A film of melt-crystallized poly(L-lactic acid) (PLLA) consisting of R-form crystals was uniaxially drawn by solid-state coextrusion at 110-170 °C using split billets of different polymers. The effects of extrusion variables, including the extrusion draw ratio (EDR), temperature (Text), and pressure (Pext), on the crystal transformation from the initial R-form to the oriented β-form crystals were studied. The crystal transformation proceeded rapidly with EDR. It was found that when coextrusion was made at a constant P ext and EDR but at different Text's, there was a Text (130 °C) where the crystal transformation proceeded most efficiently. Furtheremore, when coextrusion was made at a constant Text and EDR but at different Pext's, the transformation proceeded more efficiently at a higher Pext. These results show that crystal transformation proceeds with the EDR most efficiently for the coextrusion at a Text of 130 °C and a higher Pext. As a result of the specific effect for each of the extrusion variables on the crystal transformation and ductility, a highly oriented film consisting of β crystals alone was obtained by coextrusion to the highest EDR of ∼14 achieved at a high Text of 170 °C, near the Tm, and a high Pext of ∼50 MPa.

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Melt-crystallized, low molecular weight poly(L-lactic acid) (PLLA) consisting of ␣ crystals was uniaxially drawn by solid-state extrusion at an extrusion temperature (T ext ) of 130 -170°C. A series of extrusion-drawn samples were prepared at an optimum T ext value of 170°C, slightly below the melting temperature (T m ) of ␣ crystals (ϳ180°C). The drawn products were characterized by deformation flow profiles, differential scanning calorimetry (DSC) melting thermograms, wide-angle X-ray scattering (WAXD), and small-angle X-ray scattering as a function of the extrusion draw ratio (EDR). The deformation mode in the solid-state extrusion of semicrystalline PLLA was more variable and complex than that in the extensional deformation expected in tensile drawing, which generally gave a mixture of ␣ and ␤ crystals. The deformation profile was extensional at a low EDR and transformed to a parabolic shear pattern at a higher EDR. At a given EDR, the central portion of an extrudate showed extensional deformation and the shear component became progressively more significant, moving from the center to the surface region. The WAXD intensities of the (0010) ␣ and (003) ␤ reflections on the meridian as well as the DSC melting thermograms showed that the crystal transformation from the initial ␣ form to the oriented ␤ form proceeded rapidly with increasing EDR at an EDR greater than 4. Furthermore, WAXD showed that the crystal transformation proceeded slightly more rapidly at the sheath region than at the core region. This fact, combined with the deformation profiles (shear at the sheath and extensional at the core), indicated that the crystal transformation was promoted by shear deformation under a high pressure rather than by extensional deformation. Thus, a highly oriented rod consisting of only ␤ crystals was obtained by solid-state extrusion of melt-crystallized, low molecular weight PLLA slightly below T m . The structure and properties of the ␣and ␤-form crystals were also studied.