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

Abstract

AbstractA series of four‐armed poly(d,l‐lactide)‐block‐poly(d‐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 (Mn,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 Mn,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 Mn,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 Mn,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 Mn,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. © 2020 Society of Chemical Industry