Poly (hydroxybutyrate) (PHB) is biodegradable aliphatic polyester that is produced by a wide range of microorganisms. Basic PHB has relatively high glass transition and melting temperatures. To improve flexibility for potential food packaging applications, syntheses of PHB with various co-polymers such as Poly-(3-hydroxyvalerate) (HV) decreases the glass and melting temperatures as well as broadens the processing window since there is improved melt stability at lower processing temperatures. In this study, PHB was synthesized with different valerate contents (5, 12, and 20%) and molecular weights ranging from 150 to 600 kDa. This study focused on characterizing the thermal, mechanical, rheological, and barrier properties of PHB synthesized with different valerate contents, and second to compare these properties in PHBV with similar hydroxyvalerate content but different molecular weight. All PHBV materials displayed a glass transition between -10 to 20°C. The two melting transitions found for Aldrich 5%, 12%, and Tianan 20%, resulted from crystals formed during cooling of the samples. The melt rheology suggested thermal instability of samples as the complex viscosity decreased with increasing temperature due to a decrease in molecular weights of the materials. These results suggest that processing the copolymer below 160°C would be beneficial with low screw speed. The mechanical results indicate all PHBV materials had high elastic modulus and flexural strength with low tensile strength and elongation at break. The WVTR results indicated the polymer to be very hydrophilic resulting in higher transmission rates.
Individually PHBV and PLA polymers have serious disadvantages when compared to thermoplastics that are currently used. To address high costs and thermal instability, blends of PHBV with PLA were explored as an alternative way of acquiring novel materials with desired properties. At the start of this work, three grades of PLAs were commercially available for purchase with different D- and L- lactide ratios and molecular weights used in extruding to injection molding. Miscibility of PHBV with PLA was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). The composition of PHBV in blends ranged from 50 to 80%. The aim of this study was to characterize thermal properties of three PLA resins blended with PHBV. DSC analysis indicated the blends were immiscible due to separate melting temperatures representing individual polymers. However, minimal changes in the glass transitions were witnessed for both PHBV and PLA materials. These changes in temperature were due to weak hydrogen bonding and spherulites overlapping. The TGA analysis showed two degradation peaks for individual PHBV and PLA polymers, thus supplementing the results observed for DSC analysis. The mechanical properties of the blends were also investigated using an Instron and Rheological Solids Analyzer units. The results showed significant improvements in the elastic modulus, flexural strength, and elongation at break. In general, the viscosity decreased with increasing rotational frequency due to break-up of entanglements between polymers. At various concentrations, the melt viscosity of PHBV was improved with the addition of PLAs at 160 and 170°C. At melting temperatures, the addition of PLA caused no significant improvements in the complex viscosity.