Flame-Retardant and Form-Stable Phase-Change Composites Based on Phytic Acid/ZnO-Decorated Surface-Carbonized Delignified Wood with Superior Solar-Thermal Conversion Efficiency and Improved Thermal Conductivity.

In order to efficiently exploit solar-thermal energy, it is essential to develop form-stable phase-change material (PCM) composites simultaneously with superior solar-thermal storage efficiency, excellent flame retardancy, and improved thermal conductivity. Herein, phytic acid (PA)-modified, zinc oxide-deposited, and surface-carbonized delignified woods (PZCDWs) were constructed by alkaline boiling, PA modification, ZnO deposition, and surface carbonization. Then, novel form-stable PCMs (PZPCMs) with superior solar-thermal storage efficiency, excellent flame retardancy, and improved thermal conductivity were fabricated by impregnating n-docosane into PZCDWs under vacuum. The PZCDW aerogels can well support the n-docosane and overcome liquid leakage owing to their superior surface tension and strong capillary force. Differential scanning calorimetry results showed that PZPCMs possessed superior n-docosane encapsulation yield and high phase-change enthalpy (185.2-213.1 J/g). Decorating delignified wood by surface carbonization and ZnO deposition significantly improved the solar-thermal conversion efficiency (up to 86.2%) and thermal conductivity (193.3% increased) of PCM composites. Furthermore, with the introduction of PA into PZPCMs, the peak heat release rate and total heat release of the PCM composites decreased considerably, indicating the enhanced flame retardancy of PZPCMs. In conclusion, the novel renewable wood-based PCM composites demonstrate promising potential in solar energy harnessing and thermal modulation technologies.

Dopamine-Decorated Ti3C2Tx MXene/Cellulose Nanofiber Aerogels Supported Form-Stable Phase Change Composites with Superior Solar-Thermal Conversion Efficiency and Extremely High Thermal Storage Density.

The exploitation of from-stable phase change materials (PCMs) with superior energy storage capacity and excellent solar-thermal conversion performance is crucial for the efficient exploitation of solar energy. Herein, 2D-layered polymerized dopamine-decorated Ti3C2Tx MXene nanosheets (P-MXene) with superior photothermal effects and excellent oxidation stability were synthesized from Ti3AlC2 particles by the selective etching and self-polymerization of dopamine. Then, novel biomass-derived PCM composites, eMPCMs, were fabricated by impregnating erythritol into P-MXene/cellulose nanofiber (CNF) hybrid aerogels. The porous and interconnected 3D aerogels adequately support erythritol and resist liquid leakage during thermal storage. Differential scanning calorimetry (DSC) results showed that the eMPCMs based on P-MXene/CNF aerogels exhibited an extremely high thermal storage density (325.4-330.6 J/g) and excellent PCM loading capacity (up to 1929%). The introduction of P-MXene nanosheets into eMPCMs significantly increased the solar-thermal conversion and storage efficiency, solar-thermal-electricity conversion capacity, and thermal conductivity of the synthesized PCM composites. Moreover, the P-MXene/CNF hybrid aerogel-based PCM composites possessed excellent long-term thermal reliability and thermostability. Hence, the synthesized eMPCMs reveal tremendous potential for efficient solar-thermal storage fields.

Highly transparent, mechanical, and self-adhesive zwitterionic conductive hydrogels with polyurethane as a cross-linker for wireless strain sensors.

Zwitterionic hydrogels have attracted a myriad of research interests for their excellent flexibility and biocompatibility as flexible wearable sensors. It is desired to create E-skins that integrate high mechanical strength, sensory sensitivity, and broad adhesion, possessing potential in the fields of intelligent robots and bionic prostheses. In this work, a novel macromolecular cross-linker (MPU) based on waterborne polyurethane (WPU) was designed and applied to synthesize multifunctional conductive hydrogels (PASU-Zn hydrogels). Importantly, in the presence of MPU, the hydrogels exhibited well-balanced mechanical properties (elongation at break 1193%, tensile strength 1.02 MPa, outstanding puncture resistance, and self-recovery abilities). When assembled as wireless strain sensors, PASU-Zn sensors displayed distinguished sensing characteristics to detect mechanotransduction signals of human movements in real-time. Specifically, owing to the dipole-dipole interaction and hydrogen bonding of zwitterions and MPU, the hydrogels have remarkable self-adhesion properties to various surfaces of wood, PDMS, and pigskin, allowing them to stick to skins by themselves without using any adhesive tapes when used. It is deemed that the as-designed zwitterionic hydrogels show great promise for wearable devices and bionic skins.

