Polymer Crystallization

Polymer crystallization is a very old problem. New experiments on the early stages of polymer crystallization reveal a precursor liquid-liquid phase separation in many polymers. This raises fascinating questions about metastability and crystallization in a wide variety of sytems, not just polymers!


Kinetic Monte Carlo flow-induced nucleation in polymer melts
RS Graham and PD Olmsted, Faraday Discussions 144 (2010) 71-92.

We derive a kinetic Monte Carlo algorithm to simulate flow-induced nucleation in polymer melts. The crystallisation kinetics are modified by both stretching and orientation of the amorphous chains under flow, which is modelled by a recent non-linear tube theory. Rotation of the crystallites under flow is modelled by a simultaneous Brownian dynamics simulation. Our kinetic Monte Carlo approach is highly efficient at simulating nucleation and is tractable even at low under-cooling. The simulations predict enhanced nucleation under both transient and steady state shear. Furthermore the model predicts the growth of shish-like elongated nuclei for sufficiently fast flows, which grow by a purely kinetic mechanism.

Coarse-grained simulations of flow-induced nucleation in semi-crystalline polymers
RS Graham and PD Olmsted, Physical Review Letters 103 (2009) 115702.

We perform kinetic Monte Carlo simulations of flow-induced nucleation in polymer melts with an algorithm that is tractable even at low undercooling. The configuration of the noncrystallized chains under flow is computed with a recent nonlinear tube model. Our simulations predict both enhanced nucleation and the growth of shish-like elongated nuclei for sufficiently fast flows. The simulations predict several experimental phenomena and theoretically justify a previously empirical result for the flow-enhanced nucleation rate. The simulations are highly pertinent to both the fundamental understanding and process modeling of flow-induced crystallization in polymer melts.

Experimental observations and matching viscoelastic specific work predictions of flow-induced crystallization for molten polyethylene within two flow geometries
L Scelsi, MR Mackley, H Klein, PD Olmsted, RS Graham, OG Harlen, and TCB McLeish, Journal of Rheology 53 (2009) 859-876.

Flow-induced crystallization (FIC) behavior of a high-density polyethylene melt in two entry-exit flow geometries was investigated by direct optical observation using a multi-pass rheometer and the results compared with a viscoelastic flow simulation. A set of experiments was performed at several piston speeds using a sharp and a rounded entry-exit slit and the region of onset for visible FIC was identified in both cases. During flow narrow crystal filament regions localized at the sidewalls and in a downstream “fang” region of stress accumulation were identified. A melt flow two-dimensional numerical simulation using a Lagrangian solver, FLOWSOLVE, and an 11-mode Pom-Pom model satisfactorily matched experimental pressure difference and birefringence fringe distribution for the flow. An algorithm to calculate the specific work accumulated by each fluid element in the complex flow field was implemented within FLOWSOLVE and a method was proposed to estimate the critical specific work for the onset of visible oriented FIC. The concept of specific work applied to the numerical simulations was capable of successfully predicting the experimental regions where FIC occurred.

The specific work of flow as a criterion for orientation in polymer crystallisation
OO Mykhaylyk, P Chambon, RS Graham, JPA Fairclough, PD Olmsted, and AJ Ryan, Macromolecules 41 (1901) 2008.

Shear induced crystallization in model blends of linear and long-chain branched hydrogenated polybutadienes
EL Heeley, CM Ferneyhough, RS Graham, PD Olmsted, N Inkson, J Embery, DJ Groves, T McLeish, A Morgovan, F Meneau, W Bras, and AJ Ryan, Macromolecules 39 (2006) 5058-5071.

The early stages of crystallization in isotactic polypropylene
EJ Heeley, AV Maidens, PD Olmsted, W Bras, IP Dolbnya, JPA Fairclough, NJ Terrill, and AJ Ryan, Macromolecules 36 (2003) 3656-3665.

An experimental study of the early stages of crystallization in iPP has shown a qualitative difference between the behavior at low supercooling with that observed with a deep quench. To address previous misgivings in the limits of resolution of crystallites by wide-angle scattering, a new detector has been used that has many orders of magnitude improvements in count rate. At low degrees of undercooling there is a substantial gap between the appearance of a peak in the small-angle scattering, associated with electron density modulations, and the resolution of crystallites. This early growth in electron density has been analyzed in terms of a spinodal decomposition process and the stability limit of isotactic polypropylene determined for three different samples of varying molecular weight. The underlying physics of the early stages of crystallization are discussed and a number of scenarios eliminated; at high temperature Avrami kinetics are not observed whereas at low temperatures the structure in both the small-angle and wide-angle regimes grow contemporaneously following secondary nucleation.

A Scattering Study of Nucleation Phenomena in Polymer Crystallisation
AJ Ryan, JPA Fairclough, NJ Terrill, PD Olmsted, and WCK Poon, Faraday Discussions 112 (1999) 13-30.

Spinodal-assisted nucleation in polymer melts
PD Olmsted, WCK Poon, TCB McLeish, AJ Ryan, and NJ Terrill, Physical Review Letters 81 (1998) 373-376 (pspdfor pdfpdf)

Recent experiments have shown that in some polymer melts quenched below the melting temperature, spinodal kinetics are observed in small-angle X ray scattering before the emergence of Bragg peaks at wide angles. We explain these observations by proposing that the coupling between density and secondary order parameters (chiefly chain conformation, but also orientation) gives rise to a liquid-liquid binodal buried within the equilibrium liquid-crystal coexistence region. A simple phenomenological theory is developed to illustrate the idea. Shear is shown to enhance the kinetic role of this hidden binodal. A number of experimentally testable consequences of this model are discussed, including the effects of the `hidden' critical point on crystallization kinetics.

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