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

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 (T ext ), reached a maximum at a T ext of 200 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 C and EDR of 16. Such a drawn film consisted of highly oriented sc-crystals and had a crystallinity of c ¼ 59 wt % and a high chain orientation function of f c ¼ 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 -and -form homo-crystals. The high thermal stability of oriented sc-films was primarily ascribed to the high melting temperature and weak -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. Poly(L-lactic acid) (PLLA) attracts considerable interest from both fundamental and industrial viewpoints because it can be synthesized from renewable resources, such as corn and potato.1-3 Since PLLA is biodegradable and biocompatible, this polymer has been used for biomedical applications.4-6 Because PLLA has fairly good mechanical properties with a high melting temperature (T m ¼ 170{180 C), its application for more general purposes as an environmentally friendly and economically feasible polymer has been evaluated. [7][8][9] PLLA commonly crystallizes into two crystal modifications; i.e., the -and the -forms, 10-12 both of which have a melting temperature around 170-180 C. It has been reported that an equimolar blend of PLLA and poly(D-lactic acid) (PDLA) forms a mixture of stereo-complex crystals and -form homocrystals when it was crystallized from solutions and melt, depending on the crystallization conditions. 13,14 Homo-crystals, such as -form and -form crystals consist of only PLLA or PDLA molecules, whereas the sc-crystal is composed of PLLA and PDLA chains with opposite helical hand packed side by side. It is interesting to note that the sc-crystal shows a melting temperature of 220-230 C, which is about 50-60 C higher than those for the homo-crystals. Recently, Kimura et al. 15,16 found bacteria which produces Dlactic acid from glucose. This finding may allow to produce D-lactic acid more easily at a lower coast and enhance the application of sc-poly(lactic acid).It has been reported that when an equimolar blend of PLLA and ...

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