Processing and characterization of binary poly(hydroxybutyrate) (PHB) and poly(caprolactone) (PCL) blends with improved impact properties

The present work is focused on the development of binary blends from poly(hydroxybutyrate) (PHB) and poly(caprolactone) (PCL). Miscibility, mechanical and thermal properties as well as blends morphology are evaluated in terms of the blend composition. Binary PHB–PCL blends were manufactured by melt compounding in a twin screw co-rotating extruder and injection molded. The composition of PHB–PCL covered the full range between individual polymers at 25 wt% increments. The obtained results show that PCL acts as an impact modifier, thus leading to an increase in flexibility and ductility as the PCL content in the PHB–PCL blends increases with a noticeable increase in elongation at break and on the energy absorption in impact conditions. The tensile strength and the elastic modulus decrease with increasing PCL content in the PHB–PCL blends; nevertheless, the flexural strength and the flexural modulus reach the highest values for the PHB–PCL blends containing 25 wt% PCL, with a remarkable decrease over this composition. The analysis of fractured surfaces by field emission scanning electron microscopy and thermal properties obtained by differential scanning calorimetry (DSC) and TGA give clear evidences of the immiscibility of these two biodegradable polymers. Additionally, DSC results showed an increase in crystallinity of both PHB and PCL with regard to individual polymers for PHB–PCL blends containing 25 wt% PCL. Furthermore, an increase in the degradation onset (T ₀) of about 30 °C higher was detected for the same blends. Dynamic mechanical thermal analysis showed slightly shifted glass transition temperatures of each individual polymer, thus indicating that although both PHB and PCL are not fully miscible, some interactions between them occur.