Thermally Induced Phase Transition of Polybutene-1 from Form I ' to Form II through Melt Recrystallization: Crucial Role of Chain Entanglement

Metastable crystal phase of polymorphic polymers can transform into another phase through a melt recrystallization pathway, which involves the melting of the original polymorph and the nucleation/growth of the transformed polymorph. Both the melting and recrystallization steps are influenced by chain entanglement. However, the role of chain entanglement in the melt-recrystallized phase transition is elusive. Herein, we use polybutene-1 (PB-1) as a representative polymorphic polymer and studied the effects of molecular weight (MW) and chain entanglement on the melt-recrystallized phase transition. A series of less-entangled PB-1 with controllable entanglement degrees are prepared by freeze drying. The freeze-dried PB-1 (mainly containing form I ') melts first and then recrystallizes into form II during heating and high-temperature annealing. The crystallization rate of form II enhances with the increase of the MW and entanglement degree. The entangled PB-1 has a faster I '-to-II phase transition and forms a higher amount of form II during heating and annealing. This is mainly related to the presence of more incompletely melted structures in the intermediate melt of more-entangled samples, which facilitate the following recrystallization of form II due to the memory effects. This study provides new insight into the crucial role of chain entanglement in the meltrecrystallized phase transition of polymorphic polymers.