Water Vapor Adsorption Behavior of Thermosensitive Polymers for Desiccant Humidity Control Systems

Abstract

Desiccant humidity control systems have been garnering considerable attention in the attempt to achieve highly efficient utilization of low-temperature heat exhausted from various industries at temperatures less than 373 K. We have focused on thermosensitive polymers as new desiccants because a large amount of dehumidified water would be expected in the system because of their thermosensitivity. Our previous study focused on the water adsorption behavior of poly(N-isopropylacrylamide) (poly(NIPA)), which has a low critical solution temperature (LCST) of 306 K. In this study, poly(N-isopropylmethacrylamide) (poly(NIPMA)) and poly(2-(dimethylamino)ethyl methacrylate) (poly(DMAEMA)) cross-linked with N,N′-methylenebisacrylamide (MBAA) were investigated. These polymers are known to exhibit thermosensitivity in the temperature range of 313-319 K in water, which is a higher LCST than that of poly(NIPA). Poly(NIPMA) adsorbed water vapor linearly with increasing relative humidity. It was also observed that poly(NIPMA) prepared at MBAA concentrations of 200 mol/m 3 exhibited a thermosensitivity in the temperature range of 303-313 K in water vapor adsorption. Meanwhile, poly(DMAEMA) adsorbed little water vapor up to a relative humidity (RH) of 40%; however, it exponentially adsorbed water at RH levels higher than 40%. From the estimation results of effective water adsorptivity, we found that poly(DMAEMA) is applicable in desiccant humidity control systems when the dehumidification process is performed at high RH.

Engineered Polymer Architectures for Thermo‐Responsive Desiccants in Dehumidification Applications

“…There has been a recent interest in using thermo-responsive polymers for removing water from the air. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Previous modeling work by Zeng et al [20] and Kocher et al [25] shows that the energy efficiency of desiccant-based dehumidification can be improved using thermo-responsive (TR) desiccants, which have a switchable hydrophilicity-to-hydrophobicity behavior that results in a temperature-dependent adsorption isotherm. [26][27][28][29] This hydrophilic-to-hydrophobic transition can be realized by TR polymers with a lower critical solution temperature (LCST)-these polymers are relatively hydrophilic/hydrophobic when the temperature is below/above the LCST, respectively.…”

“…[20] Experimental TR desiccant studies have primarily targeted atmospheric water harvesting applications. [10][11][12][13][14][15][16][17][18][19] These materials have been hypothesized to become a disruptive technology if water can be captured in the vapor form and then released as a liquid (which has some synthetic challenges discussed in Section 4.3). [30] However, these works focused on 1) the IPN polymer architecture at 2) specific RH values and temperatures, rather than exploring different polymer architectures and their performance over the full range of RHs and temperatures [10][11][12][13][14][15][16][17][18][19] needed in dehumidification applications (Section S1, Supporting Information).…”

“…[10][11][12][13][14][15][16][17][18][19] These materials have been hypothesized to become a disruptive technology if water can be captured in the vapor form and then released as a liquid (which has some synthetic challenges discussed in Section 4.3). [30] However, these works focused on 1) the IPN polymer architecture at 2) specific RH values and temperatures, rather than exploring different polymer architectures and their performance over the full range of RHs and temperatures [10][11][12][13][14][15][16][17][18][19] needed in dehumidification applications (Section S1, Supporting Information). Additionally, our simulations only consider polymers that both sorb and desorb water in the vapor phase (in contrast to sorbing as a vapor and desorbing as a liquid), as the only polymers that exhibited liquid water release showed poor cyclability and liquid water release is not necessary for improved efficiency.…”

“…Thus, the PNI-PAAm does not have enough chain mobility nor unbound water molecules to undergo an LCST-like response, thereby hindering Δu mean and the overall performance of the polymer. [16,18] To achieve control over both Δu mean and the LCST set point, all TR desiccant materials besides the homopolymer control sample discussed consist of a hygroscopic component, which promotes the moisture adsorption capacity to increase chain mobility, and a TR component, which enables the temperaturedependent isotherm shift. The specific materials used are detailed in Figure 5a.…”

“…This broadly agrees with literature results. [17,18,39] A simple random copolymer is not an effective polymer architecture for a TR desiccant. A small mass fraction of hygroscopic units in the copolymer with the TR unit drastically affects the LCST, thereby affecting Δu mean and the LCST set point.…”