The accelerating effect of the star-shaped poly(d-lactide)-block-poly(l-lactide) stereoblock copolymer on PLLA melt crystallization
A star-shaped poly(D-lactide)-block-poly(L-lactide) (PDLA-PLLA) stereoblock copolymer was introduced into a poly(L-lactide) (PLLA) matrix. The impacts of the PDLA-PLLA copolymer on PLLA melt crystallization, mechanical properties and rheological properties were investigated. The research results indicated that the PDLA-PLLA copolymer could significantly accelerate the crystallization rate of PLLA. The non-isothermal crystallization results showed that the crystallization temperature shifted to a higher temperature with the increase in the addition of the PDLA-PLLA copolymer. The crystallization temperature increased to about 25 °C with the addition of 10 wt% PDLA-PLLA copolymer. The isothermal crystallization results showed that the half-time of crystallization (t0.5) decreased from 10 min to 2.5 min at 120 °C as the PDLA-PLLA copolymer fraction increased from 0 to 10 wt%. Nucleation efficiency (NE) was used to describe the nucleation efficiency of the PDLA-PLLA copolymer. The highest NE of 65% was obtained for PLLA samples with a PDLA-PLLA copolymer content of 10 wt%. As polarized optical microscopy revealed, this accelerating effect resulted from the good nucleation ability of the PDLA-PLLA copolymer. Moreover, dynamic mechanical analysis results indicated that the addition of the PDLA-PLLA copolymer enhanced the storage modulus of PLLA in the glass state. Rheological properties of nucleated PLLA showed the existence of a network structure of a stereocomplex crystallite (sc-crystallite) above 5 wt% addition of the PDLA-PLLA copolymer.