Relationship between crystallization state and degradation behavior of poly(l-lactide)/four-armed poly(d,l-lactide)-block-poly(d-lactide) blends with different poly(d-lactide) block lengths

A series of four-armed poly(d,l-lactide)-block-poly(l-lactide) (4-DL-D) copolymers were synthesized by ring-opening polymerization. By fixing the poly(d,l-lactide) (PDLLA) block length (1 kg mol(-1)) and changing the poly(d-lactide) (PDLA) block length (M-n,M-PDLA = 0, 0.5, 1.1, 1.3, 1.8 and 2.6 kg mol(-1)), the crystallization and alkaline degradation of the PLLA/4-DL-D blends were investigated. The four-armed PDLLA core of the copolymer inhibited the crystallization of PLLA, while the outer PDLA block could affect the crystallization differently when its length changed. If M-n,M-PDLA was 0 or 0.5 kg mol(-1), the crystallization of PLLA in the PLLA/4-DL-D blend was retarded markedly and the degradation rate of the blend films was much faster than that of neat PLLA film. Interestingly, when M-n,M-PDLA was 1.1 kg mol(-1) or higher, stereocomplex (SC) crystallites with different morphologies were formed, and the degradation rate of the PLLA/4-DL-D blend decreased gradually with increasing M-n,M-PDLA. In the PLLA/4-DL-D1.1 blend, the SC crystallites acted as nucleators for PLLA homocrystallites, while in the PLLA/4-DL-D1.3 blend, small isolated SC crystallites were observed inside the PLLA homospherulites. When M-n,M-PDLA was 1.8 or 2.6 kg mol(-1), a network structure of SC crystallites was formed and the degradation resistance of the films was markedly enhanced. A possible isothermal crystallization mechanism was proposed for the PLLA/4-DL-D blends, and the relationship between the crystallization state and degradation behavior was explored. This work revealed that the crystallization state, which was controlled by the PDLA block length, had a significant effect on the degradation behavior of PLLA/4-DL-D blend films. (c) 2020 Society of Chemical Industry