Effects of Orientational and Conformational Ordering on Isotactic Polypropylene Crystallization.

We employ atomistic nonequilibrium molecular dynamics (NEMD) simulations to investigate the effects of homogeneous shear and uniaxial extensional flows on the crystallization of isotactic polypropylene (iPP) oligomers. Extensional flows induce stronger, molecular-weight-dependent alignment in iPP, whereas chains in shear flows exhibit weaker alignment that is independent of chain length. While both flow types promote uniaxial alignment of polymer segments, neither induces helical conformational order in iPP. Upon quenching below the melting temperature, flow-aligned chains tend to relax toward their isotropic state unless the flow stress is maintained, indicating that flow-induced orientational order alone is insufficient to promote rapid iPP nucleation in simulations. By inducing helical order in iPP chains via dihedral restraints, conformationally ordered chains quickly develop orientational order and crystallize in simulations at elevated temperatures. Extrapolation of the melting temperatures to zero restraint recovers experimental crystal melting behavior of iPP oligomers. Using classical nucleation theory, we show that conformational ordering can drastically reduce the nucleation barrier, whereas orientational alignment alone lowers it modestly, preventing nucleation on accessible MD timescales.