This work deals with the melt blend of 60/40 PHB/PET (p-hydroxybenzoic acid and polyethylene terephthalate) and 80/20 PHB/PET copolyesters in a 50:50 weight% ratio. Specifically, the interest was to determine as to how do the mechanical properties of the injection-molded parts from the blend compare with that of 70/30 PHB/PET composition and to find out if the melt blend obtained after a single extrusion pass represents an equilibrium composition blend. To determine the anisotropic mechanical properties, injection-molded plaques were obtained by injection-molding the blend at different temperatures. It was found that the tensile properties (tensile strength, modulus and elongation at break in the machine direction) of the blend increase with the increase in the injection-molding temperature (from 300 to 320°c) and then decrease. The flexural modulus of the injection-molded plaques (at an injection-molding temperature of 330°c) of a 50:50 blend of 60/40 PHB/PET and 80/20 PHB/PET mixed only in the injection-molded unit was 2.2 x 10⁶ psi which is 40% higher than that reported for the 60/40 PHB/PET, 100% higher than reported for either 80/20 PHB/PET or 70/30 PHB/PET. To determine the equilibrium nature of the blend, samples with different residence time in the extruder and the 70/30 PHB/PET samples were analyzed by means of Differential Scanning Calorimeter (DSC), Dynamic Mechanical Analysis (DMA), Thermal Mechanical Analysis (TMA), Scanning Electron Microscope (SEM) and Rheometrics Mechanical Spectrometer (RMS), and the mechanical properties of the injection-molded plaques were also determined. The DSC thermogram of the four extrusion pass blend showed multiple melting endotherms. Similar behavior was observed for a number of samples which had been annealed above the melt temperatures for different lengths of time. The formation of multiple peaks was attributed to the incomplete transesterification reaction in the extruder. The OMA results also indicated a substantial decrease in the melting temperatures with the increase in the number of extruder passes. A similar decrease in the melting temperatures has been reported in the literature for various polyesters undergoing transesterification reaction. The TMA results showed that the modulus versus temperature profiles (softening profiles) of one, two and three extrusion pass samples were the same but that of the four extrusion pass film was different and was closer to the softening profile of the 70/30 PHB/PET film. The softening profile of the four extrusion pass sample indicated the presence of crystallites of varying degrees of development, which corresponds well with the splitting peak phenomenon observed in the corresponding DSC thermograms. Thus, it is clear that a chemical reaction is occurring in the extruder. It is suspected that the observed decrease in the flexural properties with the increase in the number of extruder passes is due the transesterification reaction occurring in the extruder. Transesterification studies were also carried out in a cone & plate geometry in the RMS in a no-shear (simple melt annealing) and shearing environment on a larger time scale. From a comparison of the thermal behavior of the blend (as a function of the reaction time) in different environments, it was concluded that the transesterification reaction proceeds faster in the presence of deformation such as occurs in the extruder or in a simple shear flow and with an increase in temperature. Finally, to compare the rheological properties of the multiple extrusion pass and the 70/30 PHB/ PET samples, dynamic frequency sweeps were performed in the RMS at different temperatures. At a given frequency, the complex viscosity, storage modulus and loss modulus increased with an increase in the number of extrusion passes. This increase is unexpected. A number of explanations have been proposed to account for this increase in the rheological properties. / Master of Science