Stereocomplex formation between enantiomeric poly(lactic acid)s. XI. Mechanical properties and morphology of solution-cast films

Films of 1:1 blend and films non-blended were prepared from poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) with a solution casting method, and the mechanical properties and morphology of the films were investigated using tensile tests, dynamic mechanical relaxation measurements, polarizing optical microscopy, differential scanning calorimetry (DSC) and X-ray diffractometry. The tensile strength, Young's modulus, and the elongation-at-break of 1:1 blend films were found to be higher than those of non-blended films when their weight-average molecular weight (M-w) was in the range 1 x 10(5)-1 x 10(6). The enthalpy of melting for stereocomplex crystallites in 1:1 blend films was higher than that of homo-crystallites when M-w of polymers was below 2 x 10(5), while this relationship was reversed when M-w increased to 1 x 10(6). Spherulites formation was suppressed in 1:1 blend films, whereas large-sized spherulites with radii of 100-1000 mu m were formed for non-blended PLLA and PDLA films, irrespective of M-w. The mechanical properties of 1:1 blend films superior to those of non-blended films were ascribed to the micro-phase structure difference generated as a result of formation of many stereocomplex crystallites which acted as intermolecular cross-links during solvent evaporation of blend solution. On the contrary, non-blended films had larger-sized spherulites of less contacting area with the surrounding spherulites. (C) 1999 Elsevier Science Ltd. All rights reserved.