Molecular dynamics simulations of polyethylene inter-crystalline phase formation

Semicrystalline microstructures consisting of both the crystalline and amorphous regions, govern the properties of many polymer materials. The formation of the semicrystalline morphology, especially the structural evolution in the amorphous regions, is still not well understood. Here we apply atomistic molecular dynamics simulations to crystal growth in model polyethylene (PE) samples. We placed two crystalline seeds into the PE melt and quenched the melt to 350K to trigger instantaneous crystallization. We quantitatively analyze the crystal growth and formation of the inter-crystalline region for samples with seeds of various inter-seed distances and relative orientations. Using the Z1+ method, we show that lamellar thickening is rapid until the lamellar thickness is about the size of the entanglement strands in the melt precursor state. Further crystallization requires entanglement relaxation, which leads to impeded crystal growth. Over time, a layer of “trapped” entanglements formed among loops and tie chains accumulate near the crystal surface and hinder further stem lengthening. Using simulations, we also quantitatively investigated the stress transmitters, which are the entangled loops and tie molecules, in the inter-crystalline region. We show a modified Huang-Brown (HB) model can estimate the tie-chain fraction between two crystalline slabs.