Stereocomplex Crystal Formation and Development of Morphology and Mechanical Property upon Drawing of a Blend of High Molecular Weight Poly(L-lactic acid)/Poly(D-lactic acid)

Abstract

The structural changes and development of morphology, crystal structure and mechanical property upon uniaxial drawing of an equimolar blend of high-molecular weight poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) were investigated. Solvent-cast film of the blend were drawn by a tensile force and solid-state coextrusion. The initial film was consisted predominantly of α-form crystals and a small amount of stereocomplex (SC) crystals. Although the amount of SC did not increase upon annealing the solvent-cast film at 200°C, it increased (with draw ratio) upon drawing by either of the draw techniques when the draw was performed in the temperature range between the melting temperature of α-form crystals (Tmα = 170-180°C) and SC (TmSC = 220 °C). Upon drawing at a given temperature, SC formed significantly more rapidly with draw ratio for the solid-state coextrusion than for the tensile drawing. Thus, an oriented product consisting of only SC was obtained by the solid-state coextrusion at Te = 200 °C to an extrusion draw ratio of 16, even for a high-molecular weight PLLA/PDLA blend, although the formation of SC in such a blend has been shown to be difficult in bulk from melt or in solution. The dynamic Young's moduli of SC samples above Tg (∼60 °C) was significantly higher than that of an α-form sample.