In spite of the industrial significance, molecular mechanism of the strain hardening\nsaliently observed in bidisperse polymeric liquids has not been elucidated yet. In this\nstudy, the multi-chain slip-link simulation (called primitive chain network simulation)\nwas performed for the bidisperse polystyrene blends for which experimental data for\nelongational viscosity have been reported earlier. The simulation reasonably reproduced\nlinear viscoelasticity and transient and steady uniaxial elongational viscosities.\nIt has been confirmed that the long chain stretch dominates the stress at the strain\nhardening as already demonstrated earlier via the tube model. The molecular analysis\nemploying the decoupling approximation revealed for the first time that there exist\ntwo molecular mechanisms to induce strain hardening in bidisperse blends. The\nmechanism switches depending on the Weissenberg number with respect to the\nRouse relaxation time of the long chain, WiRL. At WiRL < 1, the simultaneous increase of\nthe long chain orientation and stretch with increasing WiRL lifts the viscosity beyond\nthe Trouton�s viscosity. At WiRL ? 1, the isotropic short chain suppresses the stretch/\norientation-induced reduction of friction to enhance the stretch of long chain.
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