Melting and surface nucleation behavior of syndiotactic polypropylene (sPP) in crystalline Form II were studied via molecular dynamics (MD) simulation. Accelrys Material StudioR with built-in engines for initial system generation and molecular dynamic simulation (Forcite) was adopted. For the study of melting, an NPT ensemble of 2 × 2 × 15 Form II unit cells under PCFF force field and periodic boundary conditions was chosen. The MD box for the study of secondary nucleation corresponded to an NVT ensemble of 4 × 6 × 15 unit cells under DREIDING force field, with the central 5 unit cells along the c-axis artificially frozen to represent crystal surface.
Melting was observed to start from the fold surface and then spreading to the central region of the lamellar crystal. Instead of axial translation, our MD observations indicated that the melting process in sPP involves the thermal expansion of lattice, disruption of the helical conformation, followed by lateral expansion (increased inter-chain distance) and further randomized chain conformation.
For secondary nucleation, our MD simulation results indicated that trans-like conformation is preferentially formed both near the crystal surface and in the bulk matrix at a high supercooling of ΔT ≈ 150 K. Instead of stem attachment assumed in the Hoffman-Lauritzen scenario, we have observed only the increase of trans-like structures within the MD simulation time up to 55 ns. The trans-conformers in trans-like sequences were observed to extend along the chain but then became limited by sequences of gauche-like conformers. The trans-like conformers formed initially in the vicinity of (but not necessarily on) the crystal surface and then started to spread to regions deeper in the bulk matrix after an extended MD time of 35 ns. These fluctuation-induced trans-like conformers were formed in a sequential manner, piling one on top of another without full contact. These MD results are consistent with our experimental observations that the nucleation stage of sPP nucleation involves the formation of trans-rich sequences; this transformation from all-trans to helical (TTGG) sequences appears to serve as the rate determining step in sPP crystallization.

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