Crystal Transformation and Development of Tensile Properties upon Drawing of Poly(L‐lactic acid) by Solid‐State Coextrusion: Effects of Molecular Weight

Melt-crystallized films of poly(L-lactic acid) (PLLA) with M v in the range of 3.8∼46 × 10 4 consisting of α-form crystals were uniaxially drawn by solid-state coextrusion. The effects of M v , extrusion draw ratio (EDR), and extrusion temperature (T ext ) on the crystal/crystal transformation from α- to β-form crystals and the resultant tensile properties of drawn products were studied. The crystal transformation proceeded with EDR and more rapidly for the higher M v 's. Furthermore, the crystal transformation proceeded most rapidly with EDR at a T ext around 130°C, independently of the M v 's. As a result of the optimum combination of processing variables influencing the the crystal transformation (M v , T ext , and drawability), highly oriented films consisting of β-form crystals alone were obtained by coextrusion of higher M v samples at T ext 's slightly below the melting temperature (150 ∼170°C) and at higher EDR's > 11. Both the tensile modulus and strength increased rapidly with EDR. The modulus at a given EDR was slightly higher for the samples with higher M v 's. In contrast, the strength at a given EDR was remarkably higher for the higher M v 's. The highest tensile modulus of 8.0 GPa and strength of 500 MPa were obtained with the sample of the highest M v of 46 x 10 4 coextruded at 170 °C to the highest EDR of 14.