Hydrogel adsorbs more water at elevated temperature

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. Occasionally we all come across results that, at first glance, just don’t make sense. We question the methodology and equipment setup. We probe deeply and sometimes we find something unexpected. This is exactly what a team of researchers at MIT and Michigan State University encountered.

XINYUE LIU: Did we expect this? No! It kind of breaks our common sense.

LAURA LEAY: Post-doctoral researcher Xinyue Liu from MIT was involved in the surprising discovery that a hydrogel can adsorb more water at elevated temperature. This is counterintuitive: thermodynamics suggests that sorption is more favorable at a low temperature. Liu and the team were searching for a way of harvesting water from the air without using a lot of energy. They want to tackle the problem of water scarcity and find a way of generating water sustainably. To do so, they tested many different sorbents. Most sorbents, such as zeolite and silica gel have a structure that doesn’t change much when it has adsorbed water but the polyethylene glycol – or PEG – hydrogel that the team synthesized is different. While it’s semi-crystalline at 25°C, it becomes amorphous at 50°C. This structural change means that more adsorption sites are available at the higher temperature. As water is absorbed, it caused the hydrogel to swell, opening up further adsorption sites. The PEG hydrogel monomers are star-shaped, forming a network where the molecular weight can be precisely controlled. The shape of the monomer leads to very homogeneous structures, facilitating crystallization. 

XINYUE LIU: The star-shape has very uniform molecular weight in each arm, and this kind of uniformity leads to higher crystallinity in the hydrogels.

LAURA LEAY: The PEG hydrogel exhibited a water uptake of 0.050 grams per gram of polymer at 50°C and 50% relative humidity, with half this water uptake at 25°C and the same humidity. The team developed a thermodynamic theory to understand the fundamentals of the unusual water sorption behavior. The theory closely matched the experimental results, which means that it can be used to guide the continued development of materials that exhibit this surprising behavior. There is more work to do for this unusual phenomenon to find a real world application. The team is aiming for a device that can passively produce five liters of drinking water in arid conditions found in the desert. For Xinyue, this real-world application is highly motivating.

XINYUE LIU: We want to translate this kind of knowledge to solve a real-world challenge. The most surprising thing is how to connect – from my background – I’m a materials scientist and the DRI, the Device Research Laboratory, cares more about the real world application. When we think about this, it really motivates us or excites us to pursue the fundamental knowledge.  

LAURA LEAY: Another application lies in cooling systems. Just a small input of energy would cause the PEG hydrogel to cool, releasing water which would evaporate and take heat energy with it, leading to further cooling. This interesting polymer could also be used for energy storage, humidity regulation, and more. This work was published in a recent issue of Advanced Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.