Tough and Stretchable Ionogels

We report an ionogel that has best-in-class mechanical properties in multiple categories. An ionogel is a polymer network swollen with ionic liquids. The gels form in a single step and utilize common monomers. The ionogels may find applications in batteries, 3D printing, and electronic devices.

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Gels are polymer networks swollen with liquid. Gels are common in our daily life. Examples include contact lenses, tofu, and jelly. These gels are swollen with water and therefore called hydrogels. Hydrogels have been utilized in the literature for a wide range of applications such as wearable electronics, sensors and actuators, and energy storage devices (i.e., batteries and supercapacitors). However, water evaporation limits the practical use of hydrogels.

 It is possible to address this problem by replacing water with ionic liquids in the gels to produce ‘ionogels’.  Ionic liquids (ILs) – molten salts consisting of cations and anions at low temperature (generally < 100 ˚C)—have attracted much attention due to their remarkable properties such as nonvolatility (i.e. they do not evaporate), high ionic conductivity, and high electrochemical and thermal stability. Whereas hydrogels can only form with water, there are many diverse ionic liquids with very different physical and chemical properties. This means ionogels can be synthesized with diverse properties.  Ionogels may therefore be tuned for iontronics (devices that use ions instead of electrons), batteries, and materials for extreme environments (high vacuum and temperature, or electrical potential).

 Gels generally have poor mechanical properties.  They are brittle (consider how easy it is to chew tofu or how easily contact lenses tear).  While significant work has been done to toughen hydrogels, there are fewer examples of tough ionogels, which is the focus of our work. 

 One approach to toughen gels is to add ‘secondary bonds’ that can break and reform during deformation. Primary bonds are the covalent bonds in the polymer, but secondary bonds – such as ionic or hydrogen bonds – can form between the backbone of the polymer.  This strategy has produced tough hydrogels with a fracture strength (~10 MPa) and fracture energy (~10,000 J m-2) comparable to natural rubber and cartilage. Yet, most ionogels exhibit poor mechanical properties, such as fracture strength <1 MPa and fracture energy <1,000 J m-2, which are far lower than that of the tough hydrogels.

 The principles used to produce tough hydrogels have been employed to a lesser extent with ionogels.  However, the approaches to do it require multiple steps that are complex and laborious.   

 To achieve tough ionogels, we report a facile one-step method by randomly copolymerizing two common monomers.  We describe this in a paper in Nature Materials. One of the monomers produces a polymer with poor solubility in the ionic liquid, while the other is highly soluble. Consequently, when the two monomers copolymerize together with the appropriate composition, it phase separates in situ to form small ‘domains’ of poorly soluble polymer connected together by a rubbery, solvated network.  The poorly soluble (polymer-rich) regions form hydrogen bonds that dissipate energy. The solvent-rich phase enables large strain.

 The remarkable mechanical properties of the obtained copolymer ionogel is demonstrated by lifting a 1 kg weight (see the figure and this video). As a demonstrator, we copolymerize two common monomers of acrylamide and acrylic acid in ionic liquid (1-ethyl-3-methylimidazolium ethyl sulfate). Acrylamide is used in contact lenses and acrylic acid used in diapers, which is to say they are cheap and common.  The obtained ionogels show record mechanical properties among multiple categories, including fracture strength (~13 MPa), fracture energy (~24,000 J m-2), Young’s modulus (~47 MPa), and compressive strength (~33 MPa), which are orders of magnitude larger than the best in reported ionogels. More interestingly, the ultra-tough ionogels also exhibit high stretchability (~600% strain) and multiple functions, like self-healing, shape-memory, and self-recovery properties. Additionally, this method is confirmed to be a generalized strategy to toughen ionogels in a way that is far simpler than existing multi-step methods that result in weaker gels.

 This report provides insight into tailoring microscopic phase structures and hence macroscopic mechanical properties in ionogels. It is exciting that this synthetic strategy is simple yet achieves ionogels with best-in-class properties in multiple categories. Because the polymers form in a single step, it is possible to 3D print them. Finally, based on this generalized strategy, nontoxic monomers and ionic liquids could be appropriately selected to fabricate tough and biocompatible ionogels, which may expand their biomedical applications.

Michael Dickey

Professor, NC State University