  A few experiments have reported that the time development of shear stress
under fast start-up shear deformations exhibits double peaks before reaching
the steady state for bimodal blends of entangled linear polymers in specific
conditions. To analyze the molecular origin of this phenomenon, multi-chain
slip-link simulations based on the primitive chain network model were conducted
for the data of a bimodal polystyrene solution reported by Osaki et al. [J Pol
Sci B Pol Phys, 38, 2043 (2000)] Owing to the reasonable agreement with the
data and the simulation results, the stress was decomposed into contributions
from long and short-chain components and decoupled into segment number,
stretch, and orientation. The analysis revealed that the first and second peaks
correspond to the short-chain orientation and the long-chain stretch,
respectively. The results also imply that the peak positions are not affected
by the mixing of short and long chains, whereas the second peak intensity
depends on the mixing conditions in a complicated manner, dominating the
emergence of double peaks.
