Atomistic simulation of microphase separation and flow-induced crystallization above the melting point of entangled polymers under 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. 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 system under planar elongational flow at a temperature high above its quiescent melting point. Similar results for both PE melts and solutions were also found for uniaxial elongational flows, with very subtle differences arising in the thermodynamic phase diagram.