Primitive chain network simulations for elongational viscosity of bidisperse polystyrene melts
In spite of the industrial significance, molecular mechanism of the strain hardening saliently observed in bidisperse polymeric liquids has not been elucidated yet. In this study, the multi-chain slip-link simulation (called primitive chain network simulation) was performed for the bidisperse polystyrene blends for which experimental data for elongational viscosity have been reported earlier. The simulation reasonably reproduced linear viscoelasticity and transient and steady uniaxial elongational viscosities. It has been confirmed that the long chain stretch dominates the stress at the strain hardening as already demonstrated earlier via the tube model. The molecular analysis employing the decoupling approximation revealed for the first time that there exist two molecular mechanisms to induce strain hardening in bidisperse blends. The mechanism switches depending on the Weissenberg number with respect to the Rouse relaxation time of the long chain, WiRL. At WiRL < 1, the simultaneous increase of the long chain orientation and stretch with increasing WiRL lifts the viscosity beyond the Trouton’s viscosity. At WiRL ≥ 1, the isotropic short chain suppresses the stretch/orientation-induced reduction of friction to enhance the stretch of long chain.
Orientational Cross-Correlation in Entangled Binary Blends in Primitive Chain Network Simulations
It has been reported from the molecular dynamics simulation that the orientational cross-correlation (OCC) has a significant contribution to the relaxation modulus of polymers. Remarkably, the OCC contribution has been found to be universal with respect to the molecular weight and even to its distribution for unentangled polymers [Cao; Likhtman Phys. Rev. Lett. 2010, 104, 207801]. In this study, the OCC in entangled bidisperse polymer melts was evaluated by multichain slip-link simulations. The obtained OCC was similar to that in monodisperse systems in its intensity. On the other hand, the time development reflected the motion of short and long chains, and consequently the universality was not observed when the molecular weights of two components are well-separated and the long chain fraction is small. Comparison to the results obtained by the other slip-link model suggests that the cross-correlation is induced by the force balance and the fluctuation at the entanglement.
A constitutive analysis of transient and steady-state elongational viscosities of bidisperse polystyrene blends
The transient and steady-state elongational viscosity data of three bidisperse polystyrene blends were investigated recently by Nielsen et al. [J. Rheol. 50, 453–476 (2006)]. The blends contain a monodisperse high molar mass component (ML=390kg∕mol) in a matrix of a monodisperse small molar mass component (either MS=103kg∕mol or MS=52kg∕mol at two different weight fractions). The experimental data are analyzed in the framework of the molecular stress function model of Wagner et al. [J. Rheol. 49, 1317–1327 (2005)], which is based on the assumption of a strain-dependent tube diameter and the interchain pressure term of Marrucci and Ianniruberto [Macromolecules 37, 3934–3942 (2004)]. The dilution of the long chains in the matrix of the short chains is identified as the origin of a drastic increase in the tube-diameter relaxation time of the long chains, leading to a large stretching potential of the long-chain component and an increasing steady-state elongational viscosity with increasing strain rate. In ad...