Hygroscopic Polymer Research Articles

Strategies to control humidity sensitivity of azobenzene isomerisation kinetics in polymer thin films

Azobenzenes are versatile photoswitches that garner interest in applications ranging from photobiology to energy storage. Despite their great potential, transforming azobenzene-based discoveries and proof-of-concept demonstrations from the lab to the market is highly challenging. Herein we give an overview of a journey that started from a discovery of hydroxyazobenzene’s humidity sensitive isomerisation kinetics, developed into commercialization efforts of azobenzene-containing thin film sensors for optical monitoring of the relative humidity of air, and arrives to the present work aiming for better design of such sensors by understanding the different factors affecting the humidity sensitivity. Our concept is based on thermal isomerisation kinetics of tautomerizable azobenzenes in polymer matrices which, using pre-defined calibration curves, can be converted to relative humidity at known temperature. We present a small library of tautomerizable azobenzenes exhibiting humidity sensitive isomerisation kinetics in hygroscopic polymer films. We also investigate how water absorption properties of the polymer used, and the isomerisation kinetics are linked and how the azobenzene content in the thin film affects both properties. Based on our findings we propose simple strategies for further development of azobenzene-based optical humidity sensors.

Biomimetic bilayer phase change hydrogels with enhanced water adsorption and desorption for evaporative cooling

Hygroscopic hydrogels, reliable carriers for water-based evaporative materials, undergo shrinkage after evaporative cooling, and the consequent reduction in hygroscopic sites can significantly impair their recyclability in cooling period. To realize cyclic evaporative cooling for rapid regeneration and evaporation, bilayer phase change hydrogel (BPH) concept combining an upper layer of hygroscopic polymer film (CaCl2@ carboxymethyl cellulose/konjac glucomannan films, CaCl2@CMC/KGM, MAF) with a lower layer of hygroscopic hydrogel (CaCl2@poly(vinyl alcohol)/sodium tetraborate decahydrate hydrogel, CaCl2@PVA/Borax, PCH), is presented in this paper. The high liquid–vapor phase change enthalpy of BPH (1274 J/g) enables it to continuously and rapidly remove heat. Benefiting from the MAF hygroscopic porous skeleton, the saturated BPH exhibited a faster water evaporation rate (2.15 kg m−2 h−1) and comparable water vapor transmission rate (1.61 kg m−2 h−1) at 50 °C. After being heated at 50 °C for 1 h, BPH requires 9h to regain its initial mass at 25 °C, 90 % RH. Fifteen moisture adsorption–desorption cycles demonstrate the excellent cyclic regeneration capability of BPH. COMSOL Multiphysics simulation results indicate that evaporative cooling in BPH provides excellent heat dissipation (reduced by about 27 °C). BPH reduces the peak temperatures of a mobile phone by 7.8 °C while charging and 6.5 °C while gaming. This design will contribute to the future cooling of heat-related devices and global sustainability.

Physical properties of industrially produced carbon nanotube yarns for use in structural nanocomposites

Industrially-produced carbon nanotube (CNT) networks are a promising base for high-performance, continuous CNT nanocomposites, motivating widespread use in research and application development. Floating catalyst chemical vapor deposition (FCCVD) is a scalable, widely-adopted approach for synthesis of CNT networks; however, FCCVD can yield networks with organic impurities which complicate their characterization and processing. Herein, we analyze two varieties of FCCVD-synthesized commercial CNT yarn to explore the effects of densification and purification as they relate to forming infiltrated polymer composites. First, the fluid permeabilities and porosities of neat roving and chemically-stretched CNT yarns (CSYs) are estimated and compared to gas adsorption studies, which suggest high-tenacity CSYs are largely impermeable, leading to dry-core composites when polymer infiltration is attempted. Next, the presence and effects of oxygen-rich amorphous carbon impurities in the CSYs are identified and found to exist at the scale of CNT bundles, wherein the amorphous carbon behaves like a hygroscopic polymer matrix in a composite. Removal of the amorphous carbon phase in a 130%-increase in specific surface area, as quantified by BET analysis, and a statistically significant reduction in tensile properties. We discuss challenges in estimating yarn alignment and crystallinity resulting from these factors, and place our results in context of past studies analyzing intrinsic organic coatings in FCCVD-synthesized CNT networks.

