Molecular dynamics study on the stress-thermal relation of polymer melts under shear flows

Understanding the thermos-rheological properties of polymer melts under external flows is quite important in polymer processing. The stress-thermal rule (i.e., the linear relationship between the thermal conductivity tensor and the stress tensor) has been proposed based on a network theory of polymer melts. Experimental and simulation studies have confirmed it indeed holds for the anisotropic parts of the thermal conductivity tensor and the stress tensor of polymer melts under deformation. However, as far as the authors know, there have been few reports on the polymer melt under shear flows, except for a single experiments.

In this study, we investigate the stress-thermal relation of polymer melts under shear flows using a non-equilibrium molecular dynamics simulation, specifically employing the reversed non-equilibrium molecular dynamics (RNEMD) method. We made slight modifications to the source code of RNEMD in the LAMMPS package to concurrently achieve both temperature and velocity gradients.

Our main finding is that the stress-thermal rule is applicable to entangled polymer melts but not to non-entangled polymer melts. This characteristic sets it apart from the stress-optical rule, which uniformly applies to polymer melts, regardless of entanglement. Additionally, it has been revealed that once the linear stress-thermal rule is established, its coefficient in the linear relation remains independent of the length of polymer chains.