Molecular Dynamics Simulations of Diffusion in Polymers

1. Molecular Dynamics Simulations of Diffusion in Polymers Zach Eldridge Department of Mechanical Engineering University of Arkansas Fayetteville, AR 72701 USA REU Advisor: Dr. Douglas Spearot Grad Student Advisor: Mr. Alex Sudibjo Research Symposium – July 20, 2009

2. Background • What is Molecular Dynamics? • Molecular dynamics is a form of computer simulation used to observe the behavior of atoms and molecules, which cannot be easily observed during experiments. • Through the use of computer algorithms and known laws of physics, mathematics, and chemistry we are able conduct an experiment and model it through a simulation. • Through analysis of the simulation we are able to study the behavior of the material. Nano Indention

3. My Project • Objectives • The main objective of this research is to study diffusion of methane gas penetrates through PDMS (Polydimethylsiloxane) • PDMS – the most widely used silicon based organic polymer. It is composed of Oxygen, Siliocon and a methyl, CH3. Common uses include contact lenses and shampoo. • Penetrate – Methane, CH4 PDMS 1 PDMS 2 • Molecular dynamics simulation will allow us to calculate the diffusion coefficients of the penetrate through the PDMS and evaluate the role of concentration and distribution of penetrates

4. My Project • Simulations Conditions • Concentrations – percent of the total weight of the system • .264% = 15 atoms, .529% = 30 atoms, .793% = 45 atoms, 1.06% = 60 atoms • Volumes – Initial volume of methane molecules • 1000 Å3, 8000 Å3, 27000 Å3, 64000 Å3, 125000 Å3 • Temperatures – Temperature the simulation runs at • 200 K, 250 K, 300 K, 350 K, 400 K 125000 Å3 initial volume 1000 Å3 initial volume

5. Research Methods • Create simulation in Linux using Lammps code • Run simulation through a super computer, Trillion • Output visual data to Ensight • Output mean squared displacement data to Excel • Normalize data and create graphs • Use slope of trendline to calculate the diffusion coefficientD • Solve for Q, activation energy Kcal/mol, using the value ofD • Average all Q and calculate D0, diffusion constant (cm2/s) • Equations • D = (1/6)slope • Q = (Rln(D1 / D2))/( 1/T2 – 1/T1) • D0 = D1/exp(-Q/RT1)

6. Results • Equilibrium Reached at 10 ps • Low fluctuation in MSD • Equilibrium reached at 25 ps • Low fluctuation in MSD

7. Results Cont. • Equilibrium reached at 30 ps • High fluctuation in MSD

8. Results Cont.

9. Conclusion • Smaller the initial volume – longer it takes to reach equilibrium • Larger concentration – less fluctuation in MSD values • Smaller the initial concentration – longer it takes reach equilibrium • D0 is concentration dependent • The values of D0 for concentration of .793% were between 19% - 37% below the values for 1.06%.

10. Works cited • (2009, May 11). Polydimethylsiloxane. Retrieved May 26, 2009, from Wikipedia Web site: http://en.wikipedia.org/wiki/Polydimethylsiloxane • (2009, April 14). Arkansas High Performance Computing Center. Retrieved May 26, 2009, Web site: http://hpc.uark.edu/about.html • Kam Liu, Wing Ensight.com. Retrieved May 26, 2009, Web site: http://www.ensight.com/component/option,com_zoom/Itemid,41/PageNo,2/catid,4/hit,1/key,15/page,view/ • Wag.caltech.edu. Retrieved May 26, 2009, from Gallery of Polymers and Polymer Simulation Web site: http://www.wag.caltech.edu/gallery/pvcdco2.gif • Dr. Douglas Spearot – Faculty Advisor, Alex Sudibjo – Graduate Student Advisor

11. Conclusion Questions?