Stereocomplex Formation in Incompatible Racemic Chiral Polylactide Block Copolymers

Stereocomplexes in incompatible racemic chiral polylactide (PLA) block copolymers have not been widely studied. In this work, we synthesized PLLA and PDLA containing block copolymers by ring opening polymerization of L- and D-lactides from hydroxyl-terminated hydrophilic [poly(ethylene oxide) (PEO)] and hydrophobic [poly(ethylene-\textit{co}-1,2-butylene) (PEB)] oligomers, respectively. Two samples PEO-$b$-PLLA (2,000-5,400) and PEB-$b$-PDLA (4,200-5,400) were chosen. The stereocomplexes were cast from equal molar blends of above two block copolymers in chloroform solution, followed by two different thermal treatments before stereocomplex formation; The blend was either heated to 250 \r{ }C and quickly quench to 160 \r{ }C or heated to 250 \r{ }C for 15 min and quench to 160 \r{ }C for stereocomplex crystal growth. Before the formation of stereocomplexes, lamellar and cylindrical morphologies were observed in blends for the first and second thermal treatments, respectively, as evidenced by small-angle X-ray scattering (SAXS). After complete crystal growth, the 100{\%} stereocomplexes was confirmed by differential scanning calorimetry and wide-angle X-ray diffraction (WAXD). The morphologies of stereocomplexes grown from these two pre-existing microphases (lamellar vs. cylindrical) were studied by time-resolved SAXS and transmission electron microscopy (TEM).