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 (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.