Skin-conformal MXene-doped wearable sensors with self-adhesive, dual-mode sensing, and high sensitivity for human motions and wireless monitoring.

Flexible sensors have attracted extensive attention due to their excellent flexibility, biocompatibility, and information acquisition accuracy. Therefore, it is desired to fabricate a flexible sensor with high toughness and sensitivity based on conductive hydrogels to monitor human movement. In this work, MXene-(Ti3C2Tx-)WPU/PAM dual-network hydrogels (PPM hydrogels) were successfully prepared. As the first network, waterborne polyurethane (WPU) plays the role of energy dissipation and enhancement. Polyacrylamide (PAM) and WPU polymer chains form interpenetrating networks (IPNs). MXene acts as a conductive material to enhance the conductivity and for nano enhancement. The PPM hydrogels exhibited excellent mechanical characteristics (tensile ratio >600%, tensile strength 639 kPa, 1000 stretching cycles, and self-recovery rate 93.7%). Moreover, based on these hydrogels, we fabricated flexible sensors. These sensors had high sensitivity and sensing durability, and could be assembled into a human body wireless monitoring device, which possesses great potential in facial micro-expression monitoring, all-around human motion detection, and wearable electronic products. In addition, these resulting hydrogels possessed outstanding reversible adhesion to various materials (human skin, wood, PDMS, etc.) and the maximum adhesion strength can reach 305.1 N m-1 when exposed to a PDMS substrate. Therefore, PPM hydrogels could provide new inspiration for the development of wearable flexible sensors in the domain of human movements and personalized physiological health monitoring.

Recyclable, Self-Healing, and Flame-Retardant Solid-Solid Phase Change Materials Based on Thermally Reversible Cross-Links for Sustainable Thermal Energy Storage.

Conventional polymeric phase change materials (PCMs) exhibit good shape stability, large energy storage density, and satisfactory chemical stability, but they cannot be recycled and self-healed due to their permanent cross-linking structure. Additionally, the high flammability of organic PCMs seriously restricts their applications for thermal energy storage (TES). Therefore, it is urgently required to explore PCM composites exhibiting superior recyclability, good self-healing capability, and excellent flame retardancy simultaneously. Herein, tri-maleimide end-capped cyclotriphosphazene flame retardant (TMCTP) was synthesized via the nucleophilic substitution between 1,3,5,2,4,6-triazatriphosphorine-2,2,4,4,6,6-hexachloride and N-(2-hydroxyethyl)maleimide. Then, novel dynamically cross-linked PCM composites (FPCMs) with superior recyclability, good self-healing capability, and excellent flame retardancy were fabricated by bonding PEG and TMCTP to polymeric skeleton via reversible furan/maleimide Diels-Alder (DA) reaction. TMCTP, which covalently and dynamically binding in the polymeric FPCMs, acted not only as an efficient flame retardant for reducing the flammability of PCM composites but also as dynamic cross-linking skeletons for thermally induced self-healing and recycling. Differential scanning calorimetry (DSC) analysis confirmed the reversible energy storage and release ability of FPCMs. Due to its reversible DA covalent bonds, the introduction of TMCTP endowed the FPCMs with considerably increased self-healing efficiency (up to 93.1%) and recyclability efficiency (94.6%). Moreover, with the introduction of TMCTP into FPCMs, the heat release rate (HRR) and total heat release (THR) significantly decreased, while the char residue and limiting oxygen index (LOI) value increased, confirming that the flame retardancy of FPCMs greatly improved. Hence, the synthesized FPCMs show enormous potential in TES applications.

Highly Stretchable PU Ionogels with Self-Healing Capability for a Flexible Thermoelectric Generator.

With the development of thermoelectric (TE) generator, the flexible, stretchable, self-healable, and wearable TE devices have aroused great interest. Therefore, we designed a self-healable and stretchable polyurethane (PU) ionogel, composed of polyurethane main chains with double bonds in the side, cross-linkers (BDB) and nonconjugated ionic liquids (EMIM:DCA). The PU ionogels with 30 wt % ILs have a high mechanical stretchability (300%), good tensile strength (1.61 MPa), and suitable Young's modulus (0.79 MPa). The proposed materials also exhibited an excellent ionic figure of merit (ZTi) of 0.99 ± 0.3, as well as rapid self-healability in the absence of any external stimuli. The thermoelectric capability of PU ionogels kept stable under the severe condition (50% strain) and during self-healing process, which is rarely reported in recent studies. Furthermore, a stretchable and self-healable ionic thermoelectric capacitor device is also fabricated by the PU ionogels, which can efficiently convert heat into electricity.

