1. Phase separation of two-component systems in thin films
Katarzyna Bucior, Leonid Yelash, Kurt Binder
Institute of Physics Condensed Matter Theory Group KOMET 331 Johannes-Gutenberg University of Mainz, Germany

2. foamed polysulfone film [1] thin film MD simulations
Motivation and goals
industrial significance of polymer solutions
knowledge of the phase behavior
influence of confinement on phase separation
foamed polysulfone film [1]
bulk MD simulations
thin film MD simulations
[1] B. Krause et al. Macromolecules 2002, 35, 1738

3. MODEL OF A MIXTURE: coarse graining of C16H34 and CO2 molecules
C16H34- represented by flexible chain of 5 segments
(each contains roughly 3 C-C bonds)
CO2- coarse grained into a sphere

4. bulk phase diagram for C16H34/CO2 mixture [3]
bead-spring model for chain molecule FENE+LJ potential [1,2]
LJ potential for CO2-CO2 and non-bonded chain monomers interactions
bulk phase diagram for C16H34/CO2 mixture [3]
type III
type I
TcCO2
TcC16H34
cross-interactions between CO2 and C16H34 by LJ potential with ehc and shc using Lorentz-Berthelot mixing rule:
[1] K. Kremer, G. S. Grest, JCP, 92, 5057 (1990)
[2] L.G. MacDowell, P. Virnau, M. Müller, K. Binder, JCP, 117, 6360 (2002)
[3] K.Binder, M.Müller, P.Virnau, L.G. MacDowell, Adv. Polym.Sci, 176, 1 (2004)

5. Confinement:
two infinite parallel walls consisting of spherical particles
interactions between fluid particles and wall particles: U r
rcut 

6. isothermal slice through the phase diagram of C16H34/CO2 at T=486K [1]
Grand canonical MC TPT1-MSA EOS [2] spinodal curve [2]
molar fraction of CO2
pressure, bar
spinodal decomposition
simulation of phase separation in thin film geometry:
homogeneous sample in one-phase region of phase diagram (*=0.8, xCO2=0.6, T*=1.16)
system size:Lx=Ly=240, Lz=12
pressure jump to two-phase region of phase diagram (density decrease to *0.4)
system size: Lx=Ly=300, Lz=15
[1] K.Binder, M.Müller, P.Virnau, L.G. MacDowell, Adv. Polym.Sci, 176, 1 (2004) [2] L.G. MacDowell, P.Virnau, M.Müller, K.Binder, JCP, 117, 6360 (2002)

7. METHOD
MD simulations with use of ESPResSo [1] parallelized simulation package
Preparation of homogeneous sample
velocity Verlet algorithm with time step  =0.002t with time scale t=(m/)1/2
creating configuration with walls using SAW, box Lx=Ly=20 , Lz=12
NVT warming up (T*=1.16) with Langevin thermostat
switch off thermostat and stop CoM  NVE MD
replicate the box in x and y directions (Lx=Ly=20   240 , N= )
relax the periodic structure due to p.b.c.
Pressure jump
by 25% rescaling of positions of molecules in 3 directions (final system size: Lx=Ly=300, Lz=15)
Simulation of the system in NVE ensemble (multiprocessor SOFTCOMP, JUMP in Jülich)
[1]

8. time evolution of structure formation
Lx=300 (135nm)
Lz=15s
t=0-500, t=10
t= t=100
Lx=300 (135nm)
Ly=300 (135nm)
t=100
t=500
t=2000

9. DENSITY PROFILES IN Z DIRECTION
C16H34
CO2
t=0

10. time dependence of characteristic length scale
scaled real-space correlation function G(r,t):
g(r,t)- pair correlation function

11. CONCLUSIONS
Efficient coarse grained model of a real asymmetric mixture
Molecular dynamics simulation of pressure jump with use of ESPResSo
Bicontinuous structure during the spinodal decomposition in quasi 2d
Characteristic length scales as l~t1/3 (bulk: l~t1/3 to l~t)

12. Thank you for your attention!
Acknowledgements
Dr. Peter Virnau (Mainz)
Dr. Subir Das (Mainz)
Dr. Torsten Stühn (MPI Mainz)
ESPResSo: Research group of C. Holm, Max Planck Institute for Polymer Research in Mainz, Germany
CPU time in JUMP cluster and SOFTCOMP in Jülich
Thank you for your attention!

14. spinodal decomposition peak
DENSITY-DENSITY STRUCTURE FACTOR Snn
homogeneous system before quench
system after pressure quench at t=50
LJ peak
spinodal decomposition peak
number density structure factor:
partial structure factors:

15. time dependence of structure factor
3rd layer at z=3.75
5th layer at z=6.75

16. characteristic domain size R:

17. 3rd layer at z=3.75
t=10
t=50
t=100
t=200
5th layer at z=6.75

18. pressure jump: p*=0.21 to p*=0.08

19. RELATIVE CONCENTRATION DENSITY PROFILE
TOTAL DENSITY PROFILE