Stereocomplex (SC) crystallization between polymer enantiomers has opened a promising avenue for preparing high-performance materials. However, high-crystallinity SCs are difficult to achieve for high-molecular-weight (HMW) enantiomeric blends of chiral polymers [e.g., poly(lactic acid)]. Despite extensive studies, why HMW enantiomeric blends have difficulty in SC crystallization has not been clarified. Herein, we chose the HMW poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) 1/1 blend as the model system and demonstrated the crucial role of chain entanglement in regulating SC crystallization. PLLA/PDLA blends with various entanglement degrees were prepared by freeze-drying. We observed that disentangling promoted not only the crystallization rate but also the crystallinity of SCs in both the nonisothermal and isothermal processes. The less-entangled samples crystallized exclusively as the high-crystallinity SCs at different temperatures, in contrast to the predominant homocrystallization that occurred in the common entangled samples. This study provides deep insight into the SC crystallization mechanism of polymers and paves the way for future research attempting to prepare SC materials.

Stress-free two-way shape memory polymers (2W-SMPs) capable of reversible shifting between two distinct shapes are versatile platforms for the development of future smart devices. However, it is challenging to prepare stress-free 2W-SMPs with good actuation performance and shape programmability from single-component semicrystalline polymers. Herein, we demonstrate a straightforward and universal strategy for preparing 2W-SMPs through self-nucleated crystallization (SNC) of semicrystalline polymers. SNC enables the formation of two types of crystals in the 2W-SMPs, annealed and primary crystals, which function as the skeleton phase and actuation phase, respectively. We achieved a high reversible actuation strain of 17.6% and a good reprogrammability of the SNC-treated polymer networks. Complex shape transformations were obtained, and smart devices were fabricated from the SNC-treated networks by using a locally designed folding and kirigami structure. The SNC strategy provides a generalized approach to improve the 2W-shape memory behavior of semicrystalline polymers.

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.