Thermodegradation of medium-chain-length poly(3-hydroxyalkanoates) produced by Pseudomonas putida from oleic acid

Medium-chain-length poly(3-hydoxyalkanoates) (mcl-PHA), comprising six to fourteen carbon-chain-length monomers, are natural thermoplastic polyesters synthesized by fluorescent pseudomonades. In this study, mcl-PHA was produced by Pseudomonas putida from oleic acid in aerobic shake flask fermentation. Thermal degradation of mcl-PHA was performed at temperatures in the range of 160-180 degrees C. Thermodynamic parameters of mcl-PHA thermal degradation were determined where degradation activation energy. E-d and pre-exponential factor, A equal to 85.3 kJ mol(-1) and 6.07 x 10(5)s(-1), respectively: and exhibited a negative activation entropy (Delta S) of -139.4 J K-1 mol(-1). Titration was carried out to determine the carboxylic terminal concentration and used to correlate number-average molecular weight (M-n) of the polymers. Thermally-degraded PHA contained higher amount of carboxylic terminals and lower M-n compared to the initial PHA and these results coincide with the decreased M-n in GPC analysis. Thermal properties of initial and degraded mcl-PHA were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal decomposition mechanism was investigated following the analyses of the degradation products using 400-MHz H-1 NMR, FTIR spectroscopy and GC analysis. The overall decomposition reaction is the hydrolysis of ester linkages to produce hydroxyl and carboxylic terminals. A small proportion of unsaturated side chain fragments would undergo oxidative cleavage at C=C linkages, producing minor amount of low-molecular weight esters and acids. At higher temperatures, the hydroxyl terminal can undergo dehydration to form an alkenyl terminal. (C) 2010 Elsevier Ltd. All rights reserved.