Sustainable Approach to Overcome Polylactide Brittleness with Biobased Esters of Isosorbide and Fatty Acids

This work reports on the design and synthesis of sustainable plasticizers from plant-based isosorbide to enhance the intrinsic brittleness of polylactide (PLA). To keep fully biobased carbon, isosorbide was esterified with fatty acids of varying chain length, leading to isosorbide dibutyrate (IDB), dicaprylate (IDC), and dipalmitate (IDP). These esters were incorporated into PLA at different concentrations. An approach to assess PLA-plasticizer miscibility was conducted by calculating solubility parameters (δ) and the Flory–Huggins interaction parameter, χ. The effect of plasticizer type and concentration on mechanical, thermal, and thermomechanical properties, as well as on microstructure and biodegradation, was also addressed. The results indicated that IDB and IDC notably enhanced PLA toughness, reducing the PLA’s glass transition temperature (T_g) from 60.3 to 27.7 °C with 20 wt % IDC. Consequently, strain at break dramatically increased from 12.8% (PLA) to over 300% with 20 wt % IDB or IDC. In contrast, IDP exhibited limited miscibility, resulting in phase separation, though it still improved the impact strength and ductility. All formulations demonstrated exceptional disintegration in compost soil, underscoring their potential as “double green” plasticizers suitable for PLA. Since both PLA and isosorbide can be industrially derived from starch, this work places starch as a key platform for sustainable polymers.