# Effects of Orientational and Conformational Ordering on Isotactic Polypropylene Crystallization

## TL;DR

This study uses simulations to explore how different types of flow affect the crystallization of isotactic polypropylene, finding that conformational order is more important than orientational order for efficient nucleation.

## Contribution

The paper introduces a novel approach combining flow-induced alignment and conformational ordering to understand polymer crystallization mechanisms.

## Key findings

- Extensional flows cause stronger alignment in iPP compared to shear flows.
- Conformational ordering significantly reduces the nucleation barrier in iPP crystallization.
- Flow-induced orientational order alone does not promote rapid nucleation in simulations.

## Abstract

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.