Fabrication of MXene/PEI functionalized sodium alginate aerogel and its excellent adsorption behavior for Cr(VI) and Congo Red from aqueous solution.

MXene/PEI modified sodium alginate aerogel (MPA) was facilely prepared by introducing polyethylenimine (PEI) and amino functionalized Ti3C2Tx into sodium alginate (SA) aerogel matrix through cross-linking reactions. Abundant active groups of PEI coupled with in situ reduction ability of MXene dramatically promoted the removal of Cr(VI), realizing the adsorption capacity of 538.97 mg/g. MPA also possessed an ultrahigh adsorption capacity (3568 mg/g) towards Congo Red (CR), ascribing to the strong electrostatic attraction and the synergetic effect of surface adsorption and intercalation adsorption. The Cr(VI) and CR adsorption mechanisms by MPA were further validated using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Batch adsorption experiments were conducted to investigate pH impacts, kinetics, isotherms and thermodynamics. The results demonstrated that adsorption processes of both Cr(VI) and CR fitted felicitously with the Langmuir isotherm and the pseudo-second-order kinetic model. More importantly, having a double-network structure constructed by polymeric SA and PEI, the mechanical strength of the aerogel was significantly reinforced, which was easily recycled without secondary pollution and the capacity decreased few after five cycles. Furthermore, provided with outstanding antibacterial properties against S. aureus and E. coli, MPA can be extensively applied for the water treatment as a both highly efficient adsorbent and antimicrobial.

Skin-inspired nanofibrillated cellulose-reinforced hydrogels with high mechanical strength, long-term antibacterial, and self-recovery ability for wearable strain/pressure sensors.

The advent of electric skins (E-skin) with tactile sensation, flexibility, and human affinity characteristics have attracted considerable attention in extensive research fields, including intelligent robots and health monitoring, etc. To improve the intrinsic brittleness of hydrogels, a multifunctional E-skin was fabricated involving a TEMPO-NFC and a covalently cross-linked polyacrylamide (PAM) network. In this work, silver nanoparticles (AgNPs) as long-term antibacterial agent and conductive fillers were coated onto NFC nanofibers. Subsequently, this nanocomposite hydrogel was synthesized by free radical copolymerization of AM monomers with PNAg fibers as interpenetrating fibers network. Importantly with NFC present, the nanocomposite hydrogel exhibited superior mechanical performance and excellent self-recovery ability. The obtained sensor with excellent mechanical stability and sensing performance could detect mechanotransduction signal of human movements. This work provides a practicable method to prepare high antibacterial efficiency, excellent mechanical performance, and dual-modal nanocellulose-based hydrogel sensor for the broad-range application in human-motion detection and intelligence skins.

Form-stable phase change composites based on nanofibrillated cellulose/polydopamine hybrid aerogels with extremely high energy storage density and improved photothermal conversion efficiency.

The development of form-stable phase change materials (PCMs) with superior photothermal conversion efficiency and high phase change enthalpy is critical for the utilization of solar energy. In this work, nanofibrillated cellulose (NFC)/polydopamine (PDA) hybrid aerogels (NPAs) were synthesized by cation-induced gelation of NFC/PDA suspension. Then, novel form-stable PCMs with superior energy storage density and improved photothermal conversion efficiency were successfully synthesized by impregnating n-octacosane into NPAs. Differential scanning calorimetry (DSC) analysis showed that the composite PCMs exhibited extremely high phase transition enthalpy (>248 J g-1) and excellent thermal reliability. Thermogravimetric analysis (TG) showed that the composite PCMs exhibited excellent thermal stability. In photothermal experiments, PDA acted as a photon trap and effectively improved the photothermal conversion efficiency (up to 86.7%) of the composite PCMs. In conclusion, the synthesized composite PCMs displayed high phase change enthalpy and superior photothermal conversion efficiency, suggesting their promising characteristics for solar energy utilization applications.

Preparation of fluorine-free superhydrophobic and wear-resistant cotton fabric with a UV curing reaction for self-cleaning and oil/water separation.

