Molecular Rheology of Freely Jointed Chain Model Polymer Melts

                 

Principal Investigator

Peter Daivis

Department of Applied Physics,

RMIT

We aim to study the structure and properties of flowing polymer liquids by performing
simulations of their molecular motions. We use a simple molecular model that represents a polymer chain by a series of spherical beads with fixed bond lengths. Using non-equilibrium molecular dynamics simulation techniques, we are able to compute quantities such as the strain rate dependent viscosity, details of molecular shape and conformation, and various quantities related to the microscopic chain motions.
 

Co-Investigators

     

Billy Todd

CSIRO, Division of Molecular Science

Mathieu McPhie

Department of Applied Physics,

RMIT

Projects

g73 - VPP

     

   
             
                 

What are the results to date and the future of this work?

We have begun a series of computer simulation runs on our model polymer liquids to evaluate the algorithm and explore the behaviour of our model polymer molecules under shear. Results have been obtained for four and ten site (or bead) molecules, and these results indicate that the behaviour of these model molecules is qualitatively similar to that of short chain alkanes which we have studied previously. We see shear rate dependent shear thinning, molecular size changes, molecular rotation and shear rate dependent increases in the pressure and internal energy of the system. These exploratory runs have enabled us to find where the performance of the code needs to improve before systematic production runs on large systems of long molecules can begin. We intend to simulate molecules with at least 100 interaction sites. In addition, we have undertaken a detailed investigation of non-equilibrium molecular dynamics simulation techniques for the simulation of elongational flow. We have devised an oscillatory elongation simulation method which allows us to extract frequency dependent elongational viscosities, and we have also applied transient time corelation function methods to elongational flows of simple atomic liquids for the first time.

What computational techniques are used?

We use the molecular version of the SLLOD non-equilibrium molecular dynamics algorithm, with a Gaussian constraint thermostat on the molecular centre of mass translational degrees of freedom. Although the code is derived from previous alkane simulation programs, it has been completely rewritten using Fortran 90 array syntax wherever possible to aid future program

                 
Appendix B -

                 

       

development. A new version of the code, using the Fortran 90 "module" and "use" features is being developed. This will allow us to incorporate object oriented design features into the code and enable more rapid development, and a more convenient environment for code reuse in future.

Publications

B. Todd, B. Daivis, Elongational viscosities from nonequilibrium molecular dynamics simulations of oscillatory elongational flow, J. Chem. Phys., 107, 1617-1624 (1997)

B. Todd, Application of transient-time correlation functions to nonequilibrium molecular-dynamics simulations of elongational flow, Phys. Rev. E. 56, 6723-6728 (1997)

       
- Appendix B