Linear and four-armed poly( l-lactide)- block-poly( d-lactide) copolymers and their stereocomplexation with poly(lactide)s.

Linear and four-armed poly( l-lactide)- block-poly( d-lactide) (PLLA- b-PDLA) block copolymers are synthesized by ring-opening polymerization of d-lactide on the end hydroxyl of linear and four-armed PLLA prepolymers. DSC results indicate that the melting temperature and melting enthalpies of poly (lactide) stereocomplex in the copolymers are obviously lower than corresponding linear and four-armed PLLA/PDLA blends. Compared with the four-armed PLLA- b-PDLA copolymer, the similar linear PLLA- b-PDLA shows higher melting temperature (212.3 °C) and larger melting enthalpy (70.6 J g−1). After these copolymers blend with additional neat PLAs, DSC, and WAXD results show that the stereocomplex formation between free PLA molecular chain and enantiomeric PLA block is the major stereocomplex formation. In the linear copolymer/linear PLA blends, the stereocomplex crystallites (sc) as well as homochiral crystallites (hc) form in the copolymer/PLA cast films. However, in the four-armed copolymer/linear PLA blends, both sc and hc develop in the four-armed PLLA- b-PDLA/PDLA specimen, which means that the stereocomplexation mainly forms between free PDLA molecule and the inside PLLA block, and the outside PDLA block could form some microcrystallites. Although the melting enthalpies of stereocomplexes in the blends are smaller than that of neat copolymers, only two-thirds of the molecular chains participate in the stereocomplex formation, and the crystallization efficiency strengthens. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1560-1567 [ABSTRACT FROM AUTHOR]