Polymers Confined in Clay Materials

1
Polymers Confined in Clay Materials

• Gu Zhihui, Perla B. Balbuena

2
Outline

• Introduction
• Simulation details and procedure
• Results and discussion
• Conclusions

3
Introduction

• Polymeric catalytic membranes have been
  synthesized with the aim to separate H2 and
  oxidize CO in polymer electrolyte fuel cells.
• Molecular modeling is essential in understanding
  the interactions of the polymer with the
  inorganic matrix, for example, the effect of the
  polymer density on the polymer relaxation and K
  mobility.

4
Introduction

• This information is crucial because in the real
  nanocomposite membrane system, either K or Li
  present in the clay gallery is exchanged with
  Pt2 and further reduced to form Pt metal
  particles.
• Classical MD simulations were employed to study
  the structure and dynamics of PEO polymers inside
  of clay minerals with a gallery separation of
  12.8 Å.
• Selected ab initio calculations were performed in
  small portions of PEO to investigate vibrational
  spectrum of a single PEO chain.

5
Simulation details and procedure

• 3-dimensional cell built using Cerius2 contains a
  slab of mica at the bottom of the cell and a
  large vacuum space on top of that slab, which is
  then filled by PEO chains.

6
Simulation details and procedure
Cell parameters defined in cartesian coordinates.
11.7 Å 46.5 Å in the directions parallel to the
mica layer and 22.8 Å in the perpendicular
direction. A mica pore is generated with the
separation between slabs of approximately 12.8 Å.
7
Simulation details and procedure

• PEO chains were introduced into the cell and the
  chain number depends on the desired PEO densities

8
Simulation details and procedure

• The simulations were run in the canonical NVT
  ensemble, at 300 K.
• The T_Damping thermostat was used to set the
  temperature at the desired value.
• Equilibration times at least 100 ps were used and
  histograms were collected in the last 100 ps.
• All the mica atoms were fixed in their
  equilibrium positions, except for the K ions that
  were left free to interact with the polymer
  chains.

9
Simulation details and procedure
Simulation details
10
Simulation details and procedure

• The universal force-field used in the simulation
  includes the following interactions
• Intra-molecular interaction
• Bond stretching E ½ kij(r-rij )2 . rij -
  equilibrium bond length r - instantaneous
  distance kij - force constant.
• Angle bending
  . Kikj - force constant ? -
  instantaneous position.
• Torsion Ef kt( 1 cosnf), kt - force constant
  f - torsion angle

11
Simulation details and procedure

• Intermolecular interactions
• Van der Waals Aij and
  Bij - repulsive and attractive
• van der Waals parameters respectively.
• Electrostatic forces qi and qj -
  atomic charges, rij - distance
  between i and j atoms.

12
Initial and equilibrium configuration
Case 1
Case 2
Case 3
At low density, the chains may bend and adopt
various conformational changes responding to
characteristic polymer dynamics. At the highest
density, however, the chains are very much
extended parallel to the mica surface and their
motion is much restricted .
13
Simulation results - Energy
Average energy components (in Kcal/mol) for the
three cases
the Coulombic energy is the largest of all this
strongly attractive force contributes to the
stabilization of the system. The rest of the
forces are repulsive.
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Energy vs. time
Energy vs. time of the three cases during the
last period of simulation
1
2
3
15
Simulation results Local density

• Local density dXY(r) gXY(r)dbulk,Y. allows us
  to clearly identify the actual spatial variation
  of the number of Y atoms surrounding a X atom.
• C-C and C-O local densities reveal the
  chain-chain interactions.
• K-O and K-C local densities provide an
  indication of the migration of the K ions from
  the vicinity of the mica layer towards the
  carbonyl sites of PEO.

16
Simulation results Local density
Average local density of carbon atoms surrounding
other carbon atoms.
The first two sharp peaks correspond to the C-C
bonds of one chain, therefore their positions and
intensities are independent of the polymer
density
intermolecular C-C local densities are enhanced
as the bulk density increases, and two main peaks
are located at 3.1 and 3.7 Å respectively.
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Simulation results Local density
Average local density of oxygen atoms surrounding
carbon atoms. The first sharp peak corresponds
to the C-O bonds of one chain, second is a
combination of intra- and intermolecular C-O
interactions. The long-range effects are
originated by both intramolecular and chain-chain
interactions.
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Simulation results Local density
Average local density of oxygen atoms surrounding
potassium atoms
This strong electrostatic interaction is clearly
evidenced by the sharp peaks observed in the K-O
local density at approximately 2.4 Å, and the
corresponding (broader) K-C peaks in the range of
3-4 Å.
Average local density of carbon atoms surrounding
K
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Simulation results
Interactions of the potassium ion (green) with
its nearest oxygen (red) and carbon (grey) atoms
K is connected to the oxygen atom in the ether
group at a distance of 2.34 ? the four C atoms
in the first shell are located at distances
between 2.8 and 3.1 ? and the other two C atoms
are both symmetrically located at about 4.3 ?
from K. the K migrate from the immediate
proximity of the mica slab to form these strong
interactions and remain locked in these
positions.
20
Simulation result s vibrational spectrum for PEO
The PEO chain was optimized and the IR
vibrational spectrum was obtained at the HF/3-21G
level of theory by using the Gaussian98 program.
The HF/3-21G calculated frequencies were scaled
by 0.9085 .
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Simulation results vibrational spectrum for PEO
IR vibrational spectrum (HF/3-21G) for the PEO
chain above
22
Power spectrum for PEO
The PEO power spectra calculated from the MD
simulations A close similarity between the peak
positions of the single chain and those of the
confined chains. The difference is in the CH2
stretching modes, which are shifted to low wave
numbers in the confined polymers. More work is
needed to determine if this is due to our force
field or if it is an effect of confinement. An
important result is that by comparison of
Figures, the intensities of the respective modes
become dramatically reduced as the density of the
confined polymer is increased, as a consequence
of the vibrational motion becoming diminished due
to the effect of confinement.
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Conclusions

• MD simulations of PEO confined in mica pores of
  12.8 ? permits us to elucidate structural aspects
  of confined polymers of various densities.
• Strong interactions are found between the K ions
  of the clay pore and the PEO oxygen atoms
• As a result of confinement, the polymer motion
  is significantly restricted, and the chains
  become perfectly extended in a direction parallel
  to the mica pore, as the polymer density
  increases.