Outline Introduction Project goals Personnel Graduate students - - PDF document

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Outline Introduction Project goals Personnel Graduate students - - PDF document

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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

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Multiscale Simulation of the Synthesis, Assembly and Properties of Nanostructured Organic/Inorganic Hybrid Materials

Peter T. Cummings1, Sharon C. Glotzer2, John Kieffer2, Clare McCabe3, Matthew Neurock4

1University of Tennessee, 2University of Michigan, 3Colorado School of Mines, 4University of Virginia

NSF DMR Project DMR-0103399

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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
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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

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H O Si

Introduction

  • Polyhedral oligomeric silsesquioxanes (POSS)

– Initial focus on cubic POSS as basic nano building block

  • (HSiO1.5)8
  • Most experimental data

– Extremely versatile

  • Functionalized in many ways

– Functionalization affects solubility, diffusivity, rheology,…

  • Cross-linked to create network structures
  • “Alloyed” with polymer

– Nanocomposites

– Can be synthesized on large scale

  • Hybrid Plastics

[(RSiO1.5)8]

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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

  • f nano-structured materials

– Interdisciplinary team

  • Methods: Electronic structure, atomistic simulation,

mesoscale modeling, molecular theory

  • Materials: hard (silica), soft (polymer)

6 1University of Michigan 2University of Tennessee, 3University of Virginia 4Colorado School of Mines

Personnel

  • Graduate students

– Elaine Chan1, Tudor Ionescu2, Cheng-Ying Lee3, Feng Qi1, Charles Zhang1, Jinhua Zhou1

  • Post-doctoral researchers

– Jean-Sébastien Filhol3, Monica Lamm1, Hung-Chih Li2,4

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Progress to date

  • Initial foci

– Ab initio studies of POSS

  • Single POSS molecule (RSiO1.5)8H8
  • Structure of oligomers (RSiO1.5)H7R 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

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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

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Ab initio studies of POSS

  • Structure of POSS(H)8 cube

Molecular Mechanics 109.6˚ 146.1˚ 1.47 1.64 JK RFF 110.2˚ 146.9˚ 1.473 1.624 Cerius2

Compass

109.6˚ 147.5˚ 1.48 1.619 Exp. 110.0˚ 146.8˚ 1.470 1.592 UFF

109.1˚ 148.7˚ 1.462 1.650 RHF

(cc-pVdz) (GAUSSIAN 98)

109.6˚ 109.6˚ O-Si-O 145.9˚ 146.7˚ Si-O-Si 1.481 1.463 Si-H (Å) 1.654 1.630 Si-O (Å) Atomic Orbital

(DMOL)

Plane Wave

(VASP)

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HOMO LUMO

Ab initio studies of POSS

  • Frontier orbitals of POSS(H)8

6.07 Plane Wave 7.39 Atomic Orbital Gap (DFT) Method

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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
  • 1.342

1.912

  • 0.195

Electro-affinity (eV)

2POSS(F)8- 1POSS(H)82- 2POSS(H)8-

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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)

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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

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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
  • xidized) by exposure to atomic
  • xygen
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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

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Force Field Development

  • Structure of tethered POSS

– Ethyl-POSS [(SiO1.5)8H7CH2CH3]

  • Effect of tethered group on POSS structure is localized

and minor

1.647 1.650 (2) Si-O 108.3° 109.1° (1) O-Si-O 149.4° 148.7° (2) Si-O-Si 1.650 1.650 (3) Si-O 1.655 1.650 (1) Si-O tethered POSS

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Force Field Development

  • Structure of tethered POSS

– Propyl-POSS [(SiO1.5)8H7CH2CH3]

  • Effect of tethered group on POSS structure is localized

and minor

1.646 1.650 (2) Si-O 108.2° 109.1° (1) O-Si-O 149.8° 148.7° (2) Si-O-Si 1.650 1.650 (3) Si-O 1.656 1.650 (1) Si-O tethered POSS

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Force Field Development

  • Rotational barrier along Si-C bond in ethyl-

POSS

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 45 90 135 180 Rotation Angle Energy (kcal/mol)

