Abstract
The influence of solid-state drawing on the morphology of melt-spun poly(l-lactic acid) (PLLA) tapes, and the accompanying changes in mechanical and degradation behaviour have been studied. Mechanical properties are found to be strongly dependent on both applied draw ratio and drawing temperature. Moduli of these highly oriented tapes are significantly increased compared to as-extruded tapes at both ambient and elevated temperatures. Interestingly, drawing leads to a significant increase in elongation to break (~3 times) and toughness (~13 times) compared to as-extruded tapes. Structural and morphological characterization indicates strain-induced crystallization as well as an increase in orientation of the crystalline phase at small strains. Upon further stretching, an “overdrawing” regime is observed, with decreased crystalline orientation due to the breakage of existing crystals. For fixed draw ratios, a significant increase in Young’s modulus and tensile strength is observed with increasing drawing temperature, due to a higher crystallinity and orientation obtained for tapes drawn at higher temperatures. FT-IR results indicate no crystal transformation after drawing, with the α-form being observed in all tapes. Hydrolytic degradability of PLLA was significantly reduced by solid-state drawing.
Highlights
Poly(lactic acid) (PLA), which is produced from annually renewable resources such as corn and sugar beets, has attracted great interest as a sustainable and environmental-friendly bioplastic
The maximum tensile modulus achieved in this work for tapes (6.7 GPa) compares well to the moduli previously reported for melt-spun drawn poly(L-lactic acid) (PLLA) fibres, but are well below those of melt-spun polyethylene (PE), polypropylene (PP), and poly(ethylene terephthalate) (PET) with moduli ranging from 15 to 70 GPa [20]
The drawing conditions of PLLA determine the morphology of the polymer, and through this control their mechanical properties and degradation profile
Summary
Poly(lactic acid) (PLA), which is produced from annually renewable resources such as corn and sugar beets, has attracted great interest as a sustainable and environmental-friendly bioplastic. Leenslag et al [8] produced fibres with a tensile strength and modulus of 2.1 GPa and 16 GPa from PLLA with Mv = 900,000, using a solution spinning process. The obtained fibres had a modulus of 7 GPa and a tensile strength of 0.5 GPa. The investigation of Cicero et al [10] illustrates two-step melt spinning of textile grade PLA. A maximum tensile strength of the as-spun filaments has been reported of 385 MPa with a maximum modulus of 6 GPa. This paper is concerned with the understanding of structure-property relationships of PLLA films and tapes, rather than fibres, during stretching to optimize its use properties via processing. The influence of draw ratio and drawing temperature on the superstructure (orientation and crystallization, etc.) and morphology evolution of PLLA, and the accompanying changes in mechanical and degradation properties will be discussed
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