Nonequilibrium Monte Carlo Simulation of Flow-Induced Crystallization of a Short-Chain Polyethylene Liquid in Uniaxial Elongational Flow

Nonequilibrium Monte Carlo simulations were performed for an atomistic model of a dense liquid composed of linear polyethylene chains undergoing uniaxial elongational flow. The simulations were conducted at four temperatures ranging from 300 K to 450 K. At the higher temperatures of 400 K and 450 K, simulation results revealed that the polyethylene chains were stretched significantly as a function of flow strength, but that the systems remained in the liquid phase. At the lower two temperatures of 300 K and 350 K, clear evidence was obtained of a flow-induced phase transition to a crystalline solid phase. This evidence included a structure factor for the multi-chain system that compared favorably with an experimental x-ray diffraction measurement of a crystalline linear polyethylene at all relevant length scales, including Bragg peaks at the correct k values. Simulated values of the internal energy (and the configurational temperature) revealed a flow-induced jump in absolute value, reminiscent of a first-order phase transition. The heat capacity of both phases could be calculated based on the configurational temperature. A distinct flow-induced enthalpy change was also evident between the liquid and crystalline states. Monitoring the configurational temperature of the system revealed a strong flow decrease with increasing flow strength, providing a plausible microscopic physical origin (i.e., related to the local conformation environment of the chains) for the flow-induced enhancement of the crystalline (or melting) temperature that have been reported in experiments.