Flow-induced crystallization of a polyethylene liquid above the melting temperature and its nonequilibrium phase diagram

Abstract

Manufacturing of plastics is typically performed via flow processing of a polymer melt. Semicrystalline polymers, such as polyethylene (PE), play a critical role in the global plastics markets, but fundamental understanding of how process flow conditions affect their properties is lacking. Nonequilibrium molecular dynamics of a PE liquid above its melting point revealed reversible transitions from coiled, biphasic, stretched, pretransitional, and crystalline phases with applied stress in elongational flow. A nonequilibrium phase diagram was developed for the thermodynamic state space.

Flow-Induced Phase Separation and Crystallization in Entangled Polyethylene Solutions under Elongational Flow

“…The PE/hexadecane and PE/benzene solutions were simulated at temperatures deemed to be above the quiescent solution crystallization temperature, which for PE/hexadecane was estimated (see below) to be 391 K. This is above the boiling point of benzene (353 K), so the two solutions could not be simulated at the same value of temperature. Instead, we chose to simulate the PE/hexadecane solution at 450 K since we have simulated C 1000 H 2002 melts at this temperature in previous work, ,, which facilitates comparisons between the PE melt and PE solution. The simulation temperature of the benzene solution (343 K) was chosen to be below the normal boiling point of benzene but high enough to ensure that no quiescent crystallization would occur.…”

“…Development of a unifying theme to describe the underlying physics for these observations is further complicated by the experimental observations that when oligomeric solvents are employed, entangled solutions exhibit similar flow response as entangled melts within certain ranges of solvent molecular weight and solution concentration. , To begin to unravel these complexities, a few fundamental questions need to be addressed. (1) Do entangled polymeric solutions with small molecule and oligomeric solvents undergo a similar configuration hysteresis as observed in entangled melts? (2) Do they experience a configurational or chemical phase separation and crystallization under strong elongational flows? , (3) What are the rheological consequences of such flow-induced phenomena?…”

Nonequilibrium Thermodynamics of Polymeric Liquids via Atomistic Simulation

“…This inhomogeneous phase manifests as bimodal PDFs [ 52 ]. At high strain rates ( ), the chains are almost exclusively highly stretched, providing tall, narrow distributions approaching the fully extended chain length of 1290 Å, with flow-induced crystallization eventually occurring at as induced by random nucleation events [ 34 , 53 ]. Taking all into perspective, one would intuitively expect that any entropy calculations based on Gaussian statistics of a homogeneous melt would be largely inaccurate for all but the lowest .…”

“…In both cases, the change in internal energy with the strain rate is a monotonically decreasing function, which varies only mildly in the linear viscoelastic flow regime at low strain rates. As the strain rate increases, the internal energy decline accelerates as the molecules extend under the applied flow, lowering the torsional energy of the fluid as more dihedral angles assume trans configurations (rather than gauche ) and ultimately drops precipitously at high strain rates, especially in PEF where the highly stretched molecules orient and align with each other, thereby inducing a large negative change in the intermolecular LJ energy [ 14 , 53 ].…”

“…In a rubber, the entanglements (i.e., cross-links) are fixed, and no significant molecular or even segmental extension can occur; hence, the behavior of these materials is almost purely entropic, as has been verified experimentally long ago. However, in a polymer melt, there is a significant reduction in the number of entanglements under flow conditions [ 11 , 12 , 13 , 22 , 53 ], and this leads to substantial molecular and segmental extension that not only changes the intramolecular internal energy but also allows for alignment of the molecules and segments that leads to substantial changes in the intermolecular internal energy. At high flow rates, where the entanglement network is effectively degraded, these internal energy changes not only become non-negligible but can also become of comparable magnitude to the entropic changes.…”