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SELF-ASSEMBLY OF NANOPARTICLES VIA END-FUNCTIONALIZED TRIBLOCK COPOLYMERS

SELF-ASSEMBLY OF NANOPARTICLES VIA END-FUNCTIONALIZED TRIBLOCK COPOLYMERS. Rastko Sknepnek. In collaboration with: Joshua Anderson, Monica Lamm , Joerg Schmalian , and Alex Travesset. Motivation – magnetotactic bacteria

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SELF-ASSEMBLY OF NANOPARTICLES VIA END-FUNCTIONALIZED TRIBLOCK COPOLYMERS

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  1. SELF-ASSEMBLY OF NANOPARTICLES VIA END-FUNCTIONALIZED TRIBLOCK COPOLYMERS RastkoSknepnek In collaboration with: Joshua Anderson, Monica Lamm,JoergSchmalian, and Alex Travesset • Motivation – magnetotactic bacteria • Corse grained molecular dynamics • Molecular dynamics on graphics cards Schatz group meeting November 3, 2008 1/17

  2. Motivation Magnetotactic bacteria • possess ability to orient themselves in magnetic field • form magnetite naonocrystals of ~50nm in size: • each nanocrystal is surrounded by a membrane • each nanocrystal is superparamagnetic • nanocrystals form a chain which is a few hundred nanometer long 500nm (from T. Prozorov, et al. ACS Nano1, 228 (2007)) Schatz group meeting November 3, 2008 2/17

  3. Can growth and placement of magnetic nanoparticles be controlled? • Potential applications: • data storage • artificial muscles • drug delivery • block copolymers self-assemble into ordered structures at nanometer length scales • widely available and relatively easy to control • a lot of “in-house” experience with functionalizing Pluronics®. Schatz group meeting November 3, 2008 3/17

  4. Experiments on bacteria show that Mms6 peptide is present in the vesicles containing nanocrystals. (Arakakiet al., J. Biol. Chem., 278, 8745 (2003)) Experiments with Pluronic® copolymers show that Mms6 is crucial for successful growth of magnetite nanocrystals. (T. Prozorov, et al. ACS Nano1, 228 (2007)) Mms6: • 59 amino acids – 6kDa • 21 amino acid hydrophilic C-terminus and 38 amino acid hydrohibic N-terminus • structure is at present unknown • seems to form multimers (from T. Prozorov, et al. ACS Nano1, 228 (2007)) Develop a simple coarse-grained model and simulate it. What can we say about self-assembly via functionalized copolymers? Can we model this? Schatz group meeting November 3, 2008 4/17

  5. Coarse graining Reduce number of degrees of freedom by averaging sets of atoms into a single effective particle. Advantages • Simplifies the problem • Makes larger system sizes and longer times accessible to computer simulations Disadvantages • Loose information below coarse graining lengths and times • Usually possible to make only a qualitative comparison with experiments Schatz group meeting November 3, 2008 5/17

  6. Model Simple coarse-grained bead spring model with implicit solvent. Copolymer (CA5B7A5C) Nanoparticle Fully flexible bead-spring chain. Minimal energy cluster of Nnp Lennard-Jones particles (Sloane, et al. Discrete Computational Geom. 1995) 7 hydrophobic (B) 1.2Rg 2.1Rg 2.5Rg 12 hydrophilic (A) 2 functional (C) Nnp=13 Nnp=55 Nnp=75 radius of gyration Rg=2.3s Non-bonded interactions: Nanoparticle affinity eN is only tunable parameter! (set s=1, e=1, m=1) Schatz group meeting November 3, 2008 6/17

  7. Simulation details Molecular dynamics using LAMMPS. LAMMPS – S. Plimpton, J. Comp. Phys. 117, 1 (1995) (lammps.sandia.gov) Each simulated system contains: • p = 600 copolymer chains • n = 40 – 270 nanoparticles of size Nnp=13(1.2Rg), 55(2.1Rg), 75(2.5Rg) • all nanoparticles in a given system are monodisperse Explore phase diagram as a function of: • nanoparticle affinity eN (eN/kBT= 1.0, 1.5, 2.0, 2.5, 3.0) • packing fraction (f = 0.15, 0.20, 0.25, 0.30, 0.35) • NVT ensemble • reduced temperature T = 1.2 • harmonic bonds, k=330es-2, r0=0.9s • time step Dt = 0.005t (t=(ms2/e)1/2) • 107 time steps • relative nanoparticle concentration (c = 0.09, 0.12, 0.146, 0.17, 0.193, 0.215, 0.235) Schatz group meeting November 3, 2008 7/17

  8. Results Phase diagrams for Nnp=13 (1.2Rg) 1.2Rg Depending on the relative nanopaticle concentration one observes a large number of two- and three-dimensional periodic ordered structures . Square columnar order is fully suppressed and novel 3D layered hexagonal order appears. Two-dimensional square columnar order dominates phase diagram. Square columnar order yields to 2D hexagonal columnar and 3D gyroid order. nanoparticle concentration 18% 23% 10% Schatz group meeting November 3, 2008 8/17