Integrating self-hygroscopic hydrogel electrolyte and porous electrode for supercapacitor operating in variable humidity environments

The application of supercapacitors (SCs) based on the conventional polyvinyl alcohol‑potassium hydroxide (PVA-KOH) polymer electrolyte is challenged in variable humidity environments, especially in low relative humidity (RH) environments. Herein, taking advantage of the high ionic conductivity and strong hygroscopicity of highly concentrated KOH solutions, we have prepared a hygroscopic hydrogel polymer electrolyte composed of sodium alginate (SA)/PVA/poly(acrylic acid) (PAA)-KOH. The SA/PVA/PAA-KOH gel electrolyte exhibits high ionic conductivity, superior water retention and even water adsorption capability. A three-dimensional CoCu-layered double hydroxide/zeolitic imidazole framework-67 (CuCo-LDH/ZIF-67) porous electrode has also been prepared for the construction of the high-performance SC operating in variable RH environments. The CuCo-LDH/ZIF-67 porous electrode allows deep penetration of the self-hygroscopic electrolyte, facilitating electron/ion diffusion and providing fast water transport channels. As a result, our asymmetric SC show significantly improved electrochemical performance than that of SC based on conventional PVA-KOH electrolyte under different RH environments. This integration strategy can advance the practical application of SC in extreme environments.

Hygroscopic and pH indicator-based polymer coating for fabrics with responsive discoloration

Humidity-induced discoloration from surface coating is promising, but the surface suffers from low stability, due to the lack of covalent interactions between pH indicator, hygroscopic components and fabrics. Here, we report a strategy to fabricate all-polymer-based, highly hygroscopic fabrics with humidity-induced discoloration and pH-induced discoloration simply by covalently coating/polymerizing a reactive pH indicator and a hygroscopic monomer onto fabrics. The resulting fabrics show remarkable and reversible discoloration from blue to yellow, upon increasing relative humidity. The high reusability of the discoloration has been verified by repeating moisture adsorption and desorption over 20 cycles, without obvious deterioration in the discoloration behavior. Moreover, the fabrics display discoloration behaviors upon contacting aqueous solutions and vapors at different pHs. The simple coating, significant and reversible color transition, and the high reusability enable the all-polymer-based, hygroscopic fabrics to be excellent candidates for humidity indication and pH indication.

Tailoring the covalent organic frameworks based polymer materials for solar-driven atmospheric water harvesting

Atmospheric water harvesting through reticular materials is an innovation that has the potential to change the world. Here, this study offers a technique for creating a solar-powered hygroscopic polymer material for atmospheric water harvesting with the reticular materials. The results show that the porous hygroscopic polymer materials can achieve high performance with high vapor capture (up to ac. 28.8–49.7 mg/g at 28–38 %RH and 25 ℃), rapid photothermal conversion efficiency (up to 32.2 ℃ within 15 min under 1000 W/m−2 light at 25 ℃), a low desorption temperature (lower than 40 ℃), and an effective water release rate. Besides, the material also has excellent water-retention properties, which can effectively store desorbed liquid water in polymer networks for use by vegetation during water demand periods. The strategy opens new avenues for atmospheric water-harvesting materials, which will hopefully solve the global crisis of freshwater shortages.

Moisture self-regulating ionic skins with ultra-long ambient stability for self-healing energy and sensing systems

Dehydration has been a key limiting factor for the operation of conductive hydrogels in practical application. Here, we report self-healable ionic skins that can self-regulate their internal moisture level by capturing extenral moistures via hygroscopic ion-coordinated polymer backbones through antipolyelectrolyte effect. Results show the ionic skin can maintain its mechanical and electrical functions over 16 months in the ambient environment with high stretchability (fracture stretch ∼2216 %) and conductivity (23.5 mS/cm). The moisture self-regulating capability is further demonstrated by repeated exposures to harsh environments such as 200°C heating, freezing, and vacuum drying with recovered conductivity and stretchability. Their reversible ionic and hydrogen bonds also enable self-healing feature as a sample with the fully cut-through damage can restore its conductivity after 24 h at 40 % relative humidity. Utilizing the ionic skin as a building block, self-healing flexible piezoelecret sensors have been constructed to monitor physiological signals. Together with a facile transfer-printing process, a self-powered sensing system with a self-healable supercapacitor and humidity sensor has been successfully demonstrated. These results illustrate broad-ranging possibilities for the ionic skins in applications such as energy storage, wearable sensors, and human-machine interfaces.