A durable superhydrophobic, self-cleaning cotton fabric prepared with UV curing was prepared by a simple method and used for oil/water separation. Firstly, sulfhydryl silica nanoparticles on the fabric surface were prepared by the Stöber reaction (SiO2-SH@cotton). Then, the side chain hydroxyl terminated PDMS was reacted with isocyanate to form an isocyanate terminated prepolymer. The prepolymer terminated by HEMA (vinyl-terminated PDMS (PIH)) was sprayed on the fabric surface, and then the superhydrophobic coating (SiO2-S-PIH@cotton) was formed using UV curing. A series of characterization methods were used to demonstrate the properties of the modified cotton fabric. When the weight gain after PIH spraying was 1.8 wt%, the fabric reaches an optimal state (water contact angle (WCA) of 153° and a sliding angle of 7°). When used in an oil-water separation test, the highest separation efficiency reached 99.1%. In particular, the as-prepared fabric has excellent wear resistance. Compared with that before spraying, the superhydrophobicity of the as-prepared fabric has no obvious decrease after 300 cycles under 200 g of weight or after 100 cycles under 500 g of circular friction. This indicated that surface sprayed polymers have two functions: providing low surface tension and protecting the rough surface formed by silica particles. This process was time-saving, energy-saving, protected the environment, had a low material cost and a strong performance stability. It is hoped that this fabric can be used in the large-scale industrialization of oil-water separation.

Dual Triple Helix-Aptamer Probes for Mix-and-Read Detecting Antibiotics in Fish and Milk.

Antibiotic abuse in agricultural products leads to serious food safety issues. To this end, we proposed a mix-and-read and enzyme-free amplified assay for antibiotics based on a dual triple helix-aptamer probe, potentially applicable for on-site monitoring of antibiotic residues. A dual triple helix-aptamer probe can leverage the response toward target molecules without enzyme-based amplification, rendering it sensitive and robust for profiling target molecules. The proposed assay allowed mix-and-read detection of chloramphenicol with a detection limit of 0.18 nM. Besides, it accommodated for specifically resolving chloramphenicol among other antibiotics. Chloramphenicol residual in aquatic products in fish and milk can be precisely determined. Thus, the aptamer probe deems to enrich the toolbox for managing antibiotic use.

Alkylated Nanofibrillated Cellulose/Carbon Nanotubes Aerogels Supported Form-Stable Phase Change Composites with Improved n-Alkanes Loading Capacity and Thermal Conductivity.

The exploitation of phase change materials (PCMs) with excellent shape stability, considerable latent heat storage capacity, and superior thermal conductivity is essential for their applications in heat storage and thermal regulation. Here, form-stable composite PCMs based on n-octacosane, nanofibrillated cellulose (NFC), and carbon nanotubes (CNTs) were successfully obtained by impregnating n-octacosane into the alkylated NFC/CNTs hybrid aerogels. The three-dimensional interconnected porous aerogels could adequately support the melted n-octacosane and prevent the leakage problem due to strong capillary force and surface tension. After treatment with alkylated modification, the affinity between NFC/CNTs aerogels and n-alkanes was significantly improved, resulting in excellent shape stability, improved thermal reliability, and high n-alkanes loading capacity for the as-prepared composite PCMs. The differential scanning calorimetry analysis showed that composite PCMs based on the alkylated NFC/CNTs aerogels exhibited an extremely high phase change enthalpy ranging from 250.9 to 252.9 J/g. Furthermore, the thermal conductivity and photothermal conversion and storage efficiency of the synthesized PCMs were effectively enhanced by the introduction of CNTs. Thus, the synthesized composite PCMs exhibit considerable potential for practical application in heat storage and thermal regulation.

Improving the flame retardancy of waterborne polyurethanes based on the synergistic effect of P-N flame retardants and a Schiff base.

A novel reactive intumescent fire retardant hexa-[4-[(2-hydroxy-ethylimino)-methyl]-phenoxyl]-cyclotriphosphazene (HEPCP), containing both cyclotriphosphazene and Schiff base structures, is successfully prepared. The chemical structures of HEPCP and flame-retardant waterborne polyurethane (WPU) (FR-WPU) were characterized via 31P, 1H NMR and FT-IR. Thermogravimetric (TG) analysis showed that HEPCP exhibited excellent thermal stability and produced rich char residue under high temperature compared with the control sample. The Schiff base and cyclotriphosphazene had a synergistic effect on the WPU. Limiting oxygen index (LOI) values of up to 26.7% were recorded; the dripping behavior was simultaneously improved and achieved a V-1 rating in the UL-94 test by incorporating 0.5 wt% phosphorus. In contrast to the pure WPU, the peak heat release rate (pHRR) of the FR-WPU/HEPCP5 decreased by 43.8%. The char residues increased from 0.63% to 6.96%, and scanning electron microscopy (SEM) showed a relatively continuous and membranous substance, with few holes. The results of TGA-FIR, Py-GC/MS and SEM indicated that HEPCP displayed a fire-retardant mechanism in the condensed-phase. In addition, the thermomechanical behaviors and the mechanical properties indicated that both mechanical properties and T gh increased.

