Abstract
The water content of thermo-responsive hydrogels can be drastically altered by small changes in temperature because their polymer chains change from hydrophilic to hydrophobic above their low critical solution temperature (LCST). In general, such smart hydrogels have been utilized in aqueous solutions or in their wet state, and no attempt has been made to determine the phase-transition behavior of the gels in their dried states. Here we demonstrate an application of the thermo-responsive behavior of an interpenetrating polymer network (IPN) gel comprising thermo-responsive poly(N-isopropylacrylamide) and hydrophilic sodium alginate networks in their dried states. The dried IPN gel absorbs considerable moisture from air at temperatures below its LCST and oozes the absorbed moisture as liquid water above its LCST. These phenomena provide energy exchange systems in which moisture from air can be condensed to liquid water using the controllable hydrophilic/hydrophobic properties of thermo-responsive gels with a small temperature change.
Highlights
The water content of thermo-responsive hydrogels can be drastically altered by small changes in temperature because their polymer chains change from hydrophilic to hydrophobic above their low critical solution temperature (LCST)
From the discussions about moisture absorption using Langmuir and BET type models and about the activation energy for water release above the LCST, we can explain the mechanism for the absorption of moisture into the dried interpenetrating polymer network (IPN) gel and oozing it as liquid water as follows
During the moisture absorption into the IPN gel below the LCST, water molecules are adsorbed on both Alg and PNPAAm chains to the moisture absorption into general hydrophilic polymers[21–23], followed by a slight swelling of the IPN gel
Summary
The water content of thermo-responsive hydrogels can be drastically altered by small changes in temperature because their polymer chains change from hydrophilic to hydrophobic above their low critical solution temperature (LCST). Some hydrogels have a unique property that they undergo abrupt changes in their volume in response to environmental changes, such as pH3, temperature[4,5], electric field[6], light[7], and biomolecules[8,9] Such stimuli-responsive hydrogels have many potential applications as smart and soft materials in aqueous solutions or in their wet state. The phase transition of water from the gas phase to the liquid phase was achieved by moisture absorption and water oozing through a drastic change in the hydrophilicity/hydrophobicity of the thermo-responsive gels by a small temperature change This unique property demonstrates the possibility of applying thermo-responsive gels as high energy efficiency materials for condensing gaseous water to liquid water. By the simple change of the starting point, we can achieve the collection of moisture (gaseous water) as liquid water using thermoresponsive gels, followed by their innovative applications in their dried state
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