Kinetics of Nucleation and Growth of Form II to I Polymorphic Transition in Polybutene‑1 as Revealed by Stepwise Annealing

Kinetics of II to I polymorphic transformation in isotactic polybutene-1 (PB-1) and its annealing temperature and time dependencies have been investigated by means of differential scanning calorimetry and in situ wide-angle X-ray diffraction techniques. The PB-1 samples were isothermally crystallized into metastable form II crystalline modification followed by annealing at a lower temperature (T l) and at a higher temperature (T h) subsequently or at a single temperature (T s) to promote polymorphic transition from form II to I. This solid-to-solid phase transition was shown to be a two-step process including nucleation and growth suggested by the result that more form I was obtained after being annealed at T l and T h than annealed at T s for the same period. Annealing at T l benefits nucleation due to internal stress induced by unbalanced shrinkage of amorphous and crystalline phases because of their different thermal expansion coefficients, while annealing at T h is beneficial to growth owing to rapid segmental diffusion at that temperature. At a given annealing time at T l (t l) and at T h (t h), and fixing one of temperatures between T l and T h, it shows a maximum in the transformation-temperature profile that can be correlated with the optimal temperature for nucleation or growth. The phase transition was efficiently accelerated with the increase of isothermal crystallization temperature. Such dependency can be understood as a result of higher internal stress built up during cooling from higher isothermal crystallization temperature to T l. Our results decomposed the polymorphic transition into nucleation and growth for the first time providing a simple and effective way for rapid transition of form II to I in PB-1.