Cracks outrun erosion in degradable polymers

Degradable polymers are being developed for medical applications and environmental sustainability. The molecular mechanism of degradation is known: a polymer dissociates in response to a trigger, such as light, water, or biomolecules. However, the spatial and temporal processes of degradation are poorly characterized. Here we show that degradation can be highly heterogeneous, and can readily lead to cracks that outrun erosion in speed by orders of magnitude. This paper studies crack growth in poly(glycerol sebacate) (PGS), a degradable elastomer developed for medical applications. The elastomer is a polyester in which ester bonds hydrolyze in the presence of water molecules. We prepare a sample of PGS with a precut crack, apply various loads, and record the crack growth using a camera. A small load opens a crack in PGS, and provides a path for water molecules to reach the crack tip, possibly by clearing the hydrophobic debris of reaction products, enabling hydrolysis at the crack tip to outrun elsewhere. We show that the speed of the hydrolytic crack depends on relative humidity, pH, and applied load. In a fixed environment, we identify two regimes of crack growth: one is sensitive to the magnitude of the applied load, and the other is not. The hydrolytic crack causes the polymer to lose its load-carrying capacity prematurely, and fragmented polymer particles can cause severe medical complications.