Preparation of Oriented β-Form Poly(l-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.