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Abstract
The rheological behaviours of the steady sheared short-chain polypropylene (PP) fluid are studied using isobaric isothermal nonequilibrium molecular dynamics simulations. By comparing the behaviours of PP fluid with that of the linear alkane fluid of n-hexadecane (C16) having equal backbone length, we investigated the effects of the branch structure on shear thinning, rotational relaxation time, critical shear rate and potential energies. The results showed that the degree of shear thinning of the PP fluid is lower than that of the C16 fluid. With respect to different temperatures, the degree of shear thinning of the former is less sensitive than that of the latter. At the molecular level, potential energies including van der Waals nonbonding interaction and bond stretching, bond bending, and bond torsion interactions are discussed. Significantly, the varying tendency of the bending potential of the PP fluid at very high shear rates is contrary to that of the C16 fluid. We propose, therefore, that the branch structure affects the bending angle distribution such that it causes differences in the rheological behaviours of these two fluids. Furthermore, in all the molecular potentials of the PP fluid, the torsion potential of the dihedral angle is observed to be the strongest dependent upon temperature.
Acknowledgements
We gratefully acknowledge the financial support from National Science Council of the Republic of China (Grant number: NSC 98-2221-E-007-008-MY2) and CoreTech System Co., Ltd (Moldex3D). We are also thankful for the NEMD data of n-hexadecane provided and discussed by Dr Huan-Chang Tseng, who researches in Molecular Dynamics Technology Co., Ltd, Hsinchu County, Taiwan.
Notes
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