Hydrophobic Hydrogels with Fruit‐Like Structure and Functions

International audienceexperiencing solvent exchange from good to poor, a gel usually exhibits a monotonous deswelling with time if the good and poor solvents are miscible. [7-11] During this process, the good solvent easily escapes from gels driven by osmotic pressure of solvent-solvent mixing, while the poor solvent hardly enters the gel due to the demixing of polymer and poor solvent (Figure 1a). Here, we report an abnormal swelling of hydrophobic gels in water to form "hydrophobic hydrogels." We discovered that when hydrophobic gels swollen with omniphilic organic solvents are immersed in water or aqueous solutions, instead of shrinkage, the hydrophobic gels substantially swell to reach a water content as high as 99.6 wt% at most. The significant swelling of hydrophobic gels in water leads to the formation of a novel class of "hydro-phobic hydrogels" that have a unique fruit-like structure and show unique functions such as selective water absorption. To our knowledge, such abnormal swelling phenomenon of hydrophobic gels in water has never been reported before. This abnormal swelling is attributed to the rapid formation of a semipermeable skin layer on the surface of the gel by the phase separation of the hydrophobic polymer in water, which brings about a strongly asymmetrical diffusion for the organic solvent and water. The impermeable organic solvent is effectively trapped inside the gel, and thus generates a high osmotic pressure which drives the permeable Normally, a polymer network swells in a good solvent to form a gel but the gel shrinks in a poor solvent. Here, an abnormal phenomenon is reported: some hydrophobic gels significantly swell in water, reaching water content as high as 99.6 wt%. Such abnormal swelling behaviors in the nonsolvent water are observed universally for various hydrophobic organogels containing omniphilic organic solvents that have a higher affinity to water than to the hydrophobic polymers. The formation of a semipermeable skin layer due to rapid phase separation, and the asymmetric diffusion of water molecules into the gel driven by the high osmotic pressure of the organic solvent-water mixing, are found to be the reasons. As a result, the hydrophobic hydrogels have a fruit-like structure, consisting of hydrophobic skin and water-trapped micropores, to display various unique properties, such as significantly enhanced strength, surface hydrophobicity, and antidrying, despite their extremely high water content. Furthermore, the hydrophobic hydrogels exhibit selective water absorption from concentrated saline solutions and rapid water release at a small pressure like squeezing juices from fruits. These novel functions of hydrophobic hydrogels will find promising applications, e.g., as materials that can automatically take the fresh water from seawater. Polymer gels are defined as crosslinked polymer networks that are swollen in solvents. Since the last three decades, gels, especially water-based hydrogels, have embraced increasing interest for the numerous engineering and biological applications, including bioengineering, [1,2] adhesion, [3,4] soft robotics, [5] and 3D printing. [6] Ordinarily, a polymer network swells in a good solvent having a high affinity to polymer, forming a gel. Whe