Hydrolytic crack in a rubbery network

We report an experimental finding that a crack advances in a polydimethylsiloxane (PDMS), under a small stress, in a moist environment. PDMS chains consist of siloxane bonds with methyl side groups. The methyl groups make PDMS hydrophobic, and PDMS hydrolyzes extremely slowly under no stress. Nonetheless, we find that a crack advances under the combined attack of water and stress. Our experimental data support the hypothesis that PDMS is susceptible to stress-assisted hydrolysis, but show that the activation area for PDMS is smaller than that for silica by two orders of magnitude. We attribute this large difference to the molecular structures of the two materials: a glassy network and a rubbery network, respectively. This finding suggests that stress-assisted reaction should concern many elastomers and gels in engineering and medicine.