Microbial Synthesis And Characterization Of Polyhydroxyalkanoate (PHA) Block Copolymers

Polyhydroxyalkanoates (PHA) are fully biodegradable biopolymers produced byseveral microorganisms from renewable substrates. To meet the current growingrequirements of bioplastics, PHA biopolymers with diverse chemical structures andunique properties are required. Microbial block copolymerization offers the advantageof incorporating polymer microstructures that achieve some novel properties not yetseen in PHA homopolymers, blend polymer and random copolymers. In the presentstudy three types of block copolymers were biosynthesized such aspoly(3-hydroxybutyrate)-block-poly(3-hydroxyhexanoate)[P3HB-b-P3HHx],poly(3-hydroxypropionate)-block-poly(4-hydroxybutyrate)[P3HP-b-P4HB],poly(3-hydroxyhexanoate)-block-poly(3-hydroxydecanoate-co-3-hydroxydodecanoate)[P3HHx-b-P(3HD-co-HDD)]. The block copolymers were characterized by1H NMR,13C NMR, HMBC NMR, DSC, GPC and mechanical property analysis.Diblock copolymers comprising of poly-3-hydroxybutyrate (P3HB) as theshort-chain-length (SCL) PHA block covalently bonded with poly-3-hydroxyhexanoate(P3HHx) as the medium-chain-length (MCL) PHA block were for the first timesuccessfully produced in-oxidation weakened Pseudomonas putida KT2442.Differential scanning calorimetry (DSC) showed that the block copolymer had two glasstransition temperatures (Tg), one melting temperature (Tm) and one cool crystallizationtemperature (Tc), it had superior mechanical properties over random copolymerP(3HB-co-3HHx).Block copolymers consisting of elastic P4HB block with a strong and tough P3HPblock were biosynthesized to gain unique and excellent material properties, two blockcopolymers were formed from this study, including the P3HP-b-29mol%P4HB andP3HP-b-37mol%P4HB, they showed superior properties over random copolymersP(3HP-co-4HB) with similar monomer ratios. The block copolymers had two glasstransition temperatures (Tg) and two melting temperatures (Tm). In comparison to thehomopolymers P3HP and P4HB, incorporation of block microstructure resulted in thelowering of Tm, block copolymers were revealed with higher Young’s modulus, yield strengths and tension strengths, much better than the previously reported randomcopolymers of similar compositions.Medium chain length PHA (MCL PHA) diblock copolymerP3HHx-b-P(3HD-co-3HDD) consisting of a49mol%poly-3-hydroxyhexanoate (3HHx)block with another block consisting of51mol%copolymer of3-hydroxydecanoate(3HD) and3-hydroxydodecanoate (3HDD) was produced from another β-oxidationweakened strain Pseudomonas putida KT2442during the sequential addition of relatedfatty acid substrates. Thermo-mechanical properties of the block copolymer showed thatblock microstructure gained the amorphous nature of3HHx and3HD and attained thecrystallinity of3HDD, thus higher mechanical strength was realized in contrast tomedium chain length MCL PHA homopolymers and random copolymer. PHAproduction platform consisting of P. putida derivatives was also utilized for theproduction of copolyesters of P(3HB-co-3HHx) and P(3HHx-co-3HD) with a widerrange of alterable monomer contents and compositions.The purpose of this research is to microbially produce block copolymers to diversifyPHA microstructures and increase their material properties. Microbial production ofblock copolymers can covalently link together two or more homopolymers and gainnovel material properties. Thus, desired material properties can be gained based on thePHA monomer composition. In this study, Pseudomonas putida KT2442was developedas a suitable platform for the production of various new biopolymers, especiallypolyhydroxyalkanoates (PHA) block copolymers, and random copolymers. This studycould help to produce sustainable bio based polymers in order to replace petroleumbased polymers.