Molecular Dynamics Study of Star Polymer Melts under Start-up Shear

It has been established that star polymers with a large number of arms display both the conventional chain relaxation and a slower interchain relaxation due to the formation of hard cores, which is called the local structural relaxation. Here, we demonstrate that this unique relaxation property of multi-arm stars leads to unusual rheological behaviors. We study the response of star polymer melts with various arm numbers to start-up shear flow via nonequilibrium molecular dynamics simulation. Our simulation results indicate that the stress-strain response of multi-arm star polymer melts differs from that of star polymers with a small number of arms in both quantitative and qualitative manners.While the multi-arm star polymer melts exhibit an overshoot peak in the stress-strain curve both at relatively small and sufficiently large shear rates, two overshoot peaks appear when the flow strength is comparable to the relaxation time of the star polymers. This double stress overshoot is absent in linear polymers and star polymers with a small number of arms and is evidently related to the presence of the local structural relaxation in star polymers with a large number of arms. We present a detailed analysis of the conformational and dynamical properties of star polymer melts under start-up shear to gain a molecular understanding of their stress response.