The Influence of Thermal History on the Properties of Pol y( 3- hydroxybutyrate-co- 1 20/0-3-hydroxyvaIerate)

The influence of thermal history on the properties of commercial P(3HB-co-l2%-3HV) was studied. Thermogravimetric analysis and differential scanning calorimetry revealed that the plasticizer evaporated at 140°C or higher. The loss of plasticizer during thermal treatment at 170 and 180°C resulted in a slight increase of the melting temperature of the polymer. The processing time and temperature, as well as the cooling procedure influenced the thermal behaviour of the material. A decrease in molecular weight with time was found at the temperatures investigated and this significantly affected the mechanical properties of the polymer prepared at 180 and 200°C. The rate constant kd of thermal degradation was slightly higher for samples during a shape-forming process in a Minimax apparatus than during a quiescent heating process (DSC) and its value increased with temperature. Limiting the processing at 170°C to 2 min gave a material with useful properties but increasing the residence time resulted in a decrease in strength, elongation at break, molecular weight and viscosity although it did not significantly influence the modulus of elasticity. Materials prepared at 180 or 200°C were more brittle and longer residence times resulted in a deterioration of the mechanical properties.