Hydrolysis embrittles poly(lactic acid)

Abstract: The backbones of biodegradable and bioderived polymers often contain chemical bonds, such as ester and amide that are susceptible to hydrolysis. Here, we show that hydrolysis causes a transition from ductile to brittle fracture in poly(lactic acid) (PLA). Submerged in an aqueous solution and bearing a load, a sample with a precrack undergoes extensive plastic deformation when the crack grows fast, but negligible plastic deformation when the crack grows slowly. In the former, the ductile fracture creates rough and porous crack surfaces, indicating that polymer chains slip before scission. In the latter, the brittle fracture creates flat crack surfaces, indicating that polymer chains slip negligibly before scission. Furthermore, at a low load and over a broad range of pH, the velocity of a crack in PLA correlates with the rate of hydrolysis of lactic acid oligomers. Taken together, these observations demonstrate that PLA suffers hydrolytic embrittlement. The phenomenon should be taken into account in the design of—and with—biodegradable and bioderived polymers. Impact statement: In thermoplastics of high molecular weights, repeat units form long polymer chains by chemical bonds, and the long polymer chains form solids by physical interactions. Between neighboring repeat units, the chemical bonds are commonly much stronger than the physical interactions. Polymer chains slip extensively before scission in ductile fracture, but slip negligibly before scission in brittle fracture. In biodegradable and bioderived thermoplastics, repeat units often link by chemical bonds susceptible to hydrolysis. Here, we show that hydrolysis embrittles a leading bioderived thermoplastic, poly(lactic acid). Even a small load exposes a crack tip to water molecules from the environment, hydrolyzing ester bonds and breaking chains with negligible chain slip. The material has a toughness above 10⁴ J/m². However, submerged in an aqueous solution, the material can grow a crack at an energy release rate as low as 1 J/m². Hydrolytic embrittlement should be investigated for biodegradable and bioderived polymers under development for health care and sustainability. Graphical abstract: [Figure not available: see fulltext.].