Sin, Mei Chan (2012) Thermodegradation of medium-chain-length poly (3 hydroxyalkanoates), and assessment of the biopolyesters and oligoesters as plasticizer for poly(vinyl chloride) / Sin Mei Chan. PhD thesis, University of Malaya.
Abstract
Medium-chain-length poly(3-hydroxyalkanoates) (mcl-PHA) are natural polyesters of hydroxyl fatty acids, composed of monomers with 6 to 14 carbon atoms length. These biopolyesters are primarily synthesized by fluorescent pseudomonads under nutrient imbalance, as carbon and energy storage compounds. In this study, mcl-PHA were produced by Pseudomonas putida PGA1 using oleic acid (OA) and saponified palm kernel oil (SPKO) as carbon source in shake flasks and fed-batch fermentations. From thermogravimetric analysis (TGA), the mcl-PHA derived from oleic acid (PHAOA) showed a drastic decomposition at temperature above 183ºC, while mcl-PHA derived from saponified palm kernel oil (PHASPKO) above 196ºC. Therefore thermal degradation of the PHAOA was carried out at temperatures of 160°C, 170°C and 180°C; whereas thermal degradation for the PHASPKO was performed at temperatures of 160°C, 170°C, 180˚C and 190°C, respectively. Changes in the polymer structures and properties before and after thermal treatments were studied by TGA, Differential Scanning Calorimetry (DSC), Gel Permeation Chromatography (GPC), Gas Chromatography (GC), Fourier Transform Infrared (FTIR) Spectroscopy, 400-MHz Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy, and end group analysis. Acid values obtained from end group analysis showed that mcl-PHA heat-treated at higher temperatures had higher concentration of terminal carboxylic acids and lower number average molecular weight (Mn). Based on GC, FTIR and 1H-NMR spectroscopic analyses on the thermal degradation products, it is suggested that the thermal degradation mechanism of mcl-PHA involved random α-chain scission via hydrolytic ester bond cleavage, producing a mixture of lower molecular weight oligomeric hydroxyacids. Heating above 180˚C in the acidic environment would lead to the dehydration of some hydroxyl terminal groups to produce alkenoic acids. These low molecular weight oligoesters possess functional terminals and could be used as natural-based plasticizers in PVC compounds. While the common PVC plasticizers, particularly phthalates, are known to be detrimental to the environment and human health, the mcl-PHA and its oligoesters have the potential to be alternative eco-friendly plasticizers for PVC. On this premise, a series of solution-cast blends of poly(vinyl chloride)-poly(3-hydroxyalkanoates) (PVC-PHA) were prepared to assess whether mcl-PHA and its oligoesters could be acted as compatible plasticizers for PVC. Scanning Electron Microscopy (SEM), FTIR, 1H-NMR, DSC and Dynamic Mechanical Analysis (DMA) were conducted to study the microstructure, film morphology, miscibility and viscoelastic properties of the PVC-PHA blends. SEM micrographs of PVC/PHA films showed that plasticization of PVC involved the PHA penetrated in some of the porous structures of PVC, and interfused with PVC polymer segments. Both FTIR and 1H-NMR spectroscopic analyses suggested the PVC-PHA miscibility was possibly due to the specific interactions between the ester C=O group of PHA with the αH and local dipoles of chlorines of PVC. TGA study was used to investigate the thermal stability and thermo-dynamic parameters of the plasticized PVC films. Both measurements of DSC and DMA gave consistent results of a single Tg for the blends, indicating that mcl-PHA was highly miscible with PVC. Results from DMA also showed that mcl-PHA and its oligoesters could reduce the Tg of PVC, imparting elasticity to the PVC compounds and therefore decreasing the stiffness of the polymer.
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