Flow-induced conformational and orientational order of isotactic polypropylene from non-equilibrium molecular dynamics

Isotactic polypropylene (iPP) is arguably one of the most important semi-crystalline polymers. In its crystalline regions, iPP chains adopt helical conformations, with every three monomers completing one turn of the helix. These helices pack uniaxially to form crystalline lamellae, highlighting the critical role of both conformational and orientational order in iPP crystallization. By stretching polymer chains, processing flows can enhance crystallization in iPP. To better understand the role of flow in iPP crystallization, we use all-atom Non-Equilibrium Molecular Dynamics (NEMD) simulations with the OPLS-AA force field to investigate iPP crystallization across different chain lengths (18, 30, 60, and 72 monomers). We employ NEMD simulations to induce orientational and conformational ordering in the polymer chains by applying uniaxial extensional flow and shear flow at four temperatures: 350, 400, 450, and 500 K. We find that both types of flows generate orientational ordering in the iPP melt, with extensional flow producing a higher degree of ordering. However, at sufficient flow strength (Weissenberg number Wi > 10), the orientational ordering becomes independent of the strain rate and reaches a plateau. Nonetheless, the applied flows do not result in greater conformational ordering, yielding an average of only one turn per chain, with right-handed and left-handed helices occurring in equal proportions. We show that without sufficient helical order in the precursor state, high flow-induced orientational order does not promote rapid crystallization in iPP. This behavior contrasts with flow-enhanced crystallization observed in non-helical polymers, such as polyethylene.