Tough blends of poly(lactide) and amorphous poly([R,S]-3-hydroxy butyrate) – morphology and properties

Polylactide (PLA), a biodegradable polymer, produced from annually renewable natural resources, has a glass transition temperature in the range of 50–60 °C and is stiff and brittle at room temperature. In this communication we demonstrate that blending of PLA with atactic poly([R,S]-3-hydroxy butyrate) (a-PHB) leads to significant improvement of drawability and impact strength. The blends of PLA with a-PHB are biodegradable, similarly to plain PLA, and can be considered as a potential material for packaging, especially for food. The compositions containing up to 20 wt.% of a-PHB were prepared and characterized by various techniques, including DSC, TGA, SEM, WAXS, DMTA and tensile tests. PLA and a-PHB appeared partially miscible and form phase-separated blends. Tg of PLA in blends decreases slightly with increasing content of a-PHB. SEM observations revealed sub-micron a-PHB droplets with diffuse interphases, dispersed well in the PLA matrix. Partial miscibility results in a slight modification of cold crystallization and melting temperatures of PLA. Foils produced from the blends by either compression molding or extrusion appeared completely amorphous. Tensile experiments revealed a small decrease of the elastic modulus and the yield stress, while an increase of the ultimate strain with increasing content of a-PHB. These were related to the change of the deformation behavior from brittle to ductile and its micromechanisms. Easily cavitating a-PHB inclusions initiate numerous crazes in PLA matrix. These crazes become diffuse, frequently deflected and terminated by other a-PHB inclusions. At higher content of a-PHB the dense collections of diffused crazes merge to form deformation bands and eventually a macroscopic neck. Along with improvement of drawability at low deformation rate also the impact properties, probed in tensile impact test, are substantially improved. The thin-film tensile impact resistance increases from about 50 kJ/m2 in plain PLA to nearly 120 kJ/m2 in 80:20 PLA/a-PHB blend.