PHYSICAL GELATION OF A BACTERIAL THERMOPLASTIC ELASTOMER

The crystallization of a bacterial polyester, poly(beta-hydroxyoctanoate), produced by Pseudomonas oleovorans was studied by dynamic mechanical measurements. Samples were first preheated to a temperature T(p) above the melting temperature, T(m), and then supercooled for isothermal crystallization at temperatures of T(c) = 15 or 20-degrees-C. Storage and loss moduli G'(w) and G"(w) grew with crystallization time since the crystallites connected the macromolecules into a three-dimensional physical network. The final solid polymer was a thermoplastic elastomer. At the gel point the sample is characterized by the typical self-similar relaxation modulus, G(t) = St(-n) with n = 0.11, and a frequency-independent loss angle. From measurements at different crystallization times a master curve was obtained by time-cure superposition. Such master curves have the same shape for crystallization at T(c) = 15 and 20-degrees-C. The results of the rheological measurements support the suggestion that crystallization-induced physical gelation is characterized by dynamic self-similarity which was observed earlier for chemical gelation. However, when crystallizing at a higher temperature (T(c) = 30-degrees-C), no critical gel was observed macroscopically which indicated that an inhomogeneous crystallization occurred, leading to a nonuniform gelation process.