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

Abstract

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 (Text), reached a maximum at a Text of 200 °C, and at yet higher Text’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 Text of 200 °C and EDR of 16. Such a drawn film consisted of highly oriented sc-crystals and had a crystallinity of χc=59 wt % and a high chain orientation function of fc=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 °C, 20–25 °C above the melting temperatures of α- and β-form homo-crystals. The high thermal stability of oriented sc-films was primarily ascribed to the high melting temperature and weak α-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.