22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma polymerization of styrene on silver substrate: a combined ab-initio
calculations and classical molecular dynamics study
M. Zarshenas1, B. Czerwinski2, T. Leyssens3 and A. Delcorte1
1
Universite Catholique de Louvain, Institute of Condensed Matter and Nanosciences - Bio & Soft Matter
(IMCN/BSMA), Croix du Sud 1, bte L7.04.01, BE-1348 Louvain-la-Neuve, Belgium
2
Lulea University of Technology Division of Materials Science, Department of Engineering Sciences and Mathematics,
SE-971 87 Lulea, Sweden
3
Université Catholique de Louvain, Institute of Condensed Matter and Nanosciences - Molecules, Solids and Reactivity
(IMCN/MOST), Place Louis Pasteur 1, bte L4.01.03, BE-1348 Louvain-La-Neuve, Belgium
Abstract: The ab-initio calculations and classical molecular dynamics (MD) were utilized
to theoretically study the plasma polymerization of styrene on silver substrate. The stability
of all possible styrene precursors were studied by ab-initio calculations in order to guide the
MD simulations. The results of MD simulations show the significant effect of increasing
the temperature and number of precursors on increasing the number of cross-links.
Keywords: plasma polymerization, ab-initio calculation, molecular dynamics, cross-link
1. Introduction
The plasma treatment of polymers has been widely used
for the modification of the surface characteristics of
polymers. It is often intuitively assumed that plasma
polymers can be deposited on any substrate without any
influence of the properties of the substrate. However, the
deposition process is an interactive process of gas-phase
species with the top surface of the substrate material, and
the nature and the extent of this interaction are very
important factors of plasma polymerization. Without
appropriate interaction, a sufficient adhesion of the
plasma polymer to the substrate cannot be achieved [1].
The present theoretical study focuses on plasma
polystyrene (PS), because PS is a reference polymer
material with a large spectrum of applications, both from
traditional synthesis and plasma polymerization.
Therefore, a large body of experimental data is available
concerning plasma PS. For instance, the deposition of PS
films on metal substrates has been investigated by
Choudhury and coworkers, using capacitively coupled rf
plasma [2]; and the possible application of PS films as
corrosion resistance coating on metal bell has been
concluded. Yasuda and coworkers also investigated the
polymerization rate of styrene in an electrode-less glow
discharge from styrene vapour [3]. They came to the
conclusion that polymerization occurs in the vapour phase
and that the growing polymer radicals deposit on the
surface of the discharge vessel yielding highly crosslinked polymer coating. The issues of interaction with the
substrate, polymerization in the gas phase, and structure
of the deposited films really deserve more fundamental
investigations.
In the experiment of plasma
polymerization of any monomer, there are several
parameters which can be controlled. The most important
of them are: the gas flow rate, applied power, working
pressure, applied bias voltage, deposition time and
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substrate type. The various combinations of these
parameters have an impact on the final result of the
plasma polymerization process and the morphology of the
formed polymer layer.
In this study, plasma
polymerization of styrene on a silver (Ag) surface was
investigated using first-principle (ab-initio) and classical
molecular dynamics (MD). Ab-initio calculations have
been conducted in order to investigate the stability and
reactivity of the relevant species (radicals, cations and
anions). Classical molecular dynamics was used to study
mixed (styrene and radical) precursor interactions with
the surface, focusing on the induced chemical reactions.
The first-principle calculation results are used to guide
and validate our MD simulations.
2. Computational details
This study is conducted using ab-initio calculations and
MD computer simulations. The ab-initio calculations
were carried out using density functional theory (DFT) in
the framework of Gaussian 09 suite of programs. All the
investigated structures were optimized by employing the
hybrid density functional DFT/B3LYP [4] method and the
6-31G(d,p) basis set [5]. In the MD simulations the
motion of the particles is resolved by integrating
Hamilton’s equations of motion. The forces among the
particles are described by a combination of pairwise
additive and many-body potential energy functions. The
Ag–C and Ag–H interactions are described using the
Lennard-Jones potential [6].
