Characterizing the effects of ambient aging on the mechanical and physical properties of two commercially available bacterial thermoplastics

Physical aging of commercial poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate)-co-poly(3-hydroxyvalerate) (PHBV) films has been investigated in this work. Aging-related changes in mechanical properties and crystalline matrix morphology were characterized using mechanical tensile tests, gas chromatography, differential scanning calorimetry, and gel permeation chromatography. Both PHB and PHBV samples, which were verified to contain a heterogeneous mixture of processing additives, experienced significant embrittlement upon aging in a desiccated isothermal (15°C) environment set moderately above their glass transition temperatures. Significant increases in the tensile modulus of elasticity were accompanied by decreases in ultimate strain; however, the aged samples exhibited no sizable variation in ultimate tensile strength after 168 days. Melt extrusion and ambient aging did not lead to any significant reduction in polymer molecular weights. Calorimetry results suggested that aged samples exhibited only a slight increase in overall degree of crystallinity, and multiple melting peaks were observed, indicating the presence of crystals of multiple thermal stabilities. Upon aging, the major melting peaks shifted to lower melting temperatures in both PHB and PHBV. The results demonstrate that physical aging of the two industrial biopolyesters can induce substantial changes in physical and mechanical properties in PHB and PHBV when stored just above their respective glass transition temperatures, and that the governing mechanism for embrittlement in both biopolymers is restricted to the non-crystalline phase.