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.