Solar-powered porous polymer materials-enabled rapid atmospheric water harvesting and storage of liquid water

Atmospheric water harvesting (AWH) is emerging as a promising strategy to produce fresh water from abundant airborne moisture to reduce the global clean water shortage. Hydrogels are considered attractive adsorbents due to their higher water retention and tunable polymer-water interactions. Here, we report the super hygroscopic polymer material (SHPMs) using modified NH2-MIL-101(Cr) as a functional crosslinker by copolymerization with acrylic acid and 2-Acrylamide-2-methylpropanesulfonic acid, achieving a water uptake of about 28.8–49.7 mg/g at 28–38 % RH. The material can desorption quickly under sunlight, and the desorbed water can be stored in a polymer skeleton for use by vegetation or crops during periods of water demand. Besides, the Water retention and storage performance and its outdoor experiment also proved that the application of this material in agriculture, forestry and desertification control laid the foundation to improve the effective availability of soil moisture and water harvesting and conservation. With its excellent water retention and atmospheric water harvesting performance, the material offers a sustainable method for rapid water extraction in arid climates.

Hygroscopic all-polymer composite for moisture management and evaporative cooling

Adsorption-based water management and evaporative cooling personal thermal management (PTM) technologies offer great potential to achieve adaptive temperature regulation, wide applicability, and low energy consumption. However, designing high-performance and durable hygroscopic composites that combine efficient heat dissipation with wear comfort is a challenge. More recently, Xu et al. used two hygroscopic polymers and crosslinking strategies to develop moisture-absorbent fabrics with excellent hygroscopicity, durability, ductility, air permeability, washable resistance, and antibacterial properties. This work paved an intriguing PTM application prospect of an all-polymer hygroscopic composite to achieve energy-efficient moisture sorption and evaporative cooling.

Hydrochromic Nanocapsule with Real Time Visual In Situ Water Level Sensing Function and Toughening Effect for Plastic Materials

AbstractDespite the recent developments in hydrochromic materials, visual detection of water level in plastic materials without sacrificing mechanical properties remains a meaningful yet challenging topic. Currently, to optimize the sensitivity, hydrochromic agents are usually incorporated into hygroscopic matrixes to form bulk solids, which have restricted compatibility for the direct application inside other polymer materials. In this study, this challenge is tackled by nanoscale incorporation of hydrochromic agent into a hygroscopic polymer by miniemulsion polymerization. The obtained hydrochromic nanocapsule has an average diameter ≈110 nm, shows reversible hydrochromic property after 10 cycles, and can offer the first directly eye‐readable signal about the real time in situ water level in plastic materials with a water level below 4 wt.%, while achieving a 214% enhancement of the toughness at the same time. Comprehensive researches by investigating the composition and viscoelastic transition of the nanocapsules has shed light on the mechanisms concerning hydrochromic sensitivity and toughening effect. This simple methodology not only offers a new route for the nanoscale incorporation and application of different hydrochromic materials, but also inspires the development of smart nanoparticles with other stimuli‐responsive functionalities. All these aspects should render this work interesting for both scientific researches and engineering practices.

