Effect of atactic poly(3-hydroxybutyrate) block on the crystallization and degradation behavior of 6-arm poly(l-lactide)-b-atactic poly(3-hydroxybutyrate)

6-arm poly(l-lactide) (6a-PLLA) homopolymer and a series of 6-arm poly(l-lactide)-b-atactic poly(3-hydroxybutyrate) (6a-PLLA-b-PHB) diblock copolymers were synthesized by ring-opening polymerization in the presence of Zn compound as catalyst and D-mannitol as initiator. The two block segments were found to be miscible according to the single Tg detected by the DSC. The structure effect of atactic PHB (ataPHB) block on the thermal stability, crystallization kinetic, melting behavior, and enzymatic degradation of diblock copolymer were investigated. The TGA results indicated that the thermal degradation of block copolymers started from the ataPHB block and thereby enhanced the thermal stability of PLLA block. No obvious changes were observed in the nucleation mechanism and crystal structure of the PLLA block, but the amorphous ataPHB block interrupted its crystallization, and induced lower crystallization rate and imperfect crystals. The enzymatic degradation rate of PLLA domains was determined in the presence of proteinase K. It was found that the erosion rate is strongly dependent on the length of ataPHB blocks. The degradation behavior was discussed from two aspects, i.e., the molecular mobility and retardation effect from ataPHB block.