Urea Hydrogen Bonds Research Articles

Self-Healing Polyurethane-Urea Elastomers with High Strength and Toughness Based on Dynamic Hindered Urea Bonds and Hydrogen Bonds

Self-Healing Polyurethane-Urea Elastomers with High Strength and Toughness Based on Dynamic Hindered Urea Bonds and Hydrogen Bonds

Ring‐opening copolymerization of ε‐caprolactone and δ‐valerolactone to cyclic polyesters by a bimolecular system

AbstractBio‐based plastics have received much attraction because they are an alternative to petroleum‐based plastics in recent years. Among them, aliphatic polyesters have garnered the most attention due to their excellent properties. Polyester is primarily synthesized through ring opening polymerization of monomer, and the catalysts used are mostly synthesized through “click” reaction in the laboratory, which exhibits inferior activity and little commercial value. Herein, a facile approach was developed for the synthesis of high molecular‐weight cyclic aliphatic polyesters, specifically poly(valerolactone‐co‐caprolactone) with varying compositions. This involved the ring‐opening copolymerization of ε‐caprolactone (ε‐CL) and δ‐valerolactone (δ‐VL) in the absence of an initiator with commercial hydrogen bond donor triclorocarban and an organic base. The structure and properties of the prepared cyclic polyester were investigated using GPC, 13C NMR, 1H NMR, MALDI‐TOF mass spectrometry, TOF‐SIMS mass spectrometry, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X‐ray diffraction (XRD) techniques. The results show that the molecular weight of cyclic polyester can rapidly exceed 40 kDa, with a PDI of approximately 1.10. In comparison to linear polyester, it has a higher melting point and superior thermodynamic properties, thus presenting greater application potential. Finally, based on nuclear magnetic resonance titration, the potential catalytic processes and mechanisms of bimolecular systems on monomers in different environments were discussed. This study not only offers new insights into the structure and properties of cyclic polyesters, but also presents novel bimolecular catalytic systems using commercial urea hydrogen bond donors for the synthesis of cyclic polymer polyesters.

Choline Halide-Based Deep Eutectic Solvents as Biocompatible Catalysts for the Alternating Copolymerization of Epoxides and Cyclic Anhydrides.

Aliphatic polyesters have received considerable attention in recent years due to their biodegradability and biocompatible, mechanical, and thermal properties that can make them a suitable alternative to today's commercialized polymers. The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is a route to synthesize a diverse array of polyesters that could be useful in many applications. However, the catalysts used rarely consider biocompatible catalysts in the case that any are left in the polymer. To the best of our knowledge, we report the first example of using deep eutectic solvents (DESs) as biocompatible catalysts for this target ROCOP with polymerization activity for at least six diverse monomer pairs. Choline halide salts are active for this polymerization, with dried salts showing polymerization slower than that of those conducted in air. Hydrogen bonding with water is hypothesized to enhance the rate-determining step of epoxide ring opening. While the presence of water improves the rate of polymerization, it also acts as a chain transfer agent, leading to smaller molar mass polymers than intended. Combining the choline halide salts with urea or ethylene glycol hydrogen bond donors in air led to DES catalysts that reacted similarly to the salts exposed to air. However, when generating these DESs in air-free conditions, they showed similar rates of polymerization without a drop in polymer molar mass. The hydrogen bonding provided by urea and ethylene glycol seems to promote the rate increase without serving as a chain transfer agent. Results reported herein display the promising potential of biocompatible catalyst systems for this ROCOP process as well as introducing the use of hydrogen bonding to enhance polymerization rates.

3D Printing Self-Healing and Self-Adhesive Elastomers for Wearable Electronics in Amphibious Environments.

To meet the demands of challenging usage scenarios, there is an increasing need for flexible electronic skins that can operate properly not only in terrestrial environments but also extend to complex aquatic conditions. In this study, we develop an elastomer by incorporating dynamic urea bonds and hydrogen bonds into the polydimethylsiloxane backbone, which exhibits excellent autonomous self-healing and reversible adhesive performance in both dry and wet environments. A multifunctional flexible sensor with excellent sensing stability, amphibious self-healing capacity, and amphibious self-adhesive performance is fabricated through solvent-free 3D printing. The sensor has a high sensing sensitivity (GF = 45.1) and a low strain response threshold (0.25%) and can be used to detect small human movements and physiological activities, such as muscle movement, joint movement, respiration, and heartbeat. The wireless wearable sensing system assembled by coupling this device with a bluetooth transmission system is suitable for monitoring strenuous human movement in amphibious environments, such as playing basketball, cycling, running (terrestrial environments), and swimming (aquatic environments). The design strategy provides insights into enhancing the self-healing and self-adhesive properties of soft materials and promises a prospective avenue for fabricating flexible electronic skin that can work properly in amphibious environments.

