Crystallization, recrystallization and melting behaviors in supramolecular poly(l-lactic acid) bonded by metal-ligand coordination

Abstract

The effect of metal-ligand coordination on crystallization and melting behaviors of polymers has not been comprehensively investigated. In this study, a series of supramolecular poly(l-lactic acid)s (SM-PLLAs) were prepared by changing various metal ions, length of polymer chains and stoichiometric ratio of ligands to metal ions (L/M). The complexation of metal ions with ligands in SM-PLLAs was confirmed by the intrinsic viscosity and UV–visible spectroscopy. It was found that crystallization rate and crystallinity of SM-PLLAs were strongly dependent on the chain length of precursor in both non-isothermal and isothermal crystallizations. With chain length of 1.6 kg/mol, crystallization of PLLA was completely suppressed regardless of the metal ions, whereas PLLA with longer chain length of 3.0 kg/mol exhibited strong dependence of the type of metal. During the heating process, melting behaviors can be tuned by the strength of metal-ligand coordination, temperature and the degree of complexation. Unlike PLLA precursor and SM-PLLA coordinated with Cu2+ exhibited multiple melting behaviors, only a single melting peak was appeared in PLLA bonded with Co2+, indicative of suppression of recrystallization during heating. With ratio of ligand to Co2+ increased from 1:1 to 6:1, crystallization and melting behaviors were tuned by the content of free and bounded PLLA chains. As compared to Cu2+, SM-PLLA coordinated with Co2+ exhibited higher complex modulus and viscosity due to the high bond strength, as well as a more stable structure underwent heating and shear, making diffusion of polymer chains and the structural adjustment more difficult. This work may shed light on understanding the intricate interplay of metal-ligand coordination effects on the crystallization and melting behaviors of polymers, which may be helpful with the design and making out the relationship between structure and properties of supramolecular polymers.