Synergetic enhancement of mechanical and fire-resistance performance of waterborne polyurethane by introducing two kinds of phosphorus-nitrogen flame retardant.

In this work, a novel hydroxyl-terminated monomer containing phosphorus and nitrogen, tri(N, N-bis-(2-hydroxy-ethyl) acyloxoethyl) phosphate (TNAP), was synthesized successfully with phosphorus oxychloride, hydroxyethyl acrylate, and diethanolamine as raw materials, and then incorporated into flame-retarded waterborne polyurethanes to improve their flame retardancy, thermal behavior, and mechanical properties. Their structures were confirmed by nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FTIR). Besides, the thermal performance and combustion behaviors of crosslinked flame-retarded waterborne polyurethane (CFRWPU) films were evaluated through thermogravimetric analysis (TGA), thermogravimetry-FTIR, limiting oxygen index (LOI) tests, and microscale combustion calorimeter tests (MCC). Additionally, the mechanical properties were investigated by tensile stress-strain tests. These results revealed that the monomer TNAP exhibited remarkable residual char formation ability of 34.98 wt% and that TNAP-embedded FRWPU films attained an LOI value of 25.5% at a TNAP content of 4 wt%. Moreover, there was significant enhancement in tensile strength (15.81 MPa) obtained with the combined incorporation of TNAP into FRWPU. Also, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) provided the morphologies and the element distributions of char residues of CFRWPU after LOI tests. Finally, the thermal mechanical properties of CFRWPU were assessed by dynamic mechanical analysis (DMA).

Synthesis of a novel flame retardant based on DOPO derivatives and its application in waterborne polyurethane.

An intumescent flame retardant (DOPO-DAM) containing phosphorus and nitrogen was synthesized via a two-step process and then, it was incorporated into waterborne polyurethane to serve as a reactive flame retardant for preparing flame-retarded WPU (FR-WPU). The chemical structures of DOPO-DAM and FR-WPU were confirmed by 1H NMR, 31P NMR, and FTIR studies. The thermal stability, flame retardancy and mechanical properties of FR-WPU were investigated by TGA, TGA-FIR, Py-GC-MS, limiting oxygen index (LOI) test, SEM-EDS analysis, cone calorimeter test and a universal testing machine. The results showed that the conjugation of DOPO-DAM into WPU induced a slight decline in the thermal stability of FR-WPU. However, the incorporation of DOPO-DAM into WPU significantly enhanced the flame retardancy by reducing the heat release rate (HRR), total heat release (THR), smoke production rate (SPR) and total smoke production (TSP). In addition, the morphology, elemental composition and content of the residual char of the flame-retarded WPU indicated the important function of DOPO-DAM in the condensed phase. Thus, DOPO-DAM exhibited gas-phase and condensed-phase flame retardant effects on the WPU films.

The study of cationic waterborne polyurethanes modified by two different forms of polydimethylsiloxane.

Two kinds of dimethylpolysiloxane, KF-6001 and X-22-176-DX, were used to modify polyurethane. The effects of KF-6001 and X-22-176-DX on the colloidal, physico-chemical and surface properties were studied for polydimethylsiloxane modified cationic waterborne polyurethanes (SiCWPUs). The chemical structures and the surface morphologies of the SiCWPUs are characterized via Fourier transform infrared spectrometry and scanning electron microscopy. The results showed that the addition of siloxane changes the structure and surface morphology of the polyurethane. The element distributions in the polymer films were tested via X-ray photoelectron spectroscopy, and the effect of the hydrophobicity of the surfaces of the polymer films of the cationic waterborne polyurethanes was demonstrated via water contact angle tests on the surfaces of the films. As the amount of siloxane added increases, the silicon content on the surfaces of the SiCPWU1 films increases from 0% to 17.92%, and the actual silicon content on the surfaces of the films was much larger than the theoretical value. Therefore, the hydrophobicity of the membrane surface increases sharply, and the contact angle increases from 63.0° to 105.3°. Dynamic mechanical analysis indicates that the introduction of polydimethylsiloxane into the cationic aqueous polyurethane chain increases microphase separation in the polymer films. Stress-strain data showed that the mechanical properties of SiCPWU1 films were better than those of SiCPWU2 films when the same amounts of PDMS were added.