(Top) Demonstration of the proposed mechanism to slow down stress-corrosion cracking in degradable polymers. In homogeneous PDMS, the stress concentration near the crack tip accelerates the rate of hydrolysis; in a PDMS composite, the stress de-concentrates at the crack tip, resulting in a much lower rate of reaction. (Bottom) In a PDMS composite, a hydrolytic crack is arrested at the matrix/fiber interface (right), while it propagates freely in homogeneous PDMS (left).
Degradable polymers are under intense development for applications in healthcare and sustainability. Recent research has shown that these materials are susceptible to a phenomenon in which cracks grow under a small applied load due to chemical reactions at the crack tip. A team at the Harvard MRSEC led by Suo and Vlassak has exploited mechanics to design a degradable composite that resists this phenomenon. In the composite, a crack in the matrix is blunted and stresses are distributed over a long fiber segment. As a result, cracks are pinned at the interface between the matrix and embedded fibers and do not grow. The idea is demonstrated using a composite made entirely of polydimethylsiloxane (PMDS) with different crosslink densities. Experiments show that cracks propagate much more slowly in the PDMS composite than in homogeneous PDMS. It is anticipated that this concept will contribute to the development of degradable materials with enhanced reliability.
Jiao, Q., Shia, M., Yin, T., Suo, Z. and Vlassak, J., "Composites retard hydrolytic crack growth," Extreme Mechanics Letters (in review)
Zhigang Suo (MechE), and
Joost J. Vlassak (MatSci & MechE)
2020-2021 Harvard MRSEC (DMR-2011754)