Nonequilibrium Melt State of Ultra-High-Molecular-Weight Polyethylene: A Theoretical Approach on the Equilibrium Process

This work addresses the ability of the tube model to describe the rheological response of partially entangled, ultra-high-molecular-weight polyethylene (UHMW-PE) chains both in and out of equilibrium. It uses the tube model, in its usual form, to quantitatively describe the linear rheology of equilibrated UHMW-PE melts. Using a unique parameterization set for the tube model parameters, the molecular weight distribution of several samples has been determined. Concerning the transition to equilibrium, that is, entanglement recovery of the heterogeneous melt, this work examines the following two possibilities: (1) re-entanglement via ordinary reptation in dilated tubes and (2) re-entanglement by means of activated reptation. The former approach confines the chains into dilated tubes, that is, to tubes with a larger diameter than that at equilibrium, taking into account their partially entangled nature. Essentially, the model homogenizes φe, the initial (volume) fraction of entangled melt and permits molecular motions such as reptation in tubes that decrease in diameter with increasing time. This homogenization appears to work when φe is below a threshold value, which is about 0.4. For values larger than the threshold value, the proposed model performs poorly. Compared to the model prediction, the actual re-entanglement time is considerably longer, presumably because of the long time required for disentangled domains to maneuver themselves through the entangled fraction of the melt. In this regime, the activated reptation picture is more realistic. Further, the activated reptation picture appears to be applicable even below the threshold φe value, suggesting that re-entanglement occurs through activated reptation.