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Shear forces help make stretchable hydrogel

08 Apr 2019 Isabelle Dumé
The gelation mechanism
Left: The gelation mechanism through metal coordination to the pseudopolyrotaxanes; the metal ions are shown as yellow spheres. In the lower panel, which represents the hydrogel, the pseudorotaxanes are shown as blue lines. Right: Stimuli-responsive properties of the Cu(II)-based hydrogel, showing that the gel reverts to a sol overnight, and can be converted into a gel again under shear force. Courtesy: W Jiang, Southern University of Science and Technology in China

Shear forces can convert a sol into a gel. This surprising new behaviour, observed in an aqueous solution of a pseudopolyrotaxane with copper ions added to it, is similar to that in dissipative, far-from-equilibrium, biological systems capable of self-healing.

“Usually supramolecular hydrogels, like the ones we studied in this work, are destroyed by shear forces and convert into a sol,” says Wei Jiang of the Southern University of Science and Technology in China, who led this research effort. “This makes our result surprising and exciting.”

Molecular self-assembly is common in the biological world and leads to large and complex architectures that have very specific functions. Bioinspired synthetic versions of these have been built in recent decades, but they are usually stable in thermodynamic terms, unlike their biological counterparts, which work in the far-from-equilibrium state. Far-from-equilibrium means they constantly require an input of energy (or fuel) to continue functioning.

Dissipative self-assembly

This mechanism, known as dissipative self-assembly, would come in very useful in materials science because it could produce properties that exist in biological systems, such as self-healing and adaptability. The problem is that most synthetic dissipative self-assembly structures made to date are soft and have poor mechanical properties, something that limits their applications.

Jiang’s team has now self-assembled a supramolecular hydrogel under shear force. The researchers made their gel by simply adding copper ions to a solution of pseudopolyrotaxanes, which themselves are formed by threading molecular tubes on polyethylene glycol chains. When the solution is shaken vigorously for up to 30 seconds it transforms into a gel that gradually relaxes back to the sol state over time (hours to days at room temperature depending on the concentrations of the gelators) It can be converted back into a gel, however, by reapplying a shear force (the fuel). This process can be repeated many times.

Intrachain to interchain coordination

The mechanisms at play in the sol to gel transition rely on a shear-induced transition from intrachain to interchain coordination with the Cu(II) ions, explains Jiang. “The initial mixture is in a solution state because Cu(II) ions form intrachain coordination and there is no, or very little, cross-links among the pseudopolyrotaxanes. Shear forces destroy this intrachain coordination though and this results in kinetically fast interchain coordination, which leads to more cross links and thus gelation (the material becoming thicker).”

The resulting transient hydrogel has good mechanical properties, even when compared to permanent hydrogels, in that it is highly stretchable (it can be stretched up to 30 times its original length). This is in contrast to previous shear-thickening systems. It is also self-healing under a shear force as mentioned.

“The high extensibility comes from the fact that the molecular tubes are loosely packed on the polymer chains and can slide ‘frictionally’ when stretched,” explains Jiang. “This means that it might be used to prepare mechanoresponsive materials, as a first type of application, by incorporating the supramolecular coordination unit into other systems,” he tells Physics World.

The team, reporting its work in Nature Chemistry 10.1038/s41557-019-0235-8, says that it is now busy further improving the mechanical properties of its dissipative material.

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