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)

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 alpha-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 degrees 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 alpha-form crystals (T-m(alpha) = 170-180 degrees C) and SC (T-m(SC) = 220 degrees 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 T-e = 200 degrees 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 T-g (similar to 60 degrees C) was significantly higher than that of an alpha-form sample.