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

摘要

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 ℃. 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 (AS) of-139.4J 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 ~1H 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.