793 research outputs found
Primitive chain network simulations for H-polymers under fast shear
No full textBranchpoint Withdrawal (BPW) has been recognized as one of the important molecular mechanisms for the description of the dynamics of entangled branched polymers under fast flows. However, the relation to the other known molecular mechanisms has not been fully elucidated yet. In this study we performed primitive chain network (i.e., multi-chain slip-link) Brownian simulations for a melt of a well-characterized monodisperse polystyrene H-polymer, for which the linear viscoelasticity and shear viscosity growth curves at several shear rates are available in the literature. After confirming the consistency of the simulations with the rheological data, we used the simulations to analyze the molecular motion in detail. The results reveal that molecular tumbling occurs in branched polymers just as in linear ones, and that it is accelerated by BPW. Furthermore, BPW not only mitigates backbone stretch, as expected, but also arm stretch. However, because the transient startup viscosity is anyhow dominated by chain stretch dynamics rather than by molecular tumbling, our results rationalize the fact that pom-pom theories successfully ignore tumbling in shear flows
Melts of Linear Polymers in Fast Flows
No full textWe report on recent progress in our understanding of the rheological behavior of polymer melts, resulting from the work of several investigators, mostly on fast extensional flows. They contributed new data from cleverly conceived experiments, new results of Brownian and molecular dynamics simulations, and suitable theoretical models. The outcome of such collective effort is the plausible certainty that flow-induced coalignment of the Kuhn segments of the polymer modifies the friction coefficient, which may depart considerably from the equilibrium value. It is also found that the effect is sensitive to the chemistry, being different in different polymers. It so appears that the universality of the rheological behavior of polymer melts, well documented under equilibrium conditions and in relatively slow flows, is lost in fast ones. This feature, however, opens interesting perspectives of future research and application
Primitive Chain Network Simulations of Entangled Melts of Symmetric and Asymmetric Star Polymers in Uniaxial Elongational Flows
No full textIanniruberto and Marrucci developed a theory whereby entangled branched
polymers behave like linear ones in fast elongational flows. In order to test
such prediction, Huang et al. performed elongational measurements on star
polymer melts, indeed revealing that, in fast flows, the elongational viscosity
is insensitive to the molecular structure, provided the molecular weight of the
backbone is the same. Inspired by these studies, we here report on results
obtained with multi-chain slip-link simulations for symmetric and asymmetric
star polymer melts, as well as calculations of the Rouse time of the examined
branched structures. The simulations semi-quantitatively reproduce the
experimental data if the Kuhn-segment orientation-induced reduction of friction
(SORF) is accounted for. The observed insensitivity of the nonlinear
elongational viscosity to the molecular structure for the same span molar mass
may be due to several factors. In the symmetric case, the calculated Rouse time
of the star marginally differs from that of the linear molecule, so that
possible differences in the observed stress fall within the experimental
uncertainty. Secondly, it is possible that the flow-induced formation of hooked
star pairs makes the effective Rouse time of the aggregate even closer to that
of the linear polymer because the friction center moves towards the branchpoint
of the star molecule. In the asymmetric case, it is shown that the stress
contributed by the short arms is negligible with respect to that of the long
ones. However, such stress-reduction is balanced by a dilution effect whereby
the unstretched arms reduce SORF as they decrease the Kuhn-segment order
parameter of the system. As a result of that dilution, the stress contributed
by the backbone is larger. The two effects compensate one another so that the
overall stress is virtually the same as the other architectures.Comment: 19 pages, 6 figure
Simulazioni mesoscopiche della dinamica di polimeri nei fusi e nelle soluzioni concentrate
No full textQuantitative comparison of primitive chain network simulations with literature data of linear viscoelasticity for polymer melts
No full textPrimitive chain network simulations of conformational relaxation for individual molecules in the entangled state
No full text
METHOD FOR ANALYZING MOLECULAR MOTION OF BRANCHED LONG CHAIN POLYMER
No full textA method for analyzing molecular motion of a branched long chain polymer wherein the position coordinate, the orientation vector, the number of monomers in the constituent elements of a tube and counterpart tube constituent elements forming entanglement are time-developed over a unit time defined by expression 7 according to the Langevin equation defined by expression 1 on the basis of the reptation theory with regard to an inter-tube constituent element constituting each polymer produced by coarse-graining the polymers of a mixture of the branched long chain polymer material to be analyzed according to the entanglement molecular weight (step 201), the number of monomers in the constituent element of the tube is time-development according to expression 5 (step 202), the number n of monomers in the constituent element of the tube at each end of a polymer is monitored, the constituent element of the counterpart tube forming entanglement is altered according to expression 8, and geometrical position exchange is caused to take place between the entanglement and a branch point by the number of constituent element of the tube at a branch part belonging to the branch poin
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