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

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journal contribution
posted on 26.10.2017, 12:49 by Jianna Bao, Xiaohua Chang, Qing Xie, Chengtao Yu, Guorong Shan, Yongzhong Bao, Pengju Pan
Supramolecular polymers (SMPs) have quite different crystallization behavior from the conventional polymers, because of the confinement effects of supramolecular 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 (Tc). A low Tc (75–95 °C) facilitates the formation of metastable β crystals of PLLA in SM-PLLAs; while a high Tc (100–130 °C) favors the generation of α (or α′) crystals. The β crystals formed in SM-PLLAs transform into the more stable α crystals in the following heating process. We propose that the preferential formation of β 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 β-form from SM-PLLA under the crystallization conditions free of pressure, stretching, and shearing.