(527b) Atomistic Simulation of Flow-Induced Configurational Microphase Separation and Crystallization of an Entangled Polyethylene Melt in Planar and Uniaxial Elongational Flow

This presentation will focus on recent work in the MRAIL Group at the University of Tennessee consisting of united-atom simulations of entangled polyethylene solutions and melts of linear C₁₀₀₀H₂₀₀₂ undergoing both planar and uniaxial elongational flow. Flow-induced phenomena in entangled solutions of linear C₁₀₀₀H₂₀₀₂ polyethylene dissolved in n-hexadecane and benzene solvents were simulated via nonequilibrium molecular dynamics at concentrations of 14.5C* and 13.5C*, respectively, of the coil overlap concentration, C*. The simulations revealed that both solutions undergo a chemical phase separation when subject to planar extensional flow at extension rates faster than the inverse Rouse time of the solution. The onset of phase separation initiated after roughly two Hencky strain units of deformation for both solutions and attained a stationary state at about ten Hencky strain units. Furthermore, the simulations revealed that at very high extension rates the polymer phase forms semicrystalline domains regardless of the solvent; however, the critical extension rate for flow-induced crystallization appeared to be affected by a number of variables, including solution temperature and the chemical nature of the solvent. Similar qualitative behavior was observed in atomistic simulations of the C1₀₀₀H₂₀₀₂ melt under both planar and uniaxial elongational flow. As for C₁₀₀₀H₂₀₀₂ polyethylene, at intermediate extension rates (i.e., 0.3 <De<1.5) coil-stretch transition of macromolecules , qualitatively similar to that predicted for dilute polymeric solutions is observed. Furthermore, it was observed that the coiled and stretched molecules underwent a configurational microphase separation such that the coiled molecules segregated into distinct ellipsoidal domains that were surrounded by sheet-like regions of stretched macromolecules. Similar to the entangled solutions of C₁₀₀₀H₂₀₀₂ ,the monodisperse flow induced crystallization is observed at high extension rates.

Thermodynamic-like local atomistic entropy and enthalpy variables were introduced as a means to delineate and quantify the various phase phenomena in atomistic simulations of extensional flows, especially the flow-enhanced nucleation and flow-induced crystallization events. These variables measure the local ordering and energetics at the monomer level, as opposed to the global system, and hence can be used to detect and quantify flow-enhanced nucleation events on the small length and time scales that lead to flow-induced crystallization. The kinetics of the nucleating localized crystals can also be tracked using an atomistic Gibbs free energy composite variable. Based on the assumption that the global crystallization process followed a first-order reversible kinetic rate expression with a lag time, kinetic rate constants were calculated as functions of the Deborah number that allowed quantification of the flow-induced crystallization phenomenon exhibited by the simulated systems. Finally, the influence of polydispersity on the flow-induced crystallization will be discussed.