Alex Bourque1, David Nicholson1, Peng Yi2, Gregory C. Rutledge1
1Department of Chemical Engineering and 2Department of Physics
Massachusetts Institute of Technology
Cambridge, MA 02139
The properties and performance of commercial polymers depend strongly upon both the molecular scale architecture of the chains and the manner in which they are processed to form engineering materials. Polyolefins in particular owe much of their utility to the diversity of semicrystalline morphologies that can be realized. The development of semicrystalline morphology is controlled through processes of crystal nucleation and growth as the polymer is cooled from the melt, in either the presence or absence of process flows. Due to the small spatiotemporal scales involved, however, it remains a challenge to examine these processes directly and determine their underlying physical mechanisms. Using short and long n-alkanes as model materials, we perform molecular dynamics and Monte Carlo simulations to characterize homogeneous nucleation of the new crystal phase, followed by a re-examination of crystal growth as a sequence of surface nucleation and spreading processes, as proposed by Lauritzen and Hoffman (LH). We then describe a multi-scale model of polymer crystal growth that unites LH theory with the statistical theory of Johnson and Mehl, and Avrami. Finally, under typical processing conditions, one observes significantly accelerated crystallization kinetics in polymers, attributed to flow-enhanced nucleation (FEN). Using nonequilibrium molecular dynamics to study unimodal and bimodal melts of short and long chains in both shear and uniaxial elongation flows, we trace the origins of FEN to the highly nonequilibrium structure of the melt, which alters not only the driving force for nucleation but also the rate of diffusional collisions between chains. The simulation results are analyzed using a model of birth-death processes that accounts for nonequilibrium behavior during the induction period.