Retarded Crystallization and Promoted Phase Transition of Freeze-Dried Polybutene-1: Direct Evidence for the Critical Role of Chain Entanglement

Abstract

Polymorphism and crystal transition are common phenomena of semicrystalline polymers. These two behaviors are known to be controlled by the nucleation and chain mobility of polymers, both of which are constrained by the chain entanglement at the molecular level. However, the role of chain entanglement in polymorphic crystallization and crystal phase transition of polymers has not been well understood. Herein, we use isotactic polybutene-1 (PB-1) as a model polymorphic polymer and present the crucial role of chain entanglement in the polymorphic crystallization kinetics and solid−solid phase transition. A series of less-entangled PB-1 with different entanglement degrees were successfully prepared by freeze-drying the polymer dilute solution. Compared to the bulk sample and re-entangled one, chain disentangling of PB-1 suppressed the crystallization kinetics of form II but significantly increased the phase transition rate and final transition degree from form II to form I. The disentangling-promoted II−I phase transition originated from the reduced nucleation barrier and enhanced chain mobility. This work would advance the in-depth understanding on the formation and transition mechanisms of polymorphic polymer crystals at the molecular level.

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

“…The extremely low-molecular-weight or lessentangled PB-1, which contains low content of intercrystalline links, exhibits faster II-to-I phase transition than the usual samples. 12,13 Nevertheless, the II-to-I phase transition of commercialized PB-1 usually takes several weeks under ambient conditions and is accompanied by the volume shrinkage and unbalanced internal stress in the end-use materials. 3 Different from the metastable form II, the microstructure and physical properties of form I′ are rather stable when being stored at room temperature.…”

“…Polymorphic phase transition of PB-1 from form II to form I (designated as the II-to-I transition) is a typical solid-to-solid transition, which has drawn much research interest in recent years. − The II-to-I phase transition is accompanied by the changes in essential physical properties (e.g., melting temperature, density) and the enhancements of material’s mechanical properties (e.g., yield strength, modulus, and hardness). , Various methods have been developed to accelerate the II-to-I phase transition of PB-1, such as thermal treatment, stretching, copolymerization, polymer blending, and solvent treatment . Furthermore, the mechanism of II-to-I phase transition of PB-1 has been revealed at the molecular level. , It is found that the intercrystalline links (i.e., entanglements and tie chains) play a critical role in the II-to-I phase transition. The extremely low-molecular-weight or less-entangled PB-1, which contains low content of intercrystalline links, exhibits faster II-to-I phase transition than the usual samples. , …”

Cooling Condition Determines the Transition Degree at Saturation of Form II in Isotactic Polybutene-1 Confined within Nanopores

“…In the extremely low molecular weight PB-1 without any intercrystalline links, the transition is accomplished instantaneously: direct formation of Form I′ at the fixed crystallization temperature takes place . For the high molecular weight PB-1, when the intercrystalline links and entanglements are removed by freeze-drying of dilute solution, the less-entangled sample shows a faster transition …”

“…36 For the high molecular weight PB-1, when the intercrystalline links and entanglements are removed by freezedrying of dilute solution, the less-entangled sample shows a faster transition. 31 Micro-or nano-confined domains give a new perspective for understanding the crystallization and crystal transition of PB-1. It is found that Form I′ can be easily formed in some blends with PB-1 as the minor phase.…”

“…Despite the abovementioned progress, the mechanism of transition is still elusive. It is believed that the internal stress, generated by unbalanced shrinkage of the amorphous and crystalline phase, promotes the nucleation of Form I on the Form II crystal. − Jiang et al found that nucleation of the stable Form I started most likely at the crystalline side or corners of Form II, implying that the residual stress at the edges of microcrystallites is essential . The transition is also strongly related to molecular weight and crystallization temperature. , The possible explanation is related to the intercrystalline links, including tie chains and entanglements.…”