Effect of poly(l-lactide)/poly(d-lactide) block length ratio on crystallization behavior of star-shaped asymmetric poly(l(d)-lactide) stereoblock copolymers

Effect of Poly(l-lactide)/Poly(d-lactide) (PLLA/PDLA) block length ratio on the crystallization behavior of star-shaped poly(propylene oxide) block poly(d-lactide) block poly (l-lactide) (PPO-PDLA-PLLA) stereoblock copolymers with molecular weights (M-n) ranging from 6.2 x 10(4) to 1.4 x 10(5) g mol(-1) was investigated. Crystallization behaviors were studied utilizing differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). Only stereocomplex crystallites formed in isothermal crystallization at 140 to 156 degrees C for all samples. On one hand, the overall crystallization rate decreased as PLLA/PDLA block length ratio increased. As PLLA/PDLA block length ratio increased from 7:7 to 28:7, the value of half time of crystallization (t(1/2)) delayed form 2.85 to 5.31 min at 140 degrees C. On the other hand, according to the Lauritzen-Hoffman theory, the fold-surface energy (sigma(e)) was calculated. sigma(e) decreased from 77.7 to 73.3 erg/cm(2) with an increase in PLLA/PDLA block length ratio. Correspondingly increase in nucleation density was observed by the polarized optical microscope. Results indicated that the PLLA/PDLA block length ratio had a significant impact on the crystallization behavior of PPO-PDLA-PLLA copolymers. POLYM. ENG. SCI., 55:2534-2541, 2015. (c) 2015 Society of Plastics Engineers