Intrinsically self-healing crosslinked elastomer with mechanically robust based on dynamic urea bond and hydrogen bond for smart humidity sensor

Intrinsically self-healing crosslinked elastomer with mechanically robust based on dynamic urea bond and hydrogen bond for smart humidity sensor

Cross-linked self-healing polymers containing rosin moiety based on dynamic urea and multiple hydrogen bonds

In recent years, the use of renewable biomass resources to prepare self-healing polymers has become a hot research topic because of the shortage of fossil resources. Herein, a novel type of rosin-based cross-linked polymer (PR) with dynamic urea and multiple hydrogen bonds is fabricated by copolymerizing a rosin ester with a dynamic crosslinking agent poly(urethane-urea) through a simple UV-initiated reaction. Owing to its robustly dynamic bonds, the resulting PR has both good mechanical properties and ideal self-healing ability. Specifically, the PR with 25.4% biomass rosin reaches a tensile strength of up to 4.1 MPa, an elongation at break of 112%, and stress self-healing efficiency of 91.3% at 80 °C for 24 h. Remarkably, the PR with a glass transition temperature above room temperature exhibits good shape memory behavior and excellent weldability (afford 2500 g after healing at 80 °C for 5 h). Finally, by surface platinum spraying and pre-stretching treatment, strain sensors based on microcracking mechanisms are realized.

Preparation of Robust, Room‐Temperature Self‐Healable and Recyclable Polysiloxanes Based on Hierarchical Hard Domains

The contradiction between high mechanical strength and mild healing conditions has long existed in self‐healing materials, which limits the application of self‐healing materials. The preparation of robust materials with excellent healing performance under mild conditions can effectively reduce resource waste and environmental pollution. Herein, self‐healing polysiloxane elastomer materials, APDMS‐MDI‐IPDI‐Bs, based on microphase separation strategy are reported. Through the synergistic effect of the designed urea hydrogen bond and the nitrogen‐coordinated boroxine structure, the materials can maintain high mechanical properties (maximum tensile strength up to 3.35 MPa, elongation at break up to 316%), while maintaining excellent self‐healing and recyclable ability (24 h healing efficiency at room temperature can reach 94.77 ± 3.23%), and the performance can be repeated many times without decay. APDMS‐MDI‐IPDI‐Bs also exhibit unique hydrophobicity, expanding the application scenarios of materials containing boroxine structure.

Synthesis and Catalytic Activity of Bifunctional Phase-Transfer Organocatalysts Based on Camphor.

Ten novel bifunctional quaternary ammonium salt phase-transfer organocatalysts were synthesized in four steps from (+)-camphor-derived 1,3-diamines. These quaternary ammonium salts contained either (thio)urea or squaramide hydrogen bond donor groups in combination with either trifluoroacetate or iodide as the counteranion. Their organocatalytic activity was evaluated in electrophilic heterofunctionalizations of β-keto esters and in the Michael addition of a glycine Schiff base with methyl acrylate. α-Fluorination and chlorination of β-keto esters proceeded with full conversion and low enantioselectivities (up to 29% ee). Similarly, the Michael addition of a glycine Schiff base with methyl acrylate proceeded with full conversion and up to 11% ee. The new catalysts have been fully characterized; the stereochemistry at the C-2 chiral center was unambiguously determined.

Castor-oil-based, robust, self-healing, shape memory, and reprocessable polymers enabled by dynamic hindered urea bonds and hydrogen bonds

A novel biobased polyurethane with robust mechanical properties, repairability, reprocessability, and shape memory enabled by dynamic hindered urea bonds is reported. • Novel biobased polyurethanes bearing hindered urea bonds were synthesized. • The polymers possess self-healing, shape memory, and reprocessable properties. • Reversible solid/liquid transformation occurred under cooling and heating treatment. • The polymers can be used as recoverable adhesives and conductive composites. Developing biobased polyurethanes with repairability, reprocessability as well as robust mechanical properties remains a great challenge. Herein, novel, robust biobased polyurethane materials bearing hindered urea bonds (HUBs) derived from renewable castor oil are reported. The dynamic HUBs and hydrogen bonds that existed in HUBs provided these materials extremely low relaxation times (12.3 to 221 s at 100 °C) as well as excellent scratch healing efficiency (88.9–100% at 100 °C for 10 min) and recyclability (without obviously sacrificing the tensile properties at least 4 times). The selected sample also exhibited good shape memory behavior, with a shape fixity ratio above 88.4% and a shape recovery ratio above 81.3%. Remarkably, the polymers could undergo a rapid and reversible solid/liquid transformation under cooling and heating treatment. Besides, these HUBs materials demonstrated high adhesion strength (up to 2.53 MPa) when bonding stainless steel, and can be re-used for at least 5 times without significant deterioration in adhesion strength. Finally, by mixing a certain amount of carbon nanotubes (CNTs) and adjusting the compositions of HUBs, recyclable and malleable conductive composites were achieved. In general, this work presents a green, simple, and universal approach to fabricate robust, sustainable polyurethanes with multiple functions like repairability, shape memory, malleability, and recyclability.

