Development of oriented morphology and mechanical properties upon drawing of stereo-complex of poly(L-lactic acid) and poly(D-lactic acid) by solid-state coextrusion

A semicrystalline film consisting of only stereo-complex (sc) crystals was prepared by casting from a solution of an equimolar blend of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). The film was uniaxially drawn by solid-state coextrusion, and the development of oriented morphology and mechanical properties was investigated by DSC, WAXD, dynamic mechanical analyses and tensile tests. The ductility increased with extrusion temperature (T-ext), reached a maximum at a T-ext of 200 degrees C, and at yet higher T-ext's it rapidly decreased. The maximum tensile modulus and strength of 9.5 GPa and 410 MPa, respectively, were achieved for a highly drawn sc-film prepared by coextrusion at a T-ext of 200 degrees C and EDR of 16. Such a drawn film consisted of highly oriented sc-crystals and had a crystallinity of chi(c) = 59 wt % and a high chain orientation function of f(c) = 0.993. The oriented sc-films exhibited remarkably enhanced heat-resistance compared to the oriented films of PLLA homo-polymer. The dynamic storage modulus of a sc-film with an EDR of 16, measured at 11 Hz, was 9.5 GPa at room temperature and 2.7 GPa even at 200 degrees C, 20-25 degrees C above the melting temperatures of alpha- and beta-form homo-crystals. The high thermal stability of oriented sc-films was primarily ascribed to the high melting temperature and weak a-relaxation intensity for the sc-films compared to the PLLA films. No existence of a crystalline relaxation in the sc-films also contributed to improve their thermal stability.