Just add water to change nanocomposite stiffness

Composites

April 3, 2008

Sea cucumbers inspired the design of polymer nanocomposites that change their mechanical properties when exposed to water. (Courtesy of Fred Carpenter.)

The sea cucumber Cucumaria frondosa can change the rigidity of its dermal skin layer by modulating the interactions between adjacent collagen fibrils with soluble macromolecules secreted by nearby cells.

Now researchers at Case Western Reserve University have developed a nanocomposite made of cellulose nanofibers embedded in a polymer matrix that mimics the behavior of the sea cucumber dermis. The stiffness of the composite is modulated by adding or removing water [Capadona et al., Science (2008) 319, 1370].

“The nanocomposites are comprised of a rubbery polymer into which strong and rigid nanofibers are embedded so that they form a network,” says Chrisoph Weder, who led the work.

He explains that an ethylene oxide-epichlorohydrin (EO-EPI) copolymer was chosen because of its low intrinsic modulus, the ability to absorb a little bit of water, and its propensity to form well-defined, percolating nanocomposites with cellulose nanofibers. Cellulose fibers extracted from sea creatures known as tunicates were selected because of their high stiffness and surface chemistry.

“We isolated cellulose nanofibers from tunicates because these species grow cellulose fibers of particularly high aspect ratio,” Weder adds.

The nanofibers are rich in hydroxyl groups. This permits switching by turning hydrogen bond interactions on and off through the absence or presence of a competitive hydrogen bonding agent, in this case water.

“In the absence of water, the nanofibers are ‘glued’ to each other because they display strong molecular-level interactions and the nanofiber network dominates the mechanical properties of the material,” says Weder. “In this state, the material is strong and rigid.”

“If the material is exposed to water, it swells very slightly and the water molecules ‘unglue’ the nanofibers,” he continues. Water interferes with the interfiber interactions and the material becomes about 1000 times softer. “Its properties now resemble those of a rubber.”

The team also made adaptive nanocomposites using the more rigid polyvinyl acetate as the polymer matrix, giving biocompatible devices that are stiff enough to penetrate tissue then soften once embedded. Other possible applications include biomedical implants like stents and smart ‘adjustable’ casts, Weder notes.

Mark E. Greene