Abstract

Polyhydroxyalkanoates (PHA) are polyesters accumulated by bacteria as intracellular granules from renewable carbon sources. PHA have properties similar to petrochemical plastics, attracting industrial interest since they are biodegradable. PHA can be composed by short-chain-length monomers (HASCL) having C3-C5 carbons or medium-chain-length (HAMCL) when monomers are C6-C12. Monomer composition determines polymer characteristics and therefore their applications. A Pseudomonas sp recombinant (LFM 461 phaPCJTSM81), harbouring PHA synthase from Aeromonas, was cultivated in bioreactor to produce PHA for characterization. This strain is able to accumulate P(3HB-co-3HAMCL) using glucose as a single carbon source. The polymer was submitted to solvent extraction with acetone or chloroform, followed by anti-solvent purification. The resulting polymer was analysed by gas chromatography and, regardless of the solvent, a predominance of monomers of 3-hydroxybutyrate (approximately 60 mol %) followed by 3-hydroxyhexanoate monomers (about 30 mol %) were observed. The monomers of 3-hydroxyoctanoate, 3-hydroxydecanoate and 3-hydroxidodecanoate appear in smaller quantities totalling about 10 mol %. Acetone was used as solvent due to its affinity to 3HAMCL and chloroform because its ability to extract any type of PHA monomers. Results did not evidence relevant differences on monomer composition based on the extraction method, suggesting that the PHA produced may be a random copolymer of 3HB and 3HAMCL. Polymer samples were then characterized by gel permeation chromatography (GPC), Fourier transform infra-red spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). GPC analysis resulted on similar values of molecular weight and, the existence of a symmetrical peak in both solvents, indicated a single polydisperse material. FTIR spectra also did not show structural differences between the samples, which presented typical polymer structures. TGA revealed different thermo-gravimetric profiles and a difference of 10°C in the temperature of maximum degradation rate for the different extraction solvents. DSC showed two Tg values for either solvent, but the temperature associated to the second transition was different (-10.4°C and -7.8°C) and the enthalpy of fusion also revealed more discrete differences. Interpretation of the results, a priori, indicate a possibility of the material be a bloc copolymer with different composition, due to the two Tg values and the TGA results indicating different thermal stabilities and relatively narrow mass loss peaks. Complementary analyses of RMN are in progress to confirm that.