Tunable Crystallization Kinetics and Crystal Polymorphism in Semicrystalline Polymers: Insight into the Multilevel Structural Order of Polymer Melts.

The melting of semicrystalline polymers is a typical multistep process and involves a series of intermediate melt states. However, the structural characteristics of the intermediate polymer melt is unclear. Herein, we choose polymorphic -1,4-polyisoprene (PI) as a model polymer system and elucidate the structures of the intermediate polymer melt and their strong effects on the following crystallization process. We find that the metastable β crystals of the PI melt first into an intermediate state and then recrystallize in new crystals upon thermal annealing. The intermediate melt shows multilevel structural order at the chain level depending on the melting temperature. The conformationally ordered melt can memorize the initial crystal polymorph and accelerate the crystallization process, while the ordered melt without the conformational order can only enhance the crystallization rate. This work provides deep insight into the multilevel structural order of polymer melts and its strong memory effects on the crystallization process.