Cement Pastes with Hygroscopic Polymeric Additions for Potential Building Applications

The development of highly predictive analysis for designing cementitious composite with improved thermal and hygroscopic performance for building and construction poses a significant challenge. To investigate new potential applications, cement pastes have been prepared using a cement, sand, and crystallization admixture, with highly hygroscopic polymer additions (SA-PA) of sodium polyacrylate and/or recycled polyamide fibers. The porosity evolution was investigated at different curing stages and after heat treatment at 200 °C, the temperature at which the paste dehydrates quickly without structural changes. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), cyclic shear tests, thermal conductivity, and diffusivity measurements were carried out on the cement pastes to assess their microstructure. The behavior of the cement pastes varied with polymer additions and thermal treatments; ka−0.5 must be maximized in heat storage applications, where a and k are thermal diffusivity and conductivity, respectively. In contrast, the product a0.5k−1 must be maximized in energy-efficient insulation. Cement pastes with SA-PA exhibited the highest values of both 9.191 102 m−2 K−1 s0.5 W and 1.088 10−3 m2 K s−0.5 W−1, respectively. After the thermal treatment at 200 °C, SA-PA samples maintained the highest heat-storing performance of 6.258 102 m−2 K−1 s0.5 W, while the samples with SA-PA and polyamide fibers performed better in energy-efficient insulation, demonstrating performance of 2.552 10−3 m2 K s−0.5 W−1. These results, discussed in terms of pore size distribution, suggest potential applications in the building field and are valuable for designing plaster and concrete for applications such as thermal and hygroscopic control.

Use of solid thermolytic salts to facilitate microwave-induced in situ amorphization

Moisture was frequently used as dielectric heating source in classical microwave-able systems to facilitate microwave-induced in situ amorphization, however such systems may face the potential of drug hydrolysis. In this study, solid thermolytic salts were proposed to function as moisture substitutes and their feasibility and impacts on microwave-induced in situ amorphization were investigated. It was found that NH4HCO3 was a promising solid alkaline salt to facilitate both microwave-induced in situ amorphization and in situ salt formation of acidic indomethacin (IND). Moreover, it could improve the chemical stability of the drug and the dissolution performance of compacts relative to classical moisture-based compacts upon microwaving. Further mechanistic study suggested that the in situ amorphization occurred prior to the in situ salt formation, especially in formulations with low drug loadings and high solid salt mass ratios. For compacts with low polymer ratios, in situ salt formation took place subsequently, where the previously amorphized IND within compacts could interact with the NH3 gas produced in situ by the decomposition of NH4HCO3 and form the ammonium IND salt. Microwaving time showed great impacts on the decomposition of NH4HCO3 and the in situ generation of water and NH3, which indirectly affected the amorphization and salt formation of IND. In comparison to the moisture-based systems, the NH4HCO3-based system showed a number of advantages, including the reduced potential of IND hydrolysis due to the absence of absorbed moisture, a wider category of applicable polymeric carriers other than hygroscopic polymers, and an increase in drug loading up to 50% (w/w).

Rational design and easy fabrication of transparent photothermal/hygroscopic composite coatings with long-lasting antifogging performance under sunlight activation.

Hygroscopic polymers are good candidates for antifogging coatings, but their long-term effectiveness is limited by the equilibrium between water absorption and expansion. As an efficient and environmentally friendly solution, photothermal materials are being introduced into the field of antifogging. However, there is a need for enhancement in the spectral characteristics of most photothermal materials within the visible light region. In addition, photothermal antifogging coatings often exhibit a delay in heating response, which hinders their ability to promptly evaporate condensed water droplets in the absence of illumination or during initial illumination. Here, a bilayer structure design of photothermal nanomaterials/hygroscopic polymers is proposed to achieve long-term antifogging under sunlight activation. Ensuring the rapid absorption of condensed water droplets on the coating surface, while simultaneously achieving efficient photothermal conversion for a swift temperature increase over the entire coating, is key to this approach, which will not only suppress early fogging but also lead to an exponential decrease of the nucleation rate of droplets. During this process, a dynamic equilibrium is gradually established between the condensation and evaporation of fog droplets, leading to long-term antifogging properties. The light transmittance of the composite coatings reaches as high as ca. 75% in the visible light region, making them well suited for a diverse range of transparent substrate and device applications. A clear field of view can be maintained for at least 6 h under 1 sun illumination above 65 °C hot steam. The antifogging/defogging performance is effectively demonstrated even under challenging non-ideal natural conditions, such as low solar irradiation during dusk or when placed indoors behind windows.

Guar Gum-Based Macroporous Hygroscopic Polymer for Efficient Atmospheric Water Harvesting.

