The isothermal crystallization kinetics of poly(ethylene glycol)-block-poly(l-lactide) block copolymers (PEG–PLLA): effects of the block lengths of PEG and PLLA

Abstract

Poly(ethylene glycol)-block-poly(L-lactide) (PEG–PLLA) is a biodegradable copolymer, which is widely applied to medicine and drug systems, and its morphology, organization and mechanical properties have been extensively investigated. However, the crystallization kinetics was not studied systemically with different block lengths, which is significant for the performance regulation and understanding of the crystallization behavior of block copolymers. In this study, PLLA, methoxy poly(ethylene glycol)-block-PLLA (MPEG-PLLA) diblock and PLLA–PEG–PLLA triblock copolymers with various molecular masses were melted and isothermally crystallized at different temperatures (T_c), and the crystallization kinetics of the PLLA and PLLA blocks was investigated using a differential scanning calorimeter (DSC). The results indicated that the crystallization kinetics of the PLLA block strongly relies on the architecture and block length. The crystallization rate first decreased and then increased with the increase in T_c for most of the specimens. The highest crystallization rate was found at 110 °C for all investigated PLLA samples. In the case of MPEG₄-PLLA copolymers, the highest crystallization rate was observed at 100 °C. With the increase in the block length of PEG, the crystallization rate increased and the highest crystallization rate was found at lower a T_c value. High n values were found for block copolymers when crystallized at a relatively low temperature, which might be due to the dispersion of PEG in the interlamellar and interfibrillar regions of PLLA crystallites, and branched or dendritic structures developed upon subsequent crystallization.