COMPUTER-SIMULATION STUDY OF THE COMPARATIVE RHEOLOGY OF BRANCHED AND LINEAR ALKANES

We use nonequilibrium molecular dynamics simulations to investigate the rheology of branched and linear alkanes in the dense fluid state. A simplified model alkane [J.-P. Ryckaert, G. Ciccotti, and H. J. C. Berendsen, J. Comput. Phys. 23, 327 (1977); J.-P. Ryckaert and A. Bellemans, Discuss. Faraday Soc. 66,95 (1978); G. Ciccotti and J.-P. Ryckaert, Comp. Phys. Rep. 4, 345 (1986); R. Edberg, D. J. Evans, and G. P. Morriss, J. Chem. Phys. 84, 6933 (1986) ) composed of sites representing CH2 and CH3 groups with fixed bond lengths and bond angles between sites is used. The model also includes dihedral potential interactions between sites separated by three carbon bonds, and truncated and shifted Lennard-Jones potential interactions between sites on different molecules as well as those separated by more than three bonds on the same molecule. The Edberg-Evans-Morriss nonequilibrium molecular dynamics algorithm [R. Edberg, D. J. Evans, and G. P. Morriss, J. Chem. Phys. 84,6933 (1986); R. Edberg, G. P. Morriss, and D. J. Evans, J. Chem. Phys. 86, 4555 (1987)] with periodic boundary conditions was used to simulate a system of 108 13- carbon n-alkane (tridecane) molecules and the corresponding 3-armed star branched alkane (5-butyl nonane). Equilibrium and steady state Couette flow simulations were performed. Properties calculated included the viscosity, normal stress differences, radius of gyration, alignment angles, shear induced order, angular velocity and a variety of other properties related to the conformation and dynamics of the molecules. The general features of the results were in agreement with recent simulations of n-decane and n-eicosane [G. P. Morriss, P. J. Daivis, and D. J. Evans, J. Chem. Phys. 94, 7420 (1991)], but the details of the results differed for branched and linear molecules. For example, the general trend of decreasing viscosity at low shear rates followed by an increase in the viscosity at higher strain rates was once again observed, but the viscosity for the linear alkane was always less than that of the branched alkane.