Solar-driven atmospheric water harvesting technology has the advantage of not being limited by geography and has great potential in solving the freshwater crisis. Here, we first propose a purely natural and degradable superhydrophilic composite macroporous hygroscopic material by applying guar gum (GG) to atmospheric water harvesting. The material consists of GG-cellulose nanofibers (CNFs) as a porous substrate material, limiting the hygroscopic factor lithium chloride (LiCl) in its three-dimensional (3D) network structure, and carbon nanotubes (CNTs) play a photothermal conversion role. The composite material has a high light absorption rate of more than 95%, and the macroporous structure (20-60 μm) allows for rapid adsorption/desorption kinetics. At 35 °C and 90% relative humidity (RH), the moisture absorption capacity is as high as 1.94 g/g. Under 100 mW/cm2 irradiation, the absorbed water is almost completely desorbed within 3 h, and the water harvesting performance is stable in 10 cycles. Moreover, liquid water was successfully collected in an actual outdoor experiment. This work demonstrates the great potential of biomass materials in the field of atmospheric water collection and provides more opportunities for various energy and sustainable applications in the future.

Self‐Contained Moisture Management and Evaporative Cooling Through 1D to 3D Hygroscopic All‐Polymer Composites

AbstractSorption‐based moisture management and evaporative cooling represent emerging technologies with substantial potential for energy‐saving personal thermal management (PTM). However, design of high‐performance and durable hygroscopic composites combining efficient heat dissipation with wearing comfort presents significant challenges. Herein, hygroscopic 1D nanofibers and 2D fabrics are developed using two hygroscopic polymers and crosslinking strategies. The design endows the fabrics with self‐contained properties including excellent hygroscopicity, durability, ductility, breathability, washability, and antimicrobial capability. The fabrics exhibit thickness‐independent moisture sorption and equilibrium water uptake of 1.19 g g−1 in 4 h under 90% relative humidity (RH). About 80% of the absorbed water can be rapidly released through mild heating within 10 min. These high moisture sorption/desorption rates outperform the majority of composite desiccants, enabling up to five sorption/desorption cycles per day under a real sky. The hygroscopic fabrics can reduce apparent temperatures and prevent unsightly sweat stains on clothing, improving thermal and clothing comfort. Furthermore, hygroscopic 3D hierarchical matrices are printed, showcasing the potential to use small amounts of hygroscopic materials to boost moisture sorption. This work advances the controlled fabrication of hygroscopic polymer composites, ranging from 1D nanofibers and 2D fabrics to 3D matrices, highlighting their promising prospects for future sorption‐based PTM applications.

Nondestructive Analysis On 4D-Printed Hygroscopic Actuators Through Optical Flow-Based Displacement Measurements

The many advantages of additive manufacturing are particularly noticeable in the fabrication of 4D-printed actuators. Through the selection of specific printing properties and materials, hygroscopic wood polymer composites (WPCs) can be produced and their reaction to humidity can be preprogrammed to achieve the greatest deformations in the shortest time. This responsive behavior makes 4D-printed WPCs suitable for architectural applications, where they can act as passive airflow controllers to improve hygrometric conditions in indoor environments. Image analysis methods have been proven to be reliable to select the best combinations of materials and properties for hygroscopic WPCs but, in some cases, they provide only information on the curvature angles and the instrumentation and software can be expensive. This paper presents an optical flow method for tracking the displacements through a free and open-source software. Starting from a time-lapse video of the sample immersed in water, the analysis returns a matrix composed of the 3D displacement values for each pixel in consecutive frames and the velocity of the displacement, with their visual representation. Such image analysis techniques proved to be suitable to assess the different hygro-responsive behavior under water of 4D-printed WPCs through low-cost equipment without altering their configuration.

