez-Ortiz and J. J. de Pablo
J. Chem. Phys. 143, 243157 (2015).
10. A molecular view of disclination line defects in nematic liquid crystals
M. Rahimi, R. Zhang, H. Ramezani-Dakhel, A. Ramı́rez-Hernández and J. J. de Pablo
Nature Communications, Under Review (2016).
11. Demixing by a Nematic Mean Field: Coarse-Grained Simulations of Liquid Crystalline Polymers
A. Ramı́rez-Hernández, S.M. Hur, J. C. Armas-Pérez and J. J. de Pablo
J. Polym. Sci. B Polym. Phys., Under Review (2016).
12. Understanding Atomic-Scale Behavior of Liquid Crystals at Aqueous Interfaces
H. Ramezani-Dakhel, M. Sadati, M. Rahimi, A. Ramı́rez-Hernández, B. Roux and J. J. de Pablo
Journal of Chemical Theory and Computation, Under Review (2016).
13. Multichain Polymer Slip-Spring Model with Fluctuating Number of Entanglements: Density Fluctuations and Confinement
A. Ramı́rez-Hernández, L. Schneider, B. L. Peters, M. Andreev, J. D. Schieber, M. Müller and J.
J. de Pablo
J. Chem. Phys., Under Review (2016).
14. A Detailed Examination of Entangled Lamellae-Forming Block Copolymers
B. L. Peters, A. Ramı́rez-Hernández, M. Andreev, M. Kröger, M. Müller, J. D. Schieber and J. J.
de Pablo
Macromolecules, Under Review (2016).
15. Particle Dynamics in Entangled Polymer Nanocomposites: A Coarse-Grained Description
Y. Su, A. Ramı́rez-Hernández, B. L. Peters, M. Andreev and J. J. de Pablo
J. Chem. Phys., Under Review (2016).
Industry partnership:
Project URL:
Current FY Hours Used: undetermined amount
New FY Requested allocation: 2000000
Q1: 500000
Q2: 500000
Q3: 500000
Q4: 500000
Justification: Over the past year of LCRC supported research we have optimized the efficiency of our codes. For these projects, we will made use of NAMD, LAMMPS, GROMACS and in-house codes using these software packages. We achieve excellent scaling, and make near-optimal use of LCRC resources.
Storage requirements: 1TB
Thank You,
The LCRC Accounts System
