Monte-Carlo simulations of polymer crystallization in dilute solution

Abstract

Polymer crystallization in dilute solution is studied by three-dimensional Monte-Carlo simulations using the bond fluctuation model. We study monodisperse chains of moderate length, intended to model recent experiments on monodisperse alkanes with length of a few hundred carbon atoms, and we also investigate chain folding of very long polymers. For monodisperse flexible chains we observe both extended-chain and once-folded-chain crystals. The simulations illustrate the range of defects and irregularities which we expect to find in polymer crystals. The roughness of the top and bottom surfaces of the lamellae is measured. Chain ends can be seen as cilia emerging from the surfaces. Folds are found to occur with approximately equal frequency on top and bottom surfaces. Although most chain folds are aligned perpendicular to the growth direction, a significant number of chains folding parallel to the growth direction are found as defects. The simulation includes a chain stiffness parameter which has an important effect on chain folding kinetics. When chains are semi-flexible the crystals formed are extremely irregular with many defects including holes and blocks of extended chains within the folded chain lamellae. For very long chains we show that the lamellar thickness is determined by the folding kinetics. The thickness diverges as the temperature approaches the infinite chain melting point T∞. For T→T∞, the thickness is close to the theoretical minimum thickness, which indicates the dominant importance of the entropic barrier in crystallization.