Solar-powered MXene biopolymer aerogels for sorption-based atmospheric water harvesting

Solar energy-powered sorption-based atmospheric water harvesting (AWH) is an innovative approach to acquiring fresh water in water-stressed areas. Here, a method was provided for combining a polyacrylamide/chitosan skeleton and an MXene solar absorber to produce a solar-powered hygroscopic polymer aerogel for atmospheric water harvesting (AWH). Solution exchange and lyophilization were combined in the preparation method. The experiments showed that the porous aerogels enabled high-performance AWH. At 90% relative humidity and 25 °C, the water sorption capacity reached 5.0 g·g−1. At a light intensity of 1 kW·m−2 for 4 h, almost all of the adsorbed water (>98%) was released due to photothermal conversion. The tensile strength exceeded 1.3 MPa. Additionally, the water collection capacity under unstable outdoor conditions was 0.92 L·kg−1 (under unstable natural light (0.11–1.01 kW ·m−2)). This method offers a straightforward, efficient, and useful solution, even under adverse environmental conditions.

Review on Polymer Materials for Solid Desiccant Cooling System

AbstractAs an alternative form of vapor compression air conditioning devices, solid desiccant cooling (SDC) techniques have increasingly been explored recently. The overall performances of SDC primarily rely on the capability of dehumidification and regeneration of desiccant. A desiccant with a great uptake capability and excellent regeneration potential is preferred in an SDC system. Although traditional desiccants like silica gels and zeolites are able to absorb moisture at moderate levels, hygroscopic polymers show a superior ability in moisture sorption and desorption. Significant research has been conducted to investigate the hygroscopic polymers in SDC for household and industrial applications. Here, first, an introduction to SDC systems is presented, and then hygroscopic polymers from natural and synthetic origins are discussed. Synthetic polymers discussed are metal–organic frameworks (MOFs), covalent organic frameworks (COFs), covalent triazine frameworks (CTFs), amorphous porous organic polymers (POPs), polyelectrolytes, and polymer‐based composites. Their dehumidification behaviors in SDC systems, primarily desiccant‐coated heat exchanger (DCHE) systems, are compared and summarized. Binders employed in SDC systems are also summarized, as a proper binder enhances the overall performance of the desiccant system. It can be anticipated that hygroscopic polymers and binder materials would witness extensive applications in the future.

Metal-Organic Frameworks for Water Harvesting and Concurrent Carbon Capture: A Review for Hygroscopic Materials.

As water scarcity becomes a pending global issue, hygroscopic materials prove a significant solution. Thus, there is a good cause following the structure-performance relationship to review the recent development of hygroscopic materialsand provide inspirational insight intocreative materials. Herein, traditional hygroscopic materials, crystalline frameworks,polymers, and composite materials are reviewed. The similarity in working conditions of water harvesting and carbon capture makes simultaneously addressing water shortages and reduction of greenhouse effects possible. Concurrent water harvesting and carbon capture islikely to become a future challenge. Therefore, an emphasis is laid on metal-organic frameworks (MOFs) for their excellent performance in water and CO2adsorption, and representative role of micro- and mesoporous materials. Herein, the water adsorption mechanisms of MOFs are summarized, followed by a review of MOF's water stability, with a highlight on the emerging machine learning (ML) technique to predict MOF water stability and water uptake. Recent advances in the mechanistic elaboration of moisture's effects on CO2adsorption are reviewed.This review summarizes recent advances in water-harvesting porous materials with special attention on MOFs and expects to direct researchers' attention into the topic of concurrent water harvesting and carbon capture as a future challenge.

An atmospheric water harvester with fast and energy‐saving water removal and recovery

Moisture removal and water recovery from the air are vital for regulating indoor humidity and mitigating water scarcity. Most atmospheric water harvesters (AWH) focus primarily on increasing the moisture capture rate, but for it to be economical and sustainable, it is essential to consider the energy required to recover and harvest the captured water. Here, a mechanically flexible, biphilic sorption-based AWH made of green, environmentally friendly material is presented. It consists of a hygroscopic chitosan polymer embedded within a flexible, hydrophobic silica xerogel that can harvest 86.3 g water/g chitosan at 97% relative humidity and 25°C reaching saturation after 30 days (i.e. 2.88 g water/g chitosan/day). Roughly 88% of the sorbed moisture was recovered by mechanical squeezing (ca. 0.020 MPa) within 150 s. Repeated water harvesting experiments and uniaxial compression tests demonstrate that chitosan-silica xerogel is durable for long-term operations, providing a fast, reliable, and sustainable moisture removal and water harvesting tool.