Molecular Dynamics of Crystallization in a Helical Polymer Isotactic Polypropylene from the Oriented Amorphous State

Molecular simulation is attracting great interest in these days as a powerful tool to reveal the molecular mechanism of polymer crystallization. Most of the studies reported hitherto have dealt with simple model polymers like polyethylene, and the majority of polymers having more complex chemical structures remain almost untouched. In this report, we make a new challenge to the crystallization in a typical helical polymer isotactic polypropylene (iPP). We consider a relatively small system of 40 iPP oligomers each made of SO propylene monomers by use of a realistic flexible model. The crystallization of iPP has long been considered too sluggish to be investigated by molecular simulations. We here take advantage of the accelerated crystallization from the highly stretched amorphous state. By carrying out very long simulations of about 600 ns total, we succeed in observing the onset of crystallization and subsequent crystal growth. Very unexpectedly, we come to see the growth of the smectic mesophase, which is believed to grow at temperatures much lower than the melting point. For the growth of crystals of stable forms of definite chirality, such as the alpha-form or the beta-form, the chirality selection process will need a much longer time. The hypothesis is that the very fast crystallization observed here does not allow the slower processes of helical selection and gives rise to the formation of the smectic mesophase with somewhat random disposition of helices.