Methyl branch effects on rheological behaviours of short-chain polypropylene under steady shear studied via nonequilibrium molecular dynamics simulations

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 (C 16 ) 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 C 16 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 C 16 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.

Communication: Role of short chain branching in polymer structure and dynamics

“…These tendencies of branched polymers are supposed to arise from a lesser degree of chain stretch and alignment to the flow direction at a given flow strength (as compared to the linear analogues), due to a more compact molecular structure associated with their intrinsic nonlinear molecular architecture. 3,[24][25][26][27][28][29] This smaller structural deformation for branched systems decreases the magnitude of the attractive intermolecular interactions between different molecules, thus leading to an increase in the hydrostatic pressure (equal to one-third of the trace of the stress tensor σ) relative to the corresponding linear system [ Fig. 1(b)].…”