Exploring piperazine for intrinsic weather-proof, robust and self-healable poly(urethane urea) toward surface and tire protection

Herein, we report a weather-proof, robust and self-healable poly(urethane urea) elastomer via exploring piperazine (PPA) as chain extender. The amidated PPA moiety enables it to maintain high-level transparency and mechanical properties upon heat and xenon aging for 16 days. Deliberate design of short and long hard segment is implemented by using isophorone diisocyanate (IPDI) and PPA as chain extenders. The long hard segments containing strong urea hydrogen bonds play a crucial role in generating strain-induced crystallization (SIC), resulting in outstanding robustness. The reversibility of hydrogen bonds embodies in room-temperature dissociation-reformation of urethane motifs, and transformation of free and regularly hydrogen-bonded urea motifs into irregular bonding above 35 °C. This elastomer exhibits exceptional comprehensive performances in tensile strength (38.5 ± 1.3 MPa), toughness (134.6 ± 7.0 MJ/m 3 ), puncture resistance (697.9 mJ), elasticity, self-healability and adhesion, which demonstrates outstanding ambient repair of scratches, anti-puncture and self-sealing performance. • Weather-proof, robust and self-healable poly(urethane urea) elastomer. • Amidated piperazine moiety contributes to anti-aging feature. • Long hard segments contains extraordinary evolution of hydrogen bonding. • Excellent as vehicle top-paint protective coating and anti-puncture tire sealant.

Room temperature self-healing crosslinked elastomer constructed by dynamic urea bond and hydrogen bond

• A flexible strategy to develop intrinsically dynamic crosslinked elastomers with RT self-healing via a photo-initiated copolymerization is described. • The crosslinked elastomers exhibited rapid self-healing at RT, extreme stretchability of up to 1726%, high toughness, long-term stability, and recyclability. • Upon damaged, the resultant networks instantly self-heal at RT and completely self-heal within 10 min. • Self-healing and stretchable conductive device is achieved. Designing a crosslinked polymer with autonomous self-healing at room temperature (RT) remains an arduous challenge because the topological network reconstructions are governed by crosslinking. Herein, a flexible strategy is described to develop intrinsically dynamic crosslinked elastomers with RT self-healing via a photo-initiated copolymerization to address these issues. The key is to incorporate a urea–urethane structure with dynamic urea bond into the polymer network as a flexible crosslinking unit. This strategy endows the crosslinked elastomer with superior properties, including autonomously rapid self-healing at RT, extreme stretchability of up to 1726%, high toughness (∼24.45 MJ m –3 ), extended stability, and recyclability. Upon damaged, the resultant elastomers instantly self-heal at RT and completely self-heal within 10 min without the assistance of external triggers. These amazing properties of the dynamic elastomer are attributed to the synergistic effect of the robustly dynamic urea bonds and inter- and intrachain hydrogen bond interactions. Notably, the self-healing and mechanical properties of the network can be optimized by tuning the dynamic crosslinker content. Dynamic reversibility occurs at RT evidenced by rheological tests. The facile fabrication method and multifunctional properties of the self-healable and stretchable crosslinked elastomer demonstrate that it shows great potential in smart protective coating and wearable electronics industry.

Microscopic structural features of water in aqueous-reline mixtures of varying compositions.

Reline, a mixture of urea and choline chloride in a 2 : 1 molar ratio, is one of the most frequently used deep eutectic solvents. Pure reline and its aqueous solution have large scale industrial use. Owing to the presence of active hydrogen bond formation sites, urea and choline cations can disrupt the hydrogen-bonded network in water. However, a quantitative understanding of the microscopic structural features of water in the presence of reline is still lacking. We carry out extensive all-atom molecular dynamics simulations to elucidate the effect of the gradual addition of co-solvents on the microscopic arrangements of water molecules. We consider four aqueous solutions of reline, between 26.3 and 91.4 wt%. A disruption of the local hydrogen-bonded structure in water is observed upon inclusion of urea and choline chloride. The extent of deviation of the water structure from tetrahedrality is quantified using the tetrahedral order parameter (qtet). Our analyses show a monotonic increase in the structural disorder as the co-solvents are added. Increase in the qtet values are observed when highly electro-negative hetero-atoms like nitrogen, oxygen of urea and choline cations are counted as partners of the central water molecules. Further insights are drawn from the characterization of the hydrogen-bonded network in water and we observe the gradual rupturing of water-water hydrogen bonds and their subsequent replacement by the water-urea hydrogen bonds. A negligible contribution from the hydrogen bonds between water and bulky choline cations has also been found. Considering all the constituents as the hydrogen bond partners we calculate the possibility of a successful hydrogen bond formation with a central water molecule. This gives a clear picture of the underlying mechanism of water replacement by urea.

