Outline Introduction Project goals Personnel Graduate students - PDF document

1 Multiscale Simulation of the Synthesis, Assembly and Properties of Nanostructured Organic/Inorganic Hybrid Materials Peter T. Cummings 1 , Sharon C. Glotzer 2 , John Kieffer 2 , Clare McCabe 3 , Matthew Neurock 4 1 University of Tennessee, 2 University of Michigan, 3 Colorado School of Mines, 4 University of Virginia NSF DMR Project DMR-0103399 2 Outline • Introduction – Project goals • Personnel – Graduate students and post-doctoral researchers • Progress to date – Focus on • Ab initio studies • Force field development • Mesoscale models and methods • Molecular theory • Conclusions 1

3 Introduction • Background – Successful control of nano-scale materials fabrication requires understanding of atomic- and nano-scale processes taking place during self-assembly • Understanding, design, prediction and control – Recently developed hybrid organic/inorganic materials composed of nanostructured polyhedral oligomeric silsesquioxanes (POSS) molecules offer unique opportunities for creating tailored nanostructured materials 4 Introduction • Polyhedral oligomeric silsesquioxanes (POSS) – Initial focus on cubic POSS as basic nano building block • (HSiO 1.5 ) 8 H • Most experimental data – Extremely versatile O • Functionalized in many ways – Functionalization affects solubility, diffusivity, rheology,… Si • Cross-linked to create network structures • “Alloyed” with polymer – Nanocomposites – Can be synthesized on large scale • Hybrid Plastics [(RSiO 1.5 ) 8 ] 2

5 Introduction • Project Goals – Development and application of multi-scale computational framework to simulate synthesis and self- and guided assembly of hybrid organic/inorganic materials • From electronic structure methods through to mesoscale modeling – Design of new materials based on POSS molecules • Collaboration with experimentalists (Rick Laine at Michigan and Joe Lichtenhan at Hybrid Plastics for synthesis, Chris Soles and Eric Lin at NIST for characterization) – Development of strategies for controlling self-assembly of nano-structured materials – Interdisciplinary team • Methods: Electronic structure, atomistic simulation, mesoscale modeling, molecular theory • Materials: hard (silica), soft (polymer) 6 Personnel • Graduate students – Elaine Chan 1 , Tudor Ionescu 2 , Cheng-Ying Lee 3 , Feng Qi 1 , Charles Zhang 1 , Jinhua Zhou 1 • Post-doctoral researchers – Jean-Sébastien Filhol 3 , Monica Lamm 1 , Hung-Chih Li 2,4 1 University of Michigan 2 University of Tennessee, 3 University of Virginia 4 Colorado School of Mines 3

7 Progress to date • Initial foci – Ab initio studies of POSS • Single POSS molecule (RSiO 1.5 ) 8 H 8 • Structure of oligomers (RSiO 1.5 )H 7 R where R=alkane • Reactivity of POSS molecules – Force field development/verification • POSS and POSS+alkane molecule structure • Reactive force field – Development of mesoscale model and methods 8 Ab initio studies of POSS • Ab initio methods – VASP (Vienna ab initio simulation package) • Plane wave DFT using PW91 exchange correlation – DMOL (Accelrys) • Atomic orbital DFT using PW91 exchange correlation – Gaussian 98 • Molecular mechanics/dynamics on classical force fields – Universal force field (Rappé and Goddard, 1992) – Compass force field (Accelrys) – Kieffer reactive force field 4

9 Ab initio studies of POSS • Structure of POSS(H) 8 cube Molecular Mechanics Plane Atomic RHF Exp. Wave Orbital (cc-pVdz) UFF Cerius 2 JK (VASP) (DMOL) (GAUSSIAN 98) Compass RFF Si-O (Å) 1.619 1.630 1.654 1.650 1.592 1.624 1.64 1.462 Si-H (Å) 1.48 1.463 1.481 1.470 1.473 1.47 Si-O-Si 147.5˚ 146.7˚ 145.9˚ 148.7˚ 146.8˚ 146.9˚ 146.1˚ 109.1˚ O-Si-O 109.6˚ 109.6˚ 109.6˚ 110.0˚ 110.2˚ 109.6˚ 10 Ab initio studies of POSS • Frontier orbitals of POSS(H) 8 LUMO HOMO Method Gap (DFT) Atomic Orbital 7.39 Plane Wave 6.07 5

11 Ab initio studies of POSS • Electronic structure of charged POSS – Structure of LUMO of the cubic POSS • Centered in middle of POSS cavity • Able to capture an electron in center of cavity – POSS(F) 8 can very stably store electron in middle of POSS cavity • Yields molecular colored center 2 POSS(H) 8- 1 POSS(H) 82- 2 POSS(F) 8- Electro-affinity (eV) -0.195 1.912 -1.342 12 Ab initio studies of POSS • High energy collision of atomic O with POSS(H) 8 – High energy : around 7 eV • Poss structure is never broken – Very stable over the simulation time (0.5 ps) • OH group is lost • Highly reactive Si site is formed (activation of the POSS) – Insertion of a O inside structure with very complex pathway • Very high final local temperature (1200K) 6

