Photodegradation of Poly(3-hydroxybutyrate- <i>co</i> -3-hydroxyhexanoate) and Its Effects on Marine Biodegradability

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) stands out among biodegradable polymers for its key advantage of marine biodegradability. However, its photodegradation, a key factor in the environmental fate of marine plastics, remains poorly understood. This study investigated the temporal photodegradation behavior of PHBH and its effects on marine biodegradability using accelerated UV exposure and biochemical oxygen demand tests. The initial stage of photodegradation involved the structural reorganization of less-ordered polymer chains into more ordered structures. Subsequently, film thickness loss began and continued throughout the test period owing to chemical and physical processes. PHBH photodegradation occurred across the bulk polymer, with a preference for amorphous regions; the degradation rates were in the order: amorphous phases > less-ordered crystals > well-ordered crystals. The fundamental backbone structure of PHBH remained largely intact, while its amorphous fraction and hydroxyhexanoate content gradually decreased. This suggests that photodegradation proceeded via a series of homolytic cleavages at the ester linkages, producing oligomers and lower-molar-mass products that could be liberated. Prolonged photodegradation caused PHBH to fragment into microplastics, but the resulting particles were expected to exhibit the biodegradability of the parent polymer in a microbially active environment. UV-exposed PHBH showed notably enhanced biofilm formation; however, the increased surface crystallinity initially delayed this benefit. Consequently, marine biodegradability did not significantly improve until the later stages of biodegradation. These findings are critical for predicting the environmental fate of PHBH and understanding how UV exposure impacts its biodegradability in marine ecosystems.