Kinetic study of the II-I phase transition of isotactic polybutene-1

Kinetics of the solid-solid II-I phase transition of isotactic polybutene-1 was investigated. The fraction WI of phase I as a function of time ttr during the phase transition was measured by X-ray diffraction at various temperatures Ttr. The Avrami indices n of the WI-ttr plots are approximately unity for Ttr > 288 K. A bell-shaped temperature dependence of the transition rate V with the maximum transition rate at 285 K was obtained. The V-Ttr curve and the Avrami index n = 1 suggest that the rate-determining process is primary nucleation. The dependence of V on Ttr for Ttr < 283 K is described by the William-Landel-Ferry (WLF) equation, which shows that the glass transition affects the transition rate. The Avrami index decreases to n < 1 for Ttr < 283 K, indicating a broadened distribution of the transition rate caused by the spatial heterogeneity of the amorphous state at low temperatures near the glass transition. Those evidences at low temperature clearly suggest that the solid-solid phase transition is influenced by the mobility of chain folding, tie chains and cilia in the amorphous between the stacks of lamellar crystals.