13 Ab initio studies of POSS • Low energy (4eV) collision of atomic O with a POSS – Atomic O inserted in POSS structure: • Insertion in O-H bond (yielding a silanol) • Insertion in Si-O (yielding a peroxide) – Energy is transferred to vibrations of POSS stabilizing newly formed bond – Increase of oxygen ratio in the structure 14 Ab initio studies of POSS • Fragmentation of alkyl chains – Atomic O tends to insert inside Si-C bonds • Energy liberated tends to induce fragmentation of alkyl chain • POSS cube doesn’t have time to absorb energy induced by collision • Alkyl chain is destroyed (and oxidized) by exposure to atomic oxygen 7

15 Force Field Development • Force field development for atomistic simulations – Ab initio calculations to determine bond stretch, bond bending, and torsional potentials • POSS cube • Alkyl groups attached to POSS cubes – Impact on POSS cube structure – Impact of POSS on alkyl force field – Parametrization of reactive force field 16 Force Field Development • Structure of tethered POSS – Ethyl-POSS [(SiO 1.5 ) 8 H 7 CH 2 CH 3 ] • Effect of tethered group on POSS structure is localized and minor POSS tethered (1) Si-O 1.650 1.655 (2) Si-O 1.650 1.647 (3) Si-O 1.650 1.650 (1) O-Si-O 109.1° 108.3° (2) Si-O-Si 148.7° 149.4° 8

17 Force Field Development • Structure of tethered POSS – Propyl-POSS [(SiO 1.5 ) 8 H 7 CH 2 CH 3 ] • Effect of tethered group on POSS structure is localized and minor POSS tethered (1) Si-O 1.650 1.656 (2) Si-O 1.650 1.646 (3) Si-O 1.650 1.650 (1) O-Si-O 109.1° 108.2° (2) Si-O-Si 148.7° 149.8° 18 Force Field Development • Rotational barrier along Si-C bond in ethyl- POSS 1.6 1.4 1.2 Energy (kcal/mol) 1.0 0.8 0.6 0.4 0.2 0.0 0 45 90 135 180 Rotation Angle 9

19 Force Field Development • Torsional energy profile along dihedral angle Si-C-C-C in propyl-POSS 6 5 Energy (kcal/mol) 4 3 2 1 butane 0 0 45 90 135 180 Rotation Angle 20 Force Field Development • T 8 POSS dynamic properties as modeled by charge-transfer potential function – Reactive force field d (Si-O-Si) 399 n (Si-O) 465 d (O-Si-O) 566 for crosslinking experiment IR Absorption (a.u.) d (O-Si-H) 881 4 10 -18 1 n (Si-O) 1140 repulsive n (Si-H) 2276 charge transfer Charge transfer 2 10 -18 0.5 function f (r i j ) (J) 0 0 -2 10 -18 Coulomb covalent simulation attractive -4 10 -18 total -6 10 -18 0 0.1 0.2 0.3 0.4 0.5 0.6 r i j (nm) 0 500 1000 1500 2000 2500 –1 ) wavenumber (cm 10

21 Crystalline structures of T 8 POSS From x-ray analysis Simulated at 100 K Simulated at 350 K 22 Cluster of T 8 POSS cages • Density 0.78 g/cc, temperature 300K 11

23 Clustering of tethered POSS 24 “Mesoscale” simulations of Tethered POSS Development & Implementation • Key challenge in simulating assembly is long length and time scales characterizing assembly process. – To simulate assembly of POSS-based networks consisting of large collections of tethered POSS cubes, coarse-grained, classical models are being developed for simulation using three complementary methods • Molecular dynamics – Oligomer bead-spring tethers have “sticky” ends • Monte Carlo Atomistic Model Coarse-grained Model – Validates structures obtained by MD – More efficient route to equilibrium • Lattice Monte Carlo – Model adopted to 3-d cubic lattice 12

25 MD Simulations of POSS/Polymer Systems: Non-reacting POSS T 8 w/trimer tethers Small trial system. Goal: large Monte Carlo systems of code has been >1000 cubes developed to w/tethers of simulate various sizes identical model to study, e.g. to efficiently role of tether generate length in equilibrium assembly & structures for network comparison & structure. validation. E.R. Chan, M.H. Lamm, SCG 26 MD Simulations of POSS/Polymer Systems: Network formation POSS T 8 cubes with trimer tethers Non-crosslinked Crosslinked Next: • develop code for network structure analysis • validation against expt, MC simulation and LMC simulation • large-scale parallel simulations of large systems 13