(84e) Predicting the Driving Force for Multi-Stage Crystallization in Polymer Samples

We combine atomistic simulations and polymer theories to predict the free energies of mesophases and the stable crystalline phase for oligomeric isotactic polypropylene (iPP) and polyethylene (PE). In simulations, we compute the free energy of conformational ordering, including coil-to-helix transition for iPP and coil-to-rod transition for PE. Together with orientational coupling interactions between polymer segments and orientational and translation entropy upon ordering, we show that iPP predominately undergoes a one-step crystallization from the isotropic phase to the crystalline phase. However, PE can spontaneously form partially ordered nematic and rotator phases before transitioning into the crystalline state by developing conformational order. This multi-step mechanism permits rapid nucleation of entangled PE into the rotator crystals, as observed in atomistic simulations. Our predicted bulk phase transition temperatures for iPP and PE of different molecular weights agree well with experiments. Overall, our model reveals the thermodynamic driving force of orientational and conformational ordering for semicrystalline polymers.