Preparation of oriented β-form poly(L-lactic acid) by solid-state extrusion

Melt-crystallized, low molecular weight poly(L-lactic acid) (PLLA) consisting of a crystals was uniaxially drawn by solid-state extrusion at an extrusion temperature (T-ext) of 130-170 degreesC. A series of extrusion-drawn samples were prepared at an optimum T-ext value of 170 degreesC, slightly below the melting temperature (T-m) of ce crystals (similar to 180 degreesC). 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 alpha and beta 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)(alpha) and (003)(beta) reflections on the meridian as well as the DSC melting thermograms showed that the crystal transformation from the initial alpha form to the oriented beta 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 beta crystals was obtained by solid-state extrusion of melt-crystallized, low molecular weight PLLA slightly below T-m. The structure and properties of the alpha- and beta -form crystals were also studied. (C) 2001 John Wiley & Sons, Inc.