Mechanical Properties and Nanostructure in Extruded Films of Polylactide Acid and Polyethylene: Influence of Rate of Deformation

摘要

Polylactic acid (PLA) is a biodegradable aliphatic polyester, whose peculiarity is that it can be produced from renewable and sustainable sources. Due to these characteristics, PLA has been considered as an alternative to alleviate solid waste disposal problems, solving also a pollution issue. PLA is a versatile polymer which has many applications -depending on its enantiomeric constitution and molecular weight- including the textile and the medical industries. Moreover, when commercial production satisfies the demand it could be used in packaging applications and cookware 1. This research focuses on the microstructure (at the micrometer, nanometer, and Angstrom scale) and mechanical properties of extruded bags of biodegradable polylactide acid (PLA) 1,2 and compares these properties with those from hydrocarbon derived polymers, polyethylenes, high and low density. Thermal transitions were determined by differential scanning calorimetry (DSC) analysis, whereas the microstructure was determined via wide and small-angle X-ray scattering (WAXS and SAXS), and small-angle light scattering (SALS). The mechanical properties were determined under uniaxial testing at room temperature and the influence of rate of deformation on Young modulus and yield stress was investigated, varying the strain rate from 1 mm/min to 20 mm/min. The results showed for PLA bag an increase in yield stress at higher strain rates, suggesting a strain hardening phenomenon probably associated to stress-induced crystallization. Moreover, the biodegradable polymer PLA also has significantly higher yield stress and Young’s modulus relative to the PE samples. Tensile testing along and orthogonal to the extrusion direction revealed anisotropic properties in PLA, and LDPE but this was not the case for HDPE. The mechanical behavior will be discussed in correlation with the micro and nano-scale structure.