  9. Results Square columnar ordering, Nnp=13 (1.2Rg) 1.2Rg square columnar 10% 18% • two-dimensional order • two interpenetrating “line-lattices” with lattice constant 9.5s. square columnar gyroid micellar liquid micellar liquid eN/kBT hexagonal columnar • dominates phase diagram for small NP concentration cylindrical mix disordered cylinders f f Geometric interpretation (top view) • closely related to the problem of close packing of binary disks 9.5s hydrophilic (Toth, Regular figures, 1964) hydrophobic size ratio = 0.414214 concentration = 1/2 functional nanoparticle Schatz group meeting November 3, 2008 9/17

  10. Results Hexagonal columnar ordering, Nnp=13 (1.2Rg) 1.2Rg layered hexagonal square columnar • two-dimensional order • micelles form two-dimensional “line-lattice” with lattice constant 11.5s. • nanoparticles fill space in between 18% 23% gyroid gyroid micellar liquid micellar liquid eN/kBT hexagonal columnar hexagonal columnar f f Geometric interpretation (top view) • closely related to the problem of close packing of binary disks 11.5s (Toth, Regular figures, 1964) hydrophilic size ratio = 0.349198 concentration = 6/7 hydrophobic functional nanoparticle Schatz group meeting November 3, 2008 10/17

  11. Results Gyroid ordering, Nnp=13 (1.2Rg) 1.2Rg • three-dimensional order • micelles and nanoparticles form two interpenetrating gyroids. • fully connected triply periodic structures • nanoparticles stabilize gyroid over a wide parameter range layered hexagonal 18% 23% square columnar gyroid gyroid micellar liquid micellar liquid eN/kBT hexagonal columnar hexagonal columnar f f • gyroid order confirmed by structure factor • order possess Ia3d symmetry hydrophilic hydrophobic functional nanoparticle Schatz group meeting November 3, 2008 10/17

  12. Density profile Hydrophobic part Combined density isosurfaces Nanoparticles Schatz group meeting November 3, 2008 12/17

  13. Results Layered hexagonal ordering, Nnp=13 (1.2Rg) 1.2Rg layered hexagonal 23% • three-dimensional layered ordered structure • spherical micelles form simple hexagonal lattice • nanoparticles form layers that resemble honeycomb • each nanoparticle layer is stacked between two micellar layers and vice verse. gyroid micellar liquid eN/kBT hexagonal columnar f (top view) (top view) (side view) simple hexagonal lattice hydrophilic honeycomb-like layers layered structure hydrophobic functional nanoparticle Schatz group meeting November 3, 2008 13/17

  14. Results Cubic (CsCl) and square columnar orderings, Nnp=75 (2.5Rg) 21% gyroid cubic (CsCl) 2.5Rg micellar liquid square columnar (square columnar, top view) • spherical micelles and nanoparticles form two simple cubic lattices • cubic lattices are shifted by (a/2,a/2,a/2) with respect to each other forming a CsClstructure • low packing fraction  non-trivial interaction effects hydrophilic hydrophobic functional (cubic) nanoparticle Schatz group meeting November 3, 2008 14/17

  15. Summary and Conclusions • Used a simple coarse grained model to study nanoparticle self-assembly mediated by end-functionalized triblock copolymers. • Extensively studied phase diagram of the nanocomposite system as function of nanoparticle size, concentration and affinity for copolymer functional ends. • Showed that end-functionalized triblock copolymer can provide a simple, but powerful strategy for assembling nanocomposite materials • very rich phase diagram with five distinct two- and three-dimensional ordered structures • each ordered structure has unique and rich properties • easy to tune between ordered structures by changing, e.g., nanoparticle concentration End-functionalized block copolymers are shown to provide an efficient strategy for assembly of nanocomposite materials. Sknepneket al., ACS Nano2, 1259 (2008) Schatz group meeting November 3, 2008 15/17

  16. Molecular dynamics on graphics cards When science meets video games… Video games: Molecular dynamics: Process large number of pixels in real time Calculate trajectories of large number of particles • A few simple operations per pixel • Pixels are independent – full data • parallelization. • A few simple operations per particle • Once forces are known, updates of particles’ positions, velocities, etc. are independent of each. Can we use graphics cards to do molecular dynamics? Graphics cards are designed for games Advantages of graphics cards: • widely available at low cost – less than $350 for latest NVIDIA GeForce 280GTX • well supported Disadvantage: • need to learn how to write programs that run on graphics cards Schatz group meeting November 3, 2008 16/17

  17. Highly Optimized Object Oriented Molecular Dynamics - HOOMD Originally developed in Prof. Alex Travesset’s group at Iowa State University, by Joshua A. Anderson. Features: • performs general purpose molecular dynamics • optimized for speed – delivers performance equivalent of 32 fastest CPU cores. • modern object oriented design makes it highly modular and easily expandable. • it is free, released under open source license You can get HOOMD free of charge at: http://www.ameslab.gov/hoomd/ Schatz group meeting November 3, 2008 17/17

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