#### Document Type

Article

#### Publication Date

2006

#### Abstract

We present various rheological and structural properties of three polyethylene liquids, C_{50}H_{102}, C_{78}H_{158}, and C_{128}H_{258}, using nonequilibrium molecular dynamics simulations of planar elongational flow. All three melts display tension-thinning behavior of both elongational viscosities, η_{1} and η_{2}. This tension thinning appears to follow the power law with respect to the elongation rate, i.e., η ∼ *E ^{b}*, where the exponent

*b*is shown to be approximately −0.4 for η

_{1}and η

_{2}. More specifically,

*b*of η

_{1}is shown to be slightly larger than that of η

_{2}and to increase in magnitude with the chain length, while

*b*of η

_{2}appeared to be independent of the chain length. We also investigated separately the contribution of each mode to the two elongational viscosities. For all three liquids, the intermolecular Lennard-Jones (LJ), intramolecular LJ, and bond-stretching modes make positive contributions to both η

_{1}and η

_{2}, while the bond-torsional and bond-bending modes make negative contributions to both η

_{1}and η

_{2}. The contribution of each of the five modes decreases in magnitude with increasing elongation rate. The hydrostatic pressure shows a clear minimum at a certain elongation rate for each liquid, and the elongation rate at which the minimum occurs appears to increase with the chain length. The behavior of the hydrostatic pressure with respect to the elongation rate is shown to correlate with the intermolecular LJ energy from a microscopic viewpoint. On the other hand, 〈R

_{ete}

^{2}〉 and 〈R

_{g}

^{2}〉 appear to be correlated with the intramolecular LJ energy. The study of the effect of the elongational field on the conformation tensor c shows that the degree of increase of tr(c)−3 with the elongation rate becomes stronger as the chain length increases. Also, the well-known linear reaction between σ and c does not seem to be satisfactory. It seems that a simple relation between σ and c would not be valid, in general, for arbitrary flows.

#### Recommended Citation

C. Baig, B. J. Edwards, D. J. Keffer, H. D. Cochran, and V. A. Harmandaris, J. (2006). Rheological and structural studies of linear polyethylene melts under planar elongational flow using nonequilibrium molecular dynamics simulations. Chem. Phys. 124, DOI:10.1063/1.2174006

## Comments

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