Stimulus-responsive hydrogels (SRHs) are interesting materials in many respects. Their actuation (through volume change) in response to small changes in external stimuli make them attractive candidates for a range of applications. However, if an application requires the transmission of force, there can be considerable concern about gel failure. Anyone who has handled SRHs knows that they can be incredibly fragile.
Researchers in the DCML are developing models and experiments to study the resistance of SRHs to failure. We hope to understand the relationship between the "fracture toughness" of these materials, their phase state (collapsed or swollen), and the cross-link density of the underlying polymer network. This work is supported by the National Science Foundation.
The movies on the right show the progression of an edge notch in gel specimens in collapsed and swollen states. The movies do take a while to play, so you may want to accelerate them by moving the scroll on the bottom of the player. The collapsed specimen is opaque, while the swollen specimen is clear (we die them green so they're more visible). There are a couple of interesting observations that can be made on the basis of these movies alone. Note the quite large strains that are generated in the gels before the crack grows. We can also note that the flaw propagates in a self-similar pattern in the swollen specimen, while this is not true in the collapsed specimen. This appears to occur at a critical level of cross-linking, and we are actively working to better understand this phenomenon.
This work is sponsored by the National Science Foundation.