An investigation was conducted into the crystallization kinetics of PHO, poly(/3-hydroxyoctanoate), a bacterially produced copolyester containing mostly /3-hydroxyoctanoate repeating units. The crystallization rate reached a maximum at 0-5 °C. The equilibrium melting point was determined to be approximately 68 °C. The long-term crystallization study on PHO crystallized from the melt at -20, +5, and +20 °C revealed that stable but different levels of crystallinity were reached after 3-7 weeks. The melting endotherm peak continued to change shape for up to 19 weeks. After 24 weeks of crystallization, the mechanical properties of the films were evaluated. The tensile modulus ranged from 2.5 to 9 MPa, the tensile strength at break from 6 to 10 MPa, and the ultimate elongation from 450% to 300%. A high tensile set, approximately 35% after 100% elongation, was observed for PHO crystallized at all three temperatures. Unusual melting endotherm peak shapes were observed for long-term crystallized samples after undergoing large extensions.
PHO, a poly(β-hydroxyalkanoate) copolymer containing mostly β-hydroxyoctanoate repeating units, was produced in a fed batch fermentation process by Pseudomonas oleovorans when grown on sodium octanoate as the sole carbon source. The polymer from different batches—evaluated with regards to composition, molecular weight distribution, thermal transition temperatures, and decomposition temperature—was found to be highly consistent batch-to-batch. Polymer composition as a function of growth time did not change significantly once the culture reached the stationary growth phase. PHO when crystallized at room temperature from the melt, forms a physically crosslinked network with the crystalline regions acting as the physical crosslinks. The molecular weight between physical crosslinks was determined to be approximately 4000. The stress-strain properties, hardness, and tensile set of PHO were found to be within the range of values defined by a variety of commercially available thermoplastic elastomers with differing chemical structures. The tensile set of PHO was high, 35% after 100% elongation. Experimental evidence supports three possible sources of the high tensile set: permanent strain-induced orientation or displacement of the physical crosslinks, irreversible strain-induced crystallization, and deformation-induced changes of the size and purity/perfection of crystalline regions.
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