Biopolymers and Bioplastics

Biography

Biography: Gagik Ghazaryan

Abstract

Poly(L-lactic acid) (PLLA) is a slow-crystallising polyester which exhibits brittle behaviour due to relatively fast physical ageing of the amorphous phase. This embrittlement of PLLA narrows its application window in such fields where flexibility of a polymer is required (e.g., packaging). In this work we investigated the effects of thermal rejuvenation and molecular orientation of the amorphous phase on physical ageing of oriented PLLA films with emphasis on mechanical properties. Uniaxial compression testing showed that physical ageing of the amorphous phase increases the yield stress and the associated strain softening response, both contributing to the observed embrittlement of PLLA in tension. Moreover, the strain-hardening response was found not to be influenced by physical ageing. Molecular orientation of the amorphous phase at constant crystallinity was applied by uniaxial and biaxial plastic deformation just above the glass-transition temperature (at 70 °C) up to modest plastic strains of 200 %, to avoid strain-induced crystallisation. Stress-relaxation experiments, combined with tensile testing, both as a function of ageing time, have revealed that both uniaxial and biaxial plastic deformation in excess of 100 % plastic strain, decelerates and possibly prohibits the physical ageing process. The oriented monofilaments and films have improved mechanical properties, such as stiffness, strength and strain-to-break. The latter properties were not affected by physical ageing during a testing period of 40 days. In addition, plastic deformation to higher draw ratios and/or at slightly higher temperatures (90 °C), strongly enhanced crystallinity and resulted in PLLA monofilaments and films that also exhibited tough behaviour not affected by physical ageing.