Biodegradability of blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with cellulose acetate esters in activated sludge

Blends of the bacterially produced polyester poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with cellulose acetate esters (CAE) further substituted with propionyl or butyryl groups (degree of substitution: 2.60 propionyl and 0.36 acetyl or 2.59 butyryl and 0.36 acetyl, respectively) were exposed for 4 months to activated sludge to determine their biodegradability. Samples of such blends made by solution-mixing and solvent-casting had complex morphologies in which both individual components as well as a miscible blend phase were present. Additionally, the two opposite surfaces of solvent-cast films showed both physical and chemical differences. After 2 months, samples of pure PHBV had degraded by more than 98% (15 mg/cm² of surface area), whereas a pure CAE sample had degraded less than 1% (<0.2 mg/cm²). Samples containing 25% CAE lost less than 40% of their initial weights (6 mg/cm²) over the total 4-month period. Samples with 50% CAE lost up to 16% weight (2 mg/cm²), whereas those containing 75% CAE lost only slightly more weight than corresponding sterile control samples (1 mg/cm²). NMR results confirm that weight loss from samples containing 25% CAE resulted only from degradation of PHBV and that the surface of samples became enriched in CAE. Solvent-cast film samples containing equal amounts of PHBV and CAE degraded preferentially on the surface which formed at the polymer-air interface. Scanning electron microscopy and attenuated total reflectance infrared spectroscopy revealed this surface to have a rougher texture and a greater PHBV content.