## 4 Discussion
Thermal properties such as glass transition temperature (\(T_{g}\)), melting temperature (\(T_{m}\)), and thermal degradation temperature (\(T_{d}\)) are commonly evaluated for PHA polymers to determine the temperature conditions at which the polymer can be processed and utilized. MCL-3 HA is added to polymers to lower their crystallinity and melting temperature [62]. The physical and chemical properties of copolymers are largely dependent on monomer composition--i.e., the mol % of 3 HV, 3 HHx, and 3 HO subunits. Copolymers with increasing mol % (24 mol % to 71 mol %) of 3 HV display decreasing in melting temperatures (from 177 \({}^{\circ}\)C to 87 \({}^{\circ}\)C), decreasing glass transition temperatures (from 9.0 \({}^{\circ}\)C to \(-\)9.0 \({}^{\circ}\)C), and elongation-to-break ratios increased by 900 times [11; 63]. Melting temperatures and glass transition temperatures of MCL-PHAs produced by _P. putida_ LS46 from octanoate, decanoate, or biodiesel-derived fatty acids were 60 \({}^{\circ}\)C to 65 \({}^{\circ}\)C, and \(-\)33 \({}^{\circ}\)C to \(-\)35 \({}^{\circ}\)C, respectively [64]. Generally the ideal polymer should have a melting temperature far below it degradation temperature. DSC showed the different properties of PHAs produced by _P. putida_ LS46123. In the present study, co-polymers containing 3 HB and 3 HV in different proportions with MCL-monomers displayed decreased glass transition (from \(-\)34 \({}^{\circ}\)C to \(-\)36 \({}^{\circ}\)C), as well as melting temperatures from 138 \({}^{\circ}\)C to 166 \({}^{\circ}\)C. It appears that 3 HB content is not solely responsible for high melting temperature [64]. PHAs produced on octanoic acid and biodiesel fatty acid had about 50% of 3 HB but had a different melting temperature. Melting temperatures were more correlated with 3 HHx and 3 HV content and PHAs produced on hexanoic acid with 70% 3 HHx have low melting temperature than PHAs produced on octanoic acid or decanoic acid, which had 42%-48% 3 HHx. However, this it was not true for PHAs produced on biodiesel fatty acid, which had the highest content of 3 HO. All our polymers had a very high degradation temperature (248-254 \({}^{\circ}\)C), which was much higher than the melting temperature. However, the degradation temperatures of Mirel F1006 (a copolymer of 3 HB and 4-HB), Mirel 3002, and the P229 polymer were higher than our polymer [65]. Glass transition temperature is a very important parameter for stability at low temperatures. Glass transition temperature (\(T_{g}\)) varied from 4 to \(-\)44 with a change in 3 HA content from 0% to 97% in the polymer [19]. Melting temperature (\(T_{m}\)) decreased from 178 \({}^{\circ}\)C to 42 \({}^{\circ}\)C, with the change proportional to the amount of 3 HA in the polymer. Our polymers had \(T_{g}\) and \(T_{m}\) values very similar to LDPE and thus were suitable for wider application. Complex viscosity is related to shear velocity as a function of shear rate. Complex viscosity of all polymers produced in hexanoic, octanoic, nonanoic, and biodiesel fatty acid decreased with increased angular frequency. This decrease is associated with low melting stability. However, the complex viscosity of our polymers was higher than commercial polymers like Mirel F1006, Mirel 3002, and P229 [65]. Carbon substrates produced for PHAs production not only affected monomer composition but also led to the production of a polymer with diverse physical, thermal, and rheological properties.