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 a-form crystals was uniaxially drawn by solid-state coextrusion at 110-170degreesC using split billets of different polymers. The effects of extrusion variables, including the extrusion draw ratio (EDR), temperature (Teat), and pressure (P-ext), on the crystal transformation from the initial a-form to the oriented beta-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 (130degreesC) 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 130degreesC 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 beta crystals alone was obtained by coextrusion to the highest EDR of similar to14 achieved at a high T-ext of 170degreesC, near the T-m, and a high P-ext of similar to50 MPa.