The microbial degradation behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) and its compound with several polyesters such as poly(butylene adipate-co-telephtharate) (PBAT), poly(butylene succinate) (PBS), and polylactic acid (PLA) in seawater was tested by a biological oxygen demand (BOD) method. PHBHHx showed excellent biodegradation in seawater in this study. In addition, the biodegradation rate of several blends was much influenced by the weight ratio of PHBHHx in their blends and decreased in accordance with the decrement of PHBHHX ratio. The surface morphology of the sheet was important factor for controlling the biodegradation rate of PHBHHx-containing blends in seawater.

Microplastics are fragmented pieces of plastic in marine environments, and have become a serious environmental issue. However, the dynamics of the biodegradation of plastic in marine environments have not yet been elucidated in detail. Polyhydroxyalkanoates (PHAs) are biodegradable polymers that are synthesized by a wide range of microorganisms. One of the PHA derivatives, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) has flexible material properties and a low melting temperature. After an incubation in seawater samples, a significant amount of biofilms were observed on the surfaces of PHBH films, and some PHBH films were mostly or partially degraded. In the biofilms that formed on the surfaces of unbroken PHBH films, the most dominant operational taxonomic units (OTUs) showed high similarity with the genus Glaciecola in the family Alteromonadaceae. On the other hand, the dominant OTUs in the biofilms that formed on the surfaces of broken PHBH films were assigned to the families Rhodobacteraceae, Rhodospirillaceae, and Oceanospirillaceae, and the genus Glaciecola mostly disappeared. The bacterial community in the biofilms on PHBH films was assumed to have dynamically changed according to the progression of degradation. Approximately 50 colonies were isolated from the biofilm samples that formed on the PHBH films and their PHBH-degrading activities were assessed. Two out of three PHBH-degrading isolates showed high similarities to Glaciecola lipolytica and Aestuariibacter halophilus in the family Alteromonadaceae. These results suggest that bacterial strains belonging to the family Alteromonadaceae function as the principal PHBH-degrading bacteria in these biofilms.

We investigated biofilm formation on biodegradable plastics in freshwater samples. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) was covered by a biofilm after an incubation in freshwater samples. A next generation sequencing analysis of the bacterial communities of biofilms that formed on PHBH films revealed the dominance of the order Burkholderiales. Furthermore, Acidovorax and Undibacterium were the predominant genera in most biofilms. Twenty-five out of 28 PHBH-degrading isolates were assigned to the genus Acidovorax, while the other three were assigned to the genera Undibacterium and Chitinimonas. These results demonstrated that the order Burkholderiales in biofilms functions as a degrader of PHBH films.

Poly­(hydroxyalkanoate)­s (PHAs) have gained significant attention because they readily biodegrade even under environmental conditions; however, expanding the applications of these polymers to a number of fields is hindered by their brittle nature. Blending with a soft biodegradable polymer, such as poly­(ε-caprolactone) (PCL), is currently the best solution for improving the toughness of a PHA material without sacrificing biodegradability; consequently, the development of biodegradable compatibilizers is strongly desired. This Article describes a systematic investigation into the compatibilizing abilities of a number of architecturally varied block copolymers (BCPs) composed of microbially derived poly­(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P­(3HB-co-3HHx); PHBH as the A block) and PCL (as the B block), including AB-, A2B-, AB2-, ABA-, and A2BA2-type BCPs. These BCPs were precisely synthesized through the click reactions of azido-functionalized PCLs with microbially produced propargyl-end-functionalized PHBH. Transmission electron microscopy and tensile-testing experiments reveal that the mechanical properties of PHBH/PCL/BCP (= 67/23/10 wt %) ternary blends are highly dependent on the choice of BCP architecture and are closely related to their morphologies. A remarkable 5–10-fold increase in elongation at break compared to that of the BCP-free blend (PHBH/PCL = 75/25) was achieved by the addition of the ABA- or A2BA2-type BCP as the compatibilizer.

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The effect of thermal and shear histories on the crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) was studied. As with other crystalline polymers, the shear history greatly affected the crystallization rate when the shear rate was beyond a critical value, i.e., the inverse of the Rouse relaxation time. Even after the formation of extended chain crystals, spherulite texture was clearly discernable. It grew from certain points on the extended chain crystals. Consequently, a row of spherulites appeared along the flow direction. The resin temperature in the molten state was also significant. When the sample was heated to 170 °C, which is beyond the main melting peak in the differential scanning calorimetry curve, unmolten crystals did not affect the linear viscoelastic properties. They acted as effective nucleating agents for the rest of the polymer during cooling. Therefore, the shear history hardly affected the crystallization rate and the number of spherulites.

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