The potential for
hydrocarbons, AIREBO, developed by Stuart and
co-workers is used for H–H, H–C and C–C interactions
[7]. A routine we developed to identify the new
intermolecular C-C bonds created during the simulations,
which should reflect the polymerization process.
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3. Ab-initio calculations results
The stability and reactivity of styrene monomeric and
multimeric species are investigated using ab-initio
calculations. The calculated energy of the possible
neutral, anionic and cationic styrene precursors are
indicated along with their molecular structure in Fig. 1.
Following the results of the calculations, the radical with
underlined potential energy in Fig. 1 found to be the most
stable species. The different reactions and growth sites
for the addition of a monomer to a radical are investigated
in order to identify the most likely reaction pathways
(Fig. 2).
The energies of different structures,
corresponding to the addition of a monomer to a radical at
two different positions (marked as a and b in Fig. 2) were
studied. Since the potential energies of the reaction
products of a monomer with a radical are always lower
than the sum of the energies of the reactants
(∆E = -0.0243 Ha for case a and ∆E= -0.0524 Ha for case
b in Fig.2(A)), these reactions are exothermic and should
therefore occur spontaneously provided that the energy
barrier is low. As is highlighted in Fig. 2 (A) and (B), the
lowest energy position for the polymer growth reaction is
found to be the same (marked as b) each time a new
monomer is added to the structure. This reaction site
leads to a polymer that has the regular structure of linear
polystyrene.
-
+H
E=-309.5946 Ha
+H
E=-309.1303 Ha
E=-309.3830 Ha
H
E=-0.5022 Ha
-
+H
E=-309.5784 Ha
E=-309.1200 Ha
H
E=-0.5356 Ha
+H
E=-309.5767 Ha
E=-309.1166 Ha
E=-309.7632 Ha
Fig. 1. The investigated structures with specified energies
calculated with DFT/B3LYP method.
4. Molecular dynamics approach and results
In this study, due to the large complexity of the
experiment, from the point of view of its microscopic
description and the limitations of MD simulations, we can
only mimic it by the proper adjustment of the model
parameters, namely: density, velocity and chemical
composition of the plasma gas situated in the modelled
gas periodic cell. Here MD simulations of mixtures of
styrene and radicals interacting with the surface of a
chosen substrate, in order to form a plasma-polystyrene
film, were conducted. First, it was checked that the
energy differences between the radical structures obtained
from the DFT calculations were comparable to those
2
Fig. 2. Structures and energies following the reactions at
different sites. (A) Radical + monomer; (B) Radical + 2
monomers. The underlined values correspond to the
lowest potential energies.
obtained using the potential for hydrocarbons (AIREBO)
in the MD simulations.
Both methods indeed predict the top left structure of
Fig. 1 to be the most stable. Those radicals were selected
to be mixed with the styrene gas in different proportions.
Our approach was to create a substrate (blue atoms in
Fig. 3) to be bombarded by multiple impacts of styrene
and radical precursors arranged in a periodic cell (yellow
line in Fig. 3) with defined density (4.65×10-3 g/cc) and
provided with the same translational energy/velocity. The
side borders of the created system were governed by
periodic boundary conditions. To mimic cyclic or
continuous flow of the gas toward the surface, we
modified the size of the gas cell in its non-periodic
direction.
Preliminary calculations in order to optimize the system
size indicate that the deposition rate is independent of the
size of the substrate. Therefore, the smallest silver
surface with a size of (134×134) Å and a thickness of
~19 Å (9 layers of Ag atoms) was selected to keep the
number of atoms and consequently the duration of
calculations as small as possible. In order to check the
influence of the parameters on the reactions occurring at
the surface, a series of simulations were conducted,
varying initial kinetic energies of the plasma molecules
and radicals (1 - 16 eV) and proportions of styrene and
radicals in the plasma gas as (75% - 25%), respectively.
Finally, the initial series of calculations at 0 K were
compared to a series computed with a 300 K heat bath
(Langevin method). The duration of simulations varied
from 150 ps to 450 ps for different size of the cells.
