(74a) Atomistic Simulation of Flow-Enhanced Nucleation and Flow-Induced Crystallization Above the Melting Point of Entangled Polymer Melts and Solutions Under Elongational Flow

Polyethylene (PE) is a semicrystalline material possessing a microstructure containing both crystalline and amorphous regions, which can be processed in both the melt and solution state. This combination gives rise to the polymer's high strength and flexibility, which to a great extent can be tuned using the PE's degree of crystallinity as a parameter. The crystalline microdomains are formed by chain folding, wherein the long macromolecules fold back on themselves repeatedly, arranging into stacked structures called lamellae. These high strength lamellar domains are surrounded by amorphous regions of entangled polymer chains that provide flexibility and thermal/electrical resistance to the semicrystalline material.

Polyethylenes generally have normal melting points that vary greatly from low density to high density materials (0.88 - 0.96 g/cm³), typically ranging from 383-423 K for common varieties with a heat of fusion of approximately 290 J/g. Many studies point to single crystals grown from quiescent melts that possess orthorhombic or hexagonal lattices (both possessing a definite triple point with the melt phase), whereas crystals formed under mechanical stress tend to pack into a monoclinic structure. Most polymeric materials are processed in the liquid state. The intricate nonlinear coupling between fluid motion and microstructural evolution greatly complicates the fabrication of polymeric materials, and the ability to induce the desired microstructure through manipulation of the flow field is to a great extent responsible for many useful products made of polymers. Indeed, most semicrystalline fiber materials are formed via crystallization under coupled flow and thermal transport processes. Consequently, the study of flow-induced crystallization (FIC) has been a rich subject of research over the past 50 years. FIC can be applied to form semicrystalline polymers into continuous fibers and plastic sheets with microstructures that are highly dependent on the nature and strength of the applied flow field.

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.

In addition to the flow-induced phase separation and flow-induced crystallization noted above, these fluid systems also displayed random local nucleation events, consisting of highly ordered regions of approximately 20-30 Angstroms, even under quiescent conditions; these appeared randomly throughout the system for brief periods of time before disappearing. Under flow, these nucleation events were enhanced by global chain stretching, forming larger and longer-lasting nuclei, which, under high enough flow rate, would ultimately expand in size and lead to flow-induced crystallization.

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 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 due to the difference in the principal axes of the two types of elongational flow.