THE MECHANICAL-PROPERTIES OF A THERMOPLASTIC ELASTOMER PRODUCED BY THE BACTERIUM PSEUDOMONAS-OLEOVORANS

PHO, a poly(beta-hydroxyalkanoate) copolymer containing mostly beta-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.