Effects of extrusion draw ratio on the morphology, structure and mechanical properties of poly(l-lactic acid) fabricated using solid state ram extrusion

Abstract

Solid state ram extrusion (SSRE) was utilized to fabricate poly(L-lactic acid) (PLLA) and the effects of extrusion draw ratios (EDRs) on its morphology, structure, and properties were investigated. Scanning electron microscopy observation indicated that the original spherulites gradually transformed into microfibrils aligned along the extrusion direction with an increase of the EDR. The molecular orientation of SSRE PLLA proceeded with EDRs in both the sheath and core region as detected by wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering characterization. Interestingly, on the basis of the WAXD curves, the β form of PLLA was found to appear in the bulk extrudates when the EDR was beyond 4.0, where the crystal transformation from the primary α form to the β form occurred between (203)_α and (131)_β. Meanwhile, the sheath region contained a much higher amount of the β form than the core region, contributing to the existence of a larger deformation in the sheath region. A multiple melting phenomenon of PLLA consisting of α and β mixture crystals was revealed by on-line WAXD and differential scanning calorimetry (DSC) measurements, which was explained by the melting, recrystallization and remelting. Formation of the fibrous crystals and the melting re-crystallization may lead to the increase of the melting point of SSRE PLLA. Additionally, the flexural strength and flexural modulus showed a monotonic rise with the EDR, achieving 4 and 13 times higher for the specimen at an EDR of 8.2 relative to the sample at an EDR of 1.0, respectively.