Crystal transformation and development of tensile properties upon drawing of poly(L-lactic acid) by solid-state coextrusion: Effects of molecular weight

摘要

Melt-crystallized films of poly((L)-lactic acid) (PLLA) with M-v in the range of 3.8 similar to 46 X 10(4) consisting of alpha-form crystals were uniaxially drawn by solid-state coextrusion. The effects of M,, extrusion draw ratio (EDR), and extrusion temperature (Text) on the crystal/crystal transformation from alpha- to P-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 degrees C, independently of the M-v's. As a result of the optimum combination of processing variables influencing the the crystal transformation (M-v, Text, and drawability), highly oriented films consisting of P-form crystals alone were obtained by coextrusion of higher Mv samples at Text's slightly below the melting temperature (150 similar to 170 degrees 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 Mv of 46 X 104 coextruded at 170 degrees C to the highest EDR of 14.