Tension thickening, molecular shape, and flow birefringence of an H-shaped polymer melt in steady shear and planar extension

Abstract

Articles you may be interested in Nonequilibrium molecular dynamics simulation of dendrimers and hyperbranched polymer melts undergoing planar elongational flow J. Rheol. 58, 281 (2014) Despite recent advances in the design of extensional rheometers optimized for strain and stress controlled operation in steady, dynamic, and transient modes, obtaining reliable steady-state elongational data for macromolecular systems is still a formidable task, limiting today's approach to trial-and-error efforts rather than based on a deep understanding of the deformation processes occurring under elongation. Guided, in particular, by the need to understand the special rheology of branched polymers, we studied a model, unentangled H-shaped polyethylene melt using nonequilibrium molecular dynamics simulations based on a recently developed rigorous statistical mechanics algorithm. The melt has been simulated under steady shear and steady planar extension, over a wide range of deformation rates. In shear, the steady-state shear viscosity is observed to decrease monotonically as the shear rate increases; furthermore, the degree of shear thinning of the viscosity and of the first-and second-normal stress coefficients is observed to be similar to that of a linear analog of the same total chain length. By contrast, in planar extension, the primary steady-state elongational viscosity 1 is observed to exhibit a tension-thickening behavior as the elongation rate increases, which we analyze here in terms of ͑a͒ perturbations in the instantaneous intrinsic chain shape and ͑b͒ differences in the stress distribution along chain contour. The maximum in the plot of 1 with occurs when the arm-stretching mode becomes active and is followed by a rather abrupt tension-thinning behavior. In contrast, the second elongational viscosity 2 shows only a tension-thinning behavior. As an interesting point, the simulations predict the same value for the stress optical coefficient in the two flows, revealing an important rheo-optical characteristic. In agreement with experimental indications on significantly longer systems, our results confirm the importance of chain branching on the unique rheological properties of polymer melts in extension.

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

“…[1][2][3][4][5][6][7][8][9] Thus far, however, most research efforts aiming to explore the fundamental role of branches in polymer science have mainly focused on long-chain branched polymers, 2,[10][11][12][13][14][15][16] although it is equally well known that short-chain branching generally significantly affects a wide variety of physical properties such as crystallinity, melting point, modulus, and the hardness of polymeric materials. [16][17][18] From a thermodynamic viewpoint, 19 the standard approach would be to analyze the structure of polymers in solution or melt by accounting simultaneously for the energetics (polymer-polymer and polymer-solvent) and (intramolecular and intermolecular) entropy of the system, and then to determine the properties of the system based on the resulting structural information.…”

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

“…the fluctuation of bond stretching is small and therefore the bond lengths are approximate to the equilibrium bond length. The decrease in stretching potential is also seen in the works of alkane fluids [26,27].…”

“…The increase in the bending potential also occurred in linear [26] and H-shape [27] alkane fluids. On the other hand, the degree of chain stretch for the short-chain PP fluid at high shear rates is restricted by the branched structures, which causes the bending fluctuation around the equilibrium bending angle to be small.…”

“…At a molecular level, the cause of the differences in rheological behaviours between fluids from the energy transformation mechanism is due to the structure change of fluid molecules under shear. For example, the nonbonding (L -J) and torsion potential energies versus shear rate plot in Figure 5 shows that the two types of energy for both fluids increase at high shear rates, which is the same as the longer chain linear [26] and H-shape [27] alkane fluids. The rise in the potentials implies that the system energy is accumulated in the form of nonbonding and torsion potentials as speeding flow field.…”

“…On the other hand, the degree of chain stretch for the short-chain PP fluid at high shear rates is restricted by the branched structures, which causes the bending fluctuation around the equilibrium bending angle to be small. Therefore, for only four branches in a molecular chain, the H-shape fluids exhibited increasing bending potential [27]. From the above results of the potentials, we deduce that the branch effect on the bending angles at high shear rates is the major reason for the differences in rheological behaviours between branched and linear alkane fluids.…”