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( 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 ( M n,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,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,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,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,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