Molecular Rheology of Freely Jointed Chain Model Polymer Melts

                   

Principal Investigator

P J Daivis

Department of Applied Physics

RMIT

 

Co-Investigators

B D Todd

CSIRO Molecular Science

M Matin

Department of Applied Physics

RMIT

 

Projects

g73 - VPP

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.
 
               

   
               
                   

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

During 1998, we concentrated our efforts in two major areas of activity. The first was the development of an algorithm for molecular dynamics simulation of a liquid with spatially periodic boundary conditions undergoing elongational flow that is also periodic in time. Put simply, this means that, with the appropriate simulation cell orientation, elongation maps the infinite lattice of simulation cells back onto itself after a period of time known as the strain period. The practical result of this is that elongational flow can continue indefinitely, without the minimum image convention being violated. Our second area of activity was in the development of an efficient, vectorized, cell algorithm for formation of a neighbour list to be used in molecular dynamics simulations of chain molecules undergoing shear or planar elongational flow. We are now about to begin applying these techniques to the computation of the rheological properties of our model polymer liquids.

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. The code is derived from previous alkane simulation programs, and it has been

 
                   
Appendix B -

                   

       

completely rewritten using Fortran 90 array syntax wherever possible to aid future program development. The cell algorithm that we have developed for construction of the neighbour list is efficient and our interatomic force computation and summation is well vectorized, with the force subroutine achieving more than 98% vectorization.

Publications

B. D. Todd, P. J. Daivis, Nonequilibrium molecular dynamics simulations of planar elongational flow with spatially and temporally periodic boundary conditions, Phys. Rev. Lett., 81, 1118-1121 (1998)

P. J. Daivis and B. D. Todd, Frequency-dependent elongational viscosity by nonequilibrium molecular dynamics, In. J. Thermophys. 19, 1063-1072 (1998)

       
- Appendix B