Thermodynamic Origin of Multistep Polymer Crystallization

We combine atomistic simulations and theories to predict the free energy of polymer crystallization, which highlights the roles of backbone stiffness and orientational coupling between monomers in the formation of metastable mesophases. We show that polyethylene (PE) with sufficient backbone stiffness in the conformationally disordered state can develop uniaxial order through metastable nematic and rotator phases before conformationally ordering into the crystalline state. Isotactic polypropylene (iPP), with a lower backbone stiffness, undergoes one-step crystallization directly from the isotropic phase to the crystalline phase. The predicted bulk phase transition temperatures for iPP and PE of different molecular weights agree well with experiments.