Understanding flow-induced crystallization in polymers: A perspective on the role of molecular simulations

Abstract

Flow induced crystallisation in polymers is an important problem in both fundamental polymer science and industrial polymer processing. The key process of flow-induced nucleation occurs on a very rapid timescale and on a highly localised lengthscale and so is extremely difficult to observe directly in experiments. However, recent advances in molecular dynamics simulations mean that flowinduced nucleation can be simulated at an achievable computational cost. Such studies offer unrivalled time and lengthscale resolution of the nucleation process. Nevertheless, the computational cost of molecular dynamics places considerable constraints on the range of molecular weights, temperature and polydispersity that can be studied. In this review I will discuss recent progress, describe how future work might resolve or work around the constraints of molecular simulation and examine how multiscale modelling could translate molecular insight into improved polymer processing.

The Origins of Enhanced and Retarded Crystallization in Nanocomposite Polymers

“…The results reveal that for all systems, initially, the radius of gyration increased as the time proceeded; however, for some nanocomposite systems, a plateau emerged indicating that molecular extension ceased or slowed significantly. It is well known and revealed in our earlier simulations and works by others [13] that molecular extension is necessary for polymer nucleation and subsequent crystallization. Such processes are responsible for flow-induced crystallization [14,15,29].…”

“…Molecular dynamics simulations are capable to study the crystallization process and explore the kinetics of crystallization and structural effects under various conditions. As a method to calculate properties globally and locally at the nanoscale, molecular dynamics (MD) has been used by practitioners for several decades in the study of polymer crystallization [12,13]. We have used this method to study crystallization under quiescent and flow conditions and near surfaces [14,15,16].…”

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

“…[26][27][28][29][30][31][32]. Graham has recently reviewed some of the pertinent literature [33]. Although substantial progress has already been made at understanding the intimate coupling between flow history and morphology on the macroscopic and mesoscopic levels, what is currently missing is an atomic-scale understanding of the morphology and kinetics of the crystallization phenomenon and the effects of flow upon it; in particular, the experimental observation of what induces crystallization of these materials at temperatures far above the melting point and a viable theoretical means of constructing nonequilibrium phase diagrams.…”

“…Nevertheless, even at equilibrium, crystal nucleation events can be captured and analyzed via MD, which can shed light on similar nucleation kinetics induced under flow. Indeed, the majority of this work involves realistic atomistic and united-atoms models of PE [33,[36][37][38][39][40][41][42][43][44][45][46], which are known to be quite accurate for both the liquid and solid phases due to the simplicity of the interatomic forces. Notable of the NEMD and NEMC studies of FIC are those of Baig and Edwards [40,41,47] and Rutledge and co-workers [36,39,[44][45][46].…”

A fundamental rule: Determining the importance of flow prior to polymer crystallization

“…There are a number of modelling efforts focused on describing flow-enhanced crystallization under typical processing conditions 4,5 , which incorporate the formation of multiple crystal phases 6 . There also exist numerous fundamental studies investigating flow-enhanced crystallization under controlled conditions; see Ref 7 for a recent comprehensive review of the literature. Many of these fundamental studies investigate how crystals develop during an isothermal 'simple' flow, for example see Refs.…”