Study of mechanical and rheological properties, morphology, and miscibility in polylactid acid blends with thermoplastic polymers

Blends of polypropylene, polystyrene (PS), polyethylene terephthalate (PET), and high-, low- and linear low-density polyethylene (LDPE and LLDPE) with different contents on polylactid acid (PLA) were prepared. Tensile and impact properties and melt flow rate (MFR) of the mixtures were determined. Morphology was analyzed through scanning electron microscopy (SEM) and interactions were studied by measuring glass transition temperatures (T_g) as well as applying a model to quantify the composition dependence of tensile strength. Results show that there are significant differences on structure and properties among the blends. Polyolefin based blends feature heterogeneous structures with big particle sizes, which translates to blend properties with high negative deviation from the rule of mixtures (ROM), especially for LDPE and LLDPE. Blends of PET and PS feature smaller dispersed particles, resulting in blends with mechanical properties close to those of ROM. Shifting in glass transition temperatures indicate interactions between these materials, which were quantitatively determined indicating the strongest interactions for the PS/PLA pair followed by the PET/PLA mixture.