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

AbstractMelt‐crystallized films of poly(L‐lactic acid) (PLLA) with Mv in the range of 3.8 ∼ 46 × 104 consisting of α‐form crystals were uniaxially drawn by solid‐state coextrusion. The effects of Mv, extrusion draw ratio (EDR), and extrusion temperature (Text) 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 Mv's. Furthermore, the crystal transformation proceeded most rapidly with EDR at a Text around 130 °C, independently of the Mv's. As a result of the optimum combination of processing variables influencing the the crystal transformation (Mv, Text, and drawability), highly oriented films consisting of β‐form crystals alone were obtained by coextrusion of higher Mv samples at Text'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 Mv'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 Mv of 46 × 104 coextruded at 170 °C to the highest EDR of 14.