Pavani Cherukupally, Ph.D., March 2023
About 4 billion people around the world are suffering from water scarcity, which is expected to increase due to increasing global warming [1, 2]. One of the solutions to address the water scarcity challenge is to tap the 13,000 trillion of water present in the atmospheric air in the form of fog, droplets, or vapor [3]. Simple fog collection systems effectively harvest water without investing energy in highly humid environments [4], but they are ineffective in arid regions like Arizona, where the humidity is low. Alternatively, adsorbents designed to capture water vapor during colder nights and release it during the hotter day could provide a passive solution to harvest water vapor in the arid regions of the world [5,6]. Recently, porous polymeric hydrogels have been reported for water droplets harvesting from the air. The gels were fabricated with thermoresponsive polymers, such as polypyrrole chloride penetrated PNIPAM, that can switch to hydrophilic and hydrophobic structures at lower critical solution temperature (LCST) and upper critical solution temperature (UCST), respectively [7,8]. By leveraging their switchable wetting properties, the water droplets were captured during the night at higher humidity and lower temperature conditions and collected during the day at lower humidity and higher temperature conditions. However, the influence of LC/UCST on the hydrogel's chemical and morphological structures has not been investigated, as well as harvesting water vapor present in arid regions. In this work, we report the fabrication of thermoresponsive P(NIPAM-co-BzDMA) copolymeric hydrogel to collect water vapor from the air across all humidity conditions. The hydrogels were fabricated by tuning the temperatures and compositions to achieve large surface area-to-volume ratios, ordered porous structures, and excellent switch between the hydrophilic-hydrophobic wetting properties. The gels synthesized at LCST at BzDMA salt concentration of 15% could uptake 20% higher water than their counterparts. Experimental: The P(NIPAM-co-BzDMA) gels were synthesized by thermally initiated polymerization at LCST or UCST to determine the influence of their switchable hydrophilic-hydrophobic structures on the efficacy of crosslinking [9]. Then the PNIPAM was copolymerized with BzDMA salt at three concentrations of 10%, 15%, and 20% by weight. The synthesis was carried out for 4 hours under nitrogen and then freeze-dried for 12 hours in vacuum environments. The resulting porous gels were named based on their salt concentrations as P(NIPAM-co-10%BzDMA), P(NIPAM-co-15%BzDMA), and P(NIPAM-co-20%BzDMA). The surface functional groups of the hydrogels were determined using Fourier Transform Infrared Spectroscopy (FT-IR, Bruker, Germany). The morphological and crosslinking structure of the gels were evaluated using scanning electron microscopy (SEM, Hitachi Instrument, Japan). The thermoresponsive phase change behavior of the materials was verified using a Differential Scanning Calorimetry (DSC) at a heating rate of 2 C/min from 10 to 60C under a nitrogen environment (DSC, TA Instrument, USA). The switchable wetting properties of the gels were examined through water contact angles (WCA) measured at 20 and 40C. The water vapor adsorption-desorption isotherms were measured using an intelligent gravimetric analyzer (IGA, Hiden Isochema Ltd., UK). Results & Discussion: The successful synthesis of PNIPAM at LCST and UCST was confirmed from the FT-IR spectra. It showed two intense peaks at 1677 and 1563 1/cm, corresponding to C=O and N-H or C-N, respectively. The spectra also showed bimodal peaks at 1390 and 1379 1/cm from the isopropyl, peaks at 2987 and 2942 1/cm from methylene, and a broad peak at 3310 1/cm from N-H stretching [10]. Relative to LCST, the functional groups showed more intense peaks for PNIAM @UCST. It could be because the large number of oxygen radicals generated at higher temperatures could increase crosslinking of PNIPAM. The SEM images of the P(NIPAM-co-BzDMA) hydrogels showed interconnected microporous structures caused by freeze-drying [11]. Compared with PNIPAM@UCST, the PNIPAM@LCST had orderly distributed pores. The BET (Brunauer, Emmett, and Teller) surface area of the PNIPAM@UCST and PNIPAM@LCST were 2.91 mˆ2gˆ-1 and 2.67 mˆ2gˆ-1, respectively. At LCST, the slow copolymerization reaction rate and the homogeneous hydrophilic-hydrophilic structures of copolymers created orderly-distributed, uniform pores across the hydrogels. In contrast, at UCST, the fast copolymerization reaction rate and heterogeneous hydrophilic-hydrophobic networks of copolymers produced randomly-distributed, nonuniform pores. The BzDMA could react and maintain a stable morphological structure with both gels. The DSC thermograms confirmed that both PNIPAM and P(NIPAM-co-BzDMA) could change phase at approximately 32 C. Next, the WCAs measurements showed that all gels exhibit superhydrophilicity at 20C and hydrophobicity at 40 C, as intended. The adsorption-desorption isotherms of hydrogels showed an S shape curve suggesting water vapor can be harvested without losses. However, below LCST, due to superhydrophilicity of PNIPAM and P(NIPAM-co-BzDMA), the water molecules have a higher affinity to bind with the adsorbent, swell and cause a larger hysteresis loop and higher water uptake capacity. The water uptake capacity was found to be 20% highest for P(NIPAM-co-BzDMA) at 15% salt concentration than its counterparts. Conclusions: Our results suggest the influence of synthesis temperature-dependent chemical structures on the overall water vapor uptake capacities and collections was negligible. However, the gels synthesized below LCST and 15% salt concentrations slowed the copolymerization reaction rate, created uniform morphology, and delivered a higher water uptake capacity. Based on these process parameters and compositions, scalable hydrogel-based adsorbents can be designed for large-scale water vapor harvesting across all climate conditions, especially in highly-needed arid regions. References: 1. G. Meran, M. Siehlow, C. von Hirschhausen, The Economics of Water: Rules and Institutions. Springer Nature (2021). 2. M. M. Mekonnen, et al. Sci. Adv 2 (2), e1500323 (2016). 3. H. Kim, et al. Science 356 (6336), 430-434 (2017). 4. J. Ju, et al. Nat. Comm. 3 (1), 1-6 (2012). 5. N. Hanikel, et al. Nat Nanotechnol 15 (5), 348-355 (2020). 6. P.A. Kallenberger, et al. Comm. Chemistry 1(28), 1-6 (2018). 7. 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