Macromolecules, Vol.52, No.11, 4196-4208, 2019
Crystallization and Molecular Topology of Linear Semicrystalline Polymers: Simulation of Uni- and Bimodal Molecular Weight Distribution Systems
The crystallization behavior and the molecular topology of bimodal molecular weight distribution (MWD) polymers are studied using a coarse-grained molecular dynamics model with varying weight fraction of short and long chains. Extensive simulations have been performed to prepare polymer melts and obtain semicrystalline polymers by homogeneous isothermal crystallization. The incubation time (the time elapsed before the establishment of steady-state nucleation) is calculated, and the interfacial free energy is obtained using a mean first-passage time analysis. The incubation time first decreases with the weight fraction of long chains, reaches its maximum at 30%, and then increases. This results from the conflicting effects of interfacial free energy and mobility of chain segments. The interfacial free energy decreases with the weight fraction of long chains, which is attributed to the transition from intermolecular to intramolecular nucleation, whereas the chain mobility decreases with increasing long-chain content. Nevertheless, the growth rate of crystals decreases continuously with the weight fraction of long chains, mainly resulting from reduced chain sliding diffusion. We have provided insights into how bimodal MWD polymers promote both nucleation and processability. Moreover, a numerical algorithm has been proposed, tracing each chain going back and forth among crystallites, to access quantitative data of molecular topology (i.e., loop, tie, and cilia segments). It turns out that the concentration of loop and tie segments increases with the increasing weight fraction of long chains. This could be important to understand the mechanical properties of semicrystalline polymers.