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Force Field Development

  • Torsional energy profile along dihedral

angle Si-C-C-C in propyl-POSS

1 2 3 4 5 6 45 90 135 180

Rotation Angle

Energy (kcal/mol) butane

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Force Field Development

500 1000 1500 2000 2500

IR Absorption (a.u.) wavenumber (cm

–1)

d(Si-O-Si) 399 n(Si-H) 2276 n(Si-O) 1140 d(O-Si-H) 881 d(O-Si-O) 566 n(Si-O) 465

covalent attractive

  • 6 10-18
  • 4 10-18
  • 2 10-18

2 10-18 4 10-18 0.5 1 0.1 0.2 0.3 0.4 0.5 0.6

f (ri j) (J) Charge transfer ri j (nm)

Coulomb repulsive charge transfer function total

experiment simulation

  • T8 POSS dynamic properties as modeled by

charge-transfer potential function

– Reactive force field for crosslinking

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Crystalline structures of T8 POSS

From x-ray analysis Simulated at 100 K Simulated at 350 K

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Cluster of T8 POSS cages

  • Density 0.78 g/cc, temperature 300K
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Clustering of tethered POSS

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Atomistic Model Coarse-grained Model

“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

– Validates structures obtained by MD – More efficient route to equilibrium

  • Lattice Monte Carlo

– Model adopted to 3-d cubic lattice

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MD Simulations of POSS/Polymer Systems:

Non-reacting POSS T8 w/trimer tethers Goal: large systems of >1000 cubes w/tethers of various sizes to study, e.g. role of tether length in assembly & network structure.

E.R. Chan, M.H. Lamm, SCG

Monte Carlo code has been developed to simulate identical model to efficiently generate equilibrium structures for comparison & validation.

Small trial system.

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MD Simulations of POSS/Polymer Systems:

Network formation POSS T8 cubes with trimer tethers Non-crosslinked Crosslinked

  • develop code for network structure analysis
  • validation against expt, MC simulation and LMC simulation
  • large-scale parallel simulations of large systems

Next:

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Lattice Model of POSS/Polymer Network Formation:

Lattice Monte Carlo Simulation

  • Goal is to study crosslinking of POSS tethered by

polymers into POSS/Polymer networks

– Lattice Monte Carlo (LMC) is a well established method for modeling associating polymer networks, physical and chemical gelation of polymer networks, etc.

  • We are developing LMC codes to simulate

assembly of polymer-tethered POSS structures

– Efficient generation of structures and equilibrium phases.

  • Cluster moves facilitate the hierarchical ordering of

complex phases.

– Rapid prototyping of minimal models.

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Lattice MC Simulation of POSS/Polymer Network Formation:

  • Model POSS cage is rigid, and occupies 27 sites on

a cubic lattice.

  • Tethers are attached to the corners of the

POSS cage.

– Tethers modeled using bond fluctuation model.

  • One model monomer occupies 8 sites
  • n a cubic lattice.
  • Bond lengths between connected

monomers can vary.

  • Ends of tethers crosslink

via square-well attraction.

  • Investigate role of tether length
  • n cube spacing to reproduce/

interpret experiment

70 36 10 6 2 Model † 210 111 29 21 6 Experiment*

Aggregation

  • f cages*

Local

  • rdering of

cages*

Tether

length

*Soles et al., MRS Symp. Series, 628 (2000). †For polyethylene, three C-C bonds map to one model bond.

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Lattice MC Simulation of POSS/Polymer Network Formation

  • We are now

developing codes to analyze details

  • f complex network

structure

(e.g. connectivity, voids, cube spacing, rigidity, etc.)

  • To do: generalize

network analysis code to

  • ff-lattice MC and MD

M.H. Lamm & SCG, 2002 30

Mesoscale Modeling: Theory

  • Statistical Associating Fluid Theory (SAFT)

– Most successful and frequently applied molecular-theory- based equation of state

  • Application to POSS systems based on coarse-grained model
  • f functionalized POSS

– Will be used to predict solubility of functionalized POSS in polymer solvents

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Conclusions

  • Project underway

– Activity at each level of description – Integration of efforts through periodic conference calls

  • Web-based collaboration tools
  • Significant issues to be addressed

– Integration of atomistic simulation efforts

  • Multiple codes vs. single code

– Linkages

  • Upscaling and downscaling

– To what degree can it be automated?