Preferential Formation of β-Form Crystals and Temperature-Dependent Polymorphic Structure in Supramolecular Poly(L-lactic acid) Bonded by Multiple Hydrogen Bonds

Supramolecular polymers (SMPs) have quite different crystallization behavior from the conventional polymers, because of the confinement effects of supra molecular units. Crystallization of SMPs undergoes in a confined and dynamic manner. Herein, we selected the 2-ureido-4[1H]-pyrimidione (UPy)-bonded poly(L-lactic acid) (PLLA) as a model SMP and investigated the crystallization kinetics, polymorphic crystalline structure and phase transition of supramolecular PLLAs (SM-PLLAs). SM-PLLAs were synthesized by the end functionalization of hydroxyl terminated 2- and 3-arm PLLA precursors with different molecular weights. Crystallization rate and crystallinity of SM-PLLAs are strongly depressed in both nonisothermal and isothermal crystallizations, as compared to the nonfunctionalized PLLA precursors. Crystalline structure of SM-PLLAs is sensitive to the crystallization temperature (T-c). A low T-c (75-95 degrees C) facilitates the formation of metastable beta crystals of PLLA in SM-PLLAs; while a high T-c (100-130 degrees C) favors the generation of alpha (or alpha') crystals. The beta crystals formed in SM-PLLAs transform into the more stable a crystals in the following heating process. We propose that the preferential formation of beta crystals is ascribed to the strong confinements of UPy motifs and UPy UPy interactions, which may exert an orientation and stretching effects to the linked PLLA chains during crystallization. This study has shed light on the unique confined and dynamic crystallization behavior of SMPs and also paved a way to obtain the PLLA beta-form from SM-PLLA under the crystallization conditions free of pressure, stretching, and shearing.