Phase Transition Behavior of Isotactic Polypropylene Crystallized from a Homogeneous Melt

The equilibrium melting behavior of helical structures in isotactic polypropylene (iPP) was studied by a Monte Carlo simulation. It was found that the melting of iPP helical structure was an exponential process at high temperatures (T $>$ 470 K), whereas it becomes highly stretched at low temperatures (T $<$ 400 K). An estimate of the melting temperature was obtained from a novel application of the second moment of the waiting time distribution for helix melting as described by Odagaki (T. Odagaki, \textit{Phys. Rev. Lett.} \textbf{1995}, 75, 3701). The calculated melting temperature was approximately 455 K, which is in surprisingly good agreement with experimental values. A second transition temperature was also observed at a lower temperature (T $\sim$390 K), which we can not explain by equilibrium considerations. We suggest that at this temperature, once a helix forms, it does not melt within the time scale studied.