The Hydrogel Reimagined: Gel-Derived Sponges and Sheets as Absorbents for Water, Blood, and Oil
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Polymer hydrogels, i.e., crosslinked networks of polymer chains swollen in water, are well-studied materials. Superabsorbent polymer (SAP) gels that can absorb more than 100x their dry weight in water are widely used in personal hygiene products – but only in the form of microscale beads. If dry SAP gels were larger, they would either take too long to swell or would be brittle solids. This dissertation seeks to reimagine polymer gels in very different physical forms: as soft sponges or foldable, fabric-like sheets. We want these macroscale dry materials to retain the ability to absorb large amounts of liquid, either aqueous or organic. In short, we would like to make polymer gels in convenient, usable forms similar to everyday absorbents like towels and sponges.
The key to making gels as macroscale absorbents is to make the gels porous. In our first study, we devised a way to create porous gels by foam-templating. The approach involves in situ foaming of a monomer solution followed by fast polymerization. We generate the foams using a double-barrelled syringe that has acid and base in its two barrels. Gas (CO2) is formed at the mixing tip of the syringe by the acid-base reaction, and gas bubbles are stabilized by an amphiphilic polymer in one of the barrels. The monomers are then polymerized by ultraviolet (UV) light to form the gel around the bubbles, and the material is dried under ambient conditions to give a porous solid. We show that this dry, porous gel absorbs water at a rate of 20g/s until equilibrium is reached at ~ 300x of its weight. This is the fastest swelling and expansion ever achieved by a hydrogel. We convert the chemical potential energy from gel expansion into mechanical work: the gel is able to lift weights against gravity, with a power-density of 260 mW/kg.
Next, we synthesize porous gels in the form of large sheets that resemble cloth or paper towels. For this, we polymerize thin films of the foams and ambient-dry the films after plasticization. Our gel sheets are flexible, foldable, and can be cut with scissors like fabrics. At the same time, the sheets absorb more than 30x of their dry weight in various aqueous fluids (water, blood, polymer solutions). Remarkably, these gel sheets expand as they absorb water, unlike any commercial towels. The expanded sheets retain absorbed fluid when lifted upright whereas fluid drips out of commercial absorbent sheets. Because of these superior properties, our gel sheets could be used to absorb aqueous liquids in various settings such as homes, labs and hospitals. Lastly, we design oleo-sheets, which are counterparts to the above that can absorb oils, i.e., non-polar liquids. We synthesize oleo-sheets by templating foams in which the continuous phase is non-aqueous and contains hydrophobic monomers. The oleo-sheets are hydrophobic and can selectively absorb oil from water. They show a high absorption capacity (> 50 g/g) for a range of organic solvents. The sheets can also be made magnetically responsive and an oil-soaked oleo-sheet can be lifted up by a magnet. We also fabricate a ‘Janus omni-absorbent sheet’ that has two sides: one side selectively absorbs water while the other side absorbs oil/solvents. Our oleo-sheets and omni-absorbent sheets could both be used in homes, hospitals, and various industries for cleaning up different spilled liquids.