An assessment of models for flow-enhanced nucleation in an n-alkane melt by molecular simulation

Abstract

Flow-enhanced nucleation of the crystal phase under shear and uniaxial extension for a monodisperse melt of n-pentacontahectane (C150H302, or C150) chains was studied by nonequilibrium molecular dynamics (NEMD) simulation. The resulting acceleration in the crystal nucleation rate was correlated with macroscopically measurable properties of the flow field and with microscopic conformational statistics. Based on the fidelity of the observed correlations, several empirical models reported in the literature were evaluated for their abilities to account for the observed enhancement of the nucleation rate due to flow, and new models are proposed for data that do not comport with existing models. In agreement with prior reports, the nucleation rate was found to correlate well with first normal stress difference, the second invariant of the deviatoric conformation tensor, and the stretch ratio, albeit with some differences from the existing models. New models based on conformational invariants for Kuhn segments are proposed, and shown to describe the simulation data more accurately than those based on conformational behavior of entire chains. Within the applicability of the stress-optical rule, related models are proposed based on invariants of the extra stress tensor.

Stretching of Bombyx mori Silk Protein in Flow

“…Protein-based natural materials persistently inspire the development of novel human-made materials, owing to their biocompatibility, unique combinations of strength and toughness [ 1 , 2 , 3 , 4 ], low-energy processing [ 5 ] and efficient solvent recycling [ 6 ]. While the industrial production of polymer-based fibres is challenged by a highly non-trivial interdependence between the molecular level of bond-orientation-dependent nucleation, and the macroscopic level, where the temperature-dependent rheology generates stretch of entire chain segments [ 7 , 8 , 9 , 10 , 11 ], silk is processed in semi-dilute aqueous conditions [ 5 ], where nucleation can be induced through the stretch-induced disruption of the solvation layer [ 12 ]. In order to generate sufficient stretch at modest flow rates, the silk protein has evolved to contain ‘sticky’ patches (which are assumed to be consisting of ionic calcium bridges between the carboxylated side groups of aspartic and glutamic acids) that significantly slow down stretch relaxation in flow [ 6 , 13 ].…”

Molecular Dynamics Simulations of Crystal Nucleation near Interfaces in Incompatible Polymer Blends

“…Han and coworkers proposed that the inter-diffusion of polymer segments across the interfaces during the spinodal decomposition could promote chains to align perpendicular to the interfaces [ 14 , 15 , 16 ]. However, the reptative inter-diffusion may not be sufficient to impose strong alignment to polymers at the monomer or the Kuhn segment level, which is critical to accelerating crystal nucleation [ 20 , 21 , 22 ]. Nevertheless, highly incompatible semiflexible polymers tend to align parallel to interfaces, creating alignment layers of a thickness about a Kuhn length [ 23 , 24 , 25 ].…”

“…To better understand the coupling between phase separation and crystallization, we apply molecular dynamics (MD) simulations to investigate the crystal nucleation in inhomogeneous polymers. Previous authors have used MD simulations to study the kinetics and the precursors of the quiescent [ 27 , 28 , 29 , 30 , 31 , 32 , 33 ] and flow-induced [ 20 , 21 , 22 , 31 , 34 , 35 ] nucleation and crystallization in homopolymer melts. Computational studies of crystal nucleation in inhomogeneous polymer blends, however, are still lacking.…”

The Effects of Deformation on the Early Crystallization Kinetics of Basaltic Magmas

“…Although flow-enhanced nucleation in polymer melts has been well-documented experimentally and probed with molecular simulations (e.g., Nicholson and Rutledge, 2019 for a recent review), fewer studies have been performed on silicate melts. Kouchi et al (1986) deformed basaltic melts in torsion at a constant sub-liquidus temperature.…”