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bonds and reactions beyond 8 eV/molecule while the
increase of the reaction number is more gradual at 300 K.
Fig. 4. Temperature and cell size distributions of the total
number of cross-links (N cross ) obtained for the initial
kinetic energy from 1 to 16 eV and a styrene-radical
proportion of 75% - 25%.
Fig. 3. Schematic picture of the initial system
arrangement for the analysis of mixed styrene and radical
precursor interactions with the chosen substrate (Ag).
Atoms of the substrate are coloured in blue, the C and H
atoms of styrene and radical are red and white,
respectively, and the yellow frame represents the
boundary of the styrene's periodic cell.
For the calculation of bonded atoms, the threshold value
for the distance between different atoms is taken at half of
the interaction potential well (AIREBO) in the
equilibrium state, which gives 1.27, 1.495 and 1.79 Å for
H-H, C-H and C-C bonds, respectively. Our criterion for
verifying the creation of new C-C bonds is based on
comparing the distance between two carbon atoms at a
defined time with their distance at zero time. If the
distance is equal or less than 1.79 Å and those C atoms
belonged to 2 different structures (styrene-styrene,
radical-radical or styrene-radical) at the beginning of the
simulation, this C-C bond is considered as a cross-link
between the two molecules. The simulations performed
at 0 K with a rather small gas cell (1000 Å in the vertical
direction) lead to very small numbers of reacting
molecules. To increase the number of reactions, two
approaches were applied: first increasing the gas cell size
(from 1000 to 3000 Å), in order to increase the thickness
of deposited styrene and radicals on the surface; and
second, raising the temperature from 0 to 300 K to
increase the reaction rate. As it is indicated in Fig. 4 our
calculations show the significant effect of temperature on
the number of reacting molecules (created bonds).
Increasing the initial translational energy of molecules
and radicals at 0 K only leads to larger numbers of broken
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Up to 52 new C-C bonds are observed in the most
extreme case, i.e., with the initial molecule energy of
16 eV at 300 K. Increasing the size of the cell has also a
sensible impact on the number of cross-links at 300 K.
For instant by comparing the orange curve which belongs
to the 3000 Å cell and the red one which belongs to the
2000 Å one can conclude how the size of the cell
increases the number of cross-links specifically at the
higher energies.
5. Summary and outlooks
Plasma polymerization of styrene on a silver substrate
has been investigated using ab-initio calculations and
classical molecular dynamics. The energy of the possible
neutral, anionic and cationic styrene precursors were
calculated and the radical with the lowest energy was
chosen to be used in our molecular dynamics simulations.
The ab-initio results also predict the exothermic and linear
polymerization process. Our molecular dynamics results
show the substantial influence of increasing temperature
and size of the precursors cell on the number of
cross-linking; where there are almost zero cross-link at
0 K specially for the smallest cell of precursors, more
than 50 cross-links were observed at 300 K for our largest
cell. Based on our results we define our perspective as
follows: Simulations with a larger cell size (5000 Å) and
with increased temperature (up to 350 K), to increase the
reaction statistics for further analysis. In order to obtain a
thicker (poly)styrene film on the surface, an iterative
bombardment of the surface with such a gas cell is
considered, with or without relaxation time between the
bombardment stages.
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6. References
[1] H.K. Yasuda. Plasma Process. Polymers, 2, 293
(2005)
[2] A.J. Choudhury. J. Physics: Conf. Ser., 08, 012104
(2010)
[3] H.K. Yasuda. J Appl. Polymer Sci., 15, 2277 (1971)
[4] C. Lee, W. Yang and R.G. Parr. Phys. Rev. B:
Condens. Matter Mater. Phys., 37, 785 (1988)
[5] R. Krishnan, J.S. Binkley, R. Seeger and J.A. Pople.
J. Chem. Phys., 72, 650 (1980)
[6] M.P. Allen and D.J. Tildesley.
Computer
Simulation of Liquids. (Oxford: Clarendon Press)
(1987)
[7] S.J. Stuart, A.B. Tutein and J.A. Harrison. J. Chem.
Phys., 112, 6472 (2000)
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