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

Abstract

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 ( T ext ), and pressure ( P ext ), 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 P ext and EDR but at different T ext 's, there was a T ext (130 °C) where the crystal transformation proceeded most efficiently. Furtheremore, when coextrusion was made at a constant T ext and EDR but at different P ext 's, the transformation proceeded more efficiently at a higher P ext . These results show that crystal transformation proceeds with the EDR most efficiently for the coextrusion at a T ext of 130 °C and a higher P ext . 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 T ext of 170 °C, near the T m, and a high P ext of ∼50 MPa.