Multifunctional Thermoplastic Polyurea Based on the Synergy of Dynamic Disulfide Bonds and Hydrogen Bond Cross-Links.

Integrating the self-healing property with the shape-memory effect is a strategy that extends the service lifetime of shape-memory materials. However, this strategy is inadequate to reshape and recycle through the self-healing property or liquid-state remoldability. For more types of damage, solid-state plasticity is needed as a complementary mechanism to broaden the reprocessing channels of smart materials. In this study, multifunctional thermoplastic polyureas cross-linked by urea hydrogen bonds are prepared, which possess the multipathway remodeling property. The shape transition can be triggered after heating above 65 °C. The synergistic effect of dynamic disulfide bonds and hydrogen bonds causes the thermoplastic polyureas to possess characteristics similar to those of associative covalent adaptable networks. Thus, the polyureas can repair the damage or reconfigure the shape at 75 °C in 15 min by solid-state plasticity, instead of going into a viscous flow state. Soft grippers with various shapes are prepared by integration of solid-state plasticity, and the structure and function of the grippers can be repaired. The integration of solid-state plasticity and the self-healing property broadens the paths of shape-memory polymers in recyclability and reshapability.

Mechanically Robust, Elastic, and Healable Ionogels for Highly Sensitive Ultra-Durable Ionic Skins.

The fabrication of highly durable skin-mimicking sensors remains challenging because of the unavoidable fatigue and physical damage that sensors are subjected to in practical applications. In this study, ultra-durable ionic skins (I-skins) with excellent healability and high sensitivity are fabricated by impregnating ionic liquids (ILs) into a mechanically robust poly(urea-urethane) (PU) network. The PU network is composed of crystallized poly(ε-caprolactone) and flexible poly(ethylene glycol) that are dynamically cross-linked with hindered urea bonds and hydrogen bonds. Such a design endows the resultant ionogels with high mechanical strength, good elasticity, Young's modulus similar to that of natural skin, and excellent healability. The ionogel-based I-skins exhibit a high sensitivity to a wide range of strains (0.1-300%) and pressures (0.1-20 kPa). Importantly, the I-skins show a highly reproducible electrical response over 10 000 uninterrupted strain cycles. The sensing performance of the I-skins stored in open air for 200 days is almost the same as that of the freshly prepared I-skin. The fractured I-skins can be easily healed by heating at 65 °C that restores their original ultra-durable sensing performance. The long-term durability of the I-skins is attributed to the combination of non-volatility of the ILs, excellent healability, and well-designed mechanical properties.

Light-Modulated Self-Blockage of a Urea Binding Site in a Stiff-Stilbene Based Anion Receptor.

Anion binding to a receptor based on stiff‐stilbene, which is equipped with a urea hydrogen bond donating group and a phosphate or phosphinate hydrogen bond accepting group, can be controlled by light. In one photoaddressable state (E isomer) the urea binding site is available for binding, while in the other (Z isomer) it is blocked because of an intramolecular interaction with its hydrogen bond accepting motif. This intramolecular interaction is supported by DFT calculations and 1H NMR titrations reveal a significantly lower anion binding strength for the state in which anion binding is blocked. Furthermore, the molecular switching process has been studied in detail by UV/Vis and NMR spectroscopy. The presented approach opens up new opportunities toward the development of photoresponsive anion receptors.

Ion solvation scenario in an aqueous solution mixture of counteracting osmolytes: Urea and trimethylamine-N-oxide (TMAO)

Abstract In this study, we investigate that how the ion solvation scenario of different charged species (Na+, K+, Cl−, Cl0, NH4+, NMe4+ and CH3COO−) in highly concentrated urea solution (8 M) gets modified on addition of counteractive osmolyte, TMAO (upto 5.6 M). It is observed that, unlike Na+, K+ prefers to interact with oxygen of TMAO with increasing TMAO concentration. Ammonium-ion exhibits stronger hydrogen bonding interaction with water as well as with TMAO. In case of tetramethylammonium-ion, addition of TMAO seems to hinder preferable Me-Ourea interaction by Me-OTMAO interaction. Acetate-ion shows stronger hydrogen bonding interaction with water as compared to urea which increases with addition of TMAO. Whereas methyl-group of acetate ion prefers to interact with methyl-carbon of TMAO though there exists a weak hