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In this work, poly(d-lactic acid) (PDLA) multiblock copolymers with a branched structure were designed by a two-step coupling strategy. First, polyhydroxybutyrate (PHB), poly(ε-caprolactone) (PCL), and PDLA blocks were subsequently introduced by coupling to prepare a linear PDLA block copolymer (PDLA-rB), and PDLA-rB was then grafted on a cellulose acetate (CA) chain to get the CA-g-PDLA-rB copolymer (CA-rB-PDLA). PDLA-rB and CA-rB-PDLA copolymers were blended with commercial poly(l-lactic acid) (PLLA) to form the stereocomplex crystals (SCs) of poly(lactic acid) enantiomers and reduce the inherent brittleness of PLLA. For PLLA/PDLA-rB blends, the PHB-PCL block in the multiblock copolymer acts as a plasticizer to promote HC crystallization; the crystallinity increased from 17 to 17.9%. Compared with the linear blends, a higher SC crystallinity is probably attributed to the formation of the favorable interdigitation between polymer chains. Moreover, the branched chains introduced more PHB-PCL blocks and promoted the formation of multiple heterogeneous nucleation spots, which plays a role in accelerating the crystallization kinetics of HCs. The PLLA/CA-rB-PDLA blends also exhibited a greatly improved tensile toughness. With CA-rB-PDLA-5.0 loading, the strain at break of PLLA/CA-rB-PDLA was increased 4 times. Scanning electron microscopy investigation revealed that the formation of dense cavities and fibrous network structure can resist local strain, resulting in high toughness. It is further proved by tan delta (δ) curves that the great improvement in toughness of the PLLA/CA-rB-PDLA blends is attributed to its high crystallinity. These results revealed that branched structures play a significant role in improving the crystallization and mechanical properties of PLLA.
Wu et al. (Tue,) studied this question.