Plasticization of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with an Oligomeric Polyester: Miscibility and Effect of the Microstructure and Plasticizer Distribution on Thermal and Mechanical Properties

[Image: see text] In the last few decades, many efforts have been made to make poly(3-hydroxybutyrate) (PHB) and its copolymers more suitable for industrial production and large-scale use. Plasticization, especially using biodegradable oligomeric plasticizers, has been one of the strategies for this purpose. However, PHB and its copolymers generally present low miscibility with plasticizers. An understanding of the plasticizer distribution between the mobile and rigid amorphous phases and how this influences thermal, mechanical, and morphological properties remains a challenge. Herein, formulations of poly(hydroxybutyrate-co-valerate) (PHBV) plasticized with an oligomeric polyester based on lactic acid, adipic acid, and 1,2-propanediol (PLAP) were prepared by melt extrusion. The effects of the PLAP content on the processability, miscibility, and microstructure of the semicrystalline PHBV and on the thermal, morphological, and mechanical properties of the formulations were investigated. The compositions of the mobile and rigid amorphous phases of the PHBV/PLAP formulations were easily estimated by combining dynamic mechanical data and the Fox equation, which showed a heterogeneous distribution of PLAP in these two phases. An increase in the PLAP mass fraction in the formulations led to progressive changes in the composition of the amorphous phases, an increase of both crystalline lamellae and interlamellar layer thickness, and a decrease in the melting and glass transition temperatures as well as the PHBV stiffness. The Flory–Huggins interaction parameter varied with the formulation composition in the range of −0.299 to −0.081. The critical PLAP mass fraction of 0.37 obtained from thermodynamic data is close to the value estimated from dynamic mechanical analysis (DMA) data and the Fox equation. The mechanical properties showed a close relationship with the distribution of PLAP in the rigid and mobile amorphous phases as well as with the microstructure of the crystalline phase of PHBV in the formulations.