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  • Polymer/Substrate Interfaces: Simulation of Structure, Properties and Behavior

    Dhiraj K. Mahajan,a Anatoly V. Berezkin,b P. Ulrich Biedermann, b Fathollah Varnik,a

    Alexander Hartmaier a

    a ICAMS, Ruhr-Universität Bochum, b Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf

    Role of Substrate Roughness

    Linear/cross-linked polyurethane film on the ZnO surface • ZnO(0001)-Zn-O/OH single crystal surface: well

    characterized by experiment, stable at ambient conditions. • Polyurethane. The behavior of some typical molecules and

    polymer fragments are considered to extend existing OPLS-AA force filed to the polyurethane case.

    water methyl acetate methyl isocyanate N-methyl-acetamide

    , , , …

    LEED images obtained at 63 eV

    Polyurethane Network: Curing, Structure and Properties

    +

    nPPO = 2, 5, 8

    H3C

    CH2

    CH2

    CH2

    O

    O

    O

    CH2

    CH2

    CH2

    C H

    C H

    C H

    CH3 OH

    CH3 OH

    CH3 OH

    k

    l

    m

    branched polypropylene ether triol (Desmophen 1380 BT)

    = 5.3

    nPPO

    nPPO

    nPPO

    NN CC

    O O

    4,4'-methylene diphenyl diisocyanate

    +

    +

    +

    Coarse-Grain Network model

    All-atom network model

    Conversion vs. time

    Network properties

    Curing kinetics Branching degree

    32

    3

    2 2

    ss sBD +

    =

    Glass transition temperature Stress-strain curves Young modulus

    Objective • Metals are usually coated with polymers against corrosion.

    The objective of this project is to develop an atomistic understanding of the interphase/interface structure of polymer/metal oxide contacts and the diffusion mechanism of ions and small molecules (H2O, O2) along the interface as a guide for future material design.

    • Multiscale computer simulation • Kinetics of curing during polymer coating formation; • Interfacial structure of coating; • Permeability of coating and mechanisms of substrate

    corrosion.

    DFT calculations - structure, charge, and conformation of small molecules; - molecule-surface interactions

    Atomistic simulation (MD) - adsorption phenomena; - diffusivity, viscoity, density, cohesion energy of components;

    Calibration of atomistic force-field (OPLS)

    Coarse-grained level (MD) - kinetics of curing; - coating strucutre; - corrosion mechanisms;

    Parameterization of coarse-grained model

    ba ck

    -m ap

    pi ng

    to

    a to

    m is

    tic d

    et ai

    ls Method

    Main Questions

    Materials

    Adsorption of Small Organic Molecules (Density Functional Theory + Classical Simulations)

    Water, methyl acetate, methyl isocyanate.

    Adsorbed configurations and energies of adsroprtion

    Ads. energy OPLS-AA

    Ads. energy DFT

    Adsorbed configurations

    Correction of force field

    Iterative improvement of FF

    Adsorption energy of H20 (test molecule)

    Adsorption energy of methyl acetate

    Comparison of classical and DFT adsorption simulations

    Polyurethane Structure at ZnO Interface

    Objective • Polymer/substrate coating systems are generally characterized

    by rough substrate surfaces with undulations ranging from few nanometers to several micrometers. It is known that the surface roughness strengthens the polymer/substrate bonding either by increasing the effective contact area or by mechanical interlocking of polymer between surface undulations. However, little is known regarding the role of relative dimensions of polymer chains with respect to surface undulations in effecting the polymer bonding. Such information can be used in substrate surface engineering for improving bonding without changing the interface chemistry, which forms the objective of this part of the project.

    Bonded interaction

    Non-bonded interaction

    Coarse-grained Polymer molecule

    Molecular Dynamics Simulation Details

    Effect of Substrate Roughness

    • An ideal coating system is obtained by equilibrating the coarse-grained polymer molecules between thin atomic layer at the top and planar atomic substrate at the bottom.

    • After polymer equilibration, coating systems with varying substrate roughness is obtained by removing atoms from the planar substrate in regular pattern with dimensions of the order of Rg.

    • Polymer molecules interacts with the atomic substrate and top layer using non-bonded Lennard-Jones potential.

    • Zero pressure initial configuration of different coating systems is obtained by allowing motion of top layer atoms in z-direction.

    Various rough substrates studied

    Polymer/substrate system

    Statistical dimensions of polymer molecule

    cm

    x

    y z

    Failed configuration of various coating systems at similar strain under Mode I loading

    Mode I loading applied to coating system

    B

    C

    A

    Rough substrate

    Deformation behavior of various coating systems under Mode I loading with different substrate roughness.

    Work of separation versus roughness factor for different substrate roughness samples shows a vital role of mechanical interlocking in strengthening the interface in contrast to the contribution of increased effective area of contact as shown by a linear fit.

    x

    y z

    Method • Coarse-grained Molecular Dynamics simulations

    Rough surfaces strengthen the interface by increasing the effective contact area

    Mechanical interlocking is another mechanism responsible for strengthening the interface

    http://www.specialch em4adhesives.com/re sources/adhesionguid e/index.aspx?id=theo ry4

    R. Van Tijum, PhD thesis, University of Groningen, 2006

    Polymer/substrate interactions play an important role in coatings and composite materials, applications of which includes renewable energy production structures, sandwich panels used in automobiles and packaging. Desired properties of these materials include high adhesion strength of polymer with substrate and corrosion resistance of the substrate, among others. Polymer/substrate interfaces are essentially characterized by the network structure of the polymer and substrate roughness that plays a vital role in imparting the desired properties to these materials. The objective of the present project is to understand the mechanism(s) responsible for adhesion of polymer on substrates and to quantify the role of substrate roughness as well as polymer network structure on the desired properties of the coating system. At this stage of the project, the individual roles of substrate roughness and network structure are studied separately using multi-scale simulation methodology. The outcome of these complementary studies will improve our understanding of polymer/substrate interface structure, properties and behavior and may be used as guide for future material design.

    Introduction

    Polymer/Solid interfaces finds several technological applications such as renewable energy production structures, sandwich panel for automobiles and packaging.

    http://www.thyssenkrupp.com/en/bildstrecke/17/ http://www.thyssenkrupp.com/de/produkte/energie.html

    Measure of roughness

    References

    • F. Varnik and K. Binder, Multiscale modeling of polymers at interfaces, Int. J. Mat Res, 100, 1494-1502, (2009).

    • J. Rottler and M.O. Robbins, Molecular simulations of deformation and failure in bonds formed by glassy polymer adhesives. J. Adh. Sc. Tech. 17, 369-381, (2003).

    • A. V. Berezkin, P. U. Biedermann and A. A. Auer, Mesoscale simulation of network formation and structure, combining molecular dynamics and kinetic Monte Carlo approaches, In proceeding: European Polymer Congress 2011, 1378.

    Slide Number 1

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