Preparation of Oriented β-Form Poly(l-lactic acid) by Solid-State Coextrusion:  Effect of Extrusion Variables

A film of melt-crystallized poly(l-lactic acid) (PLLA) consisting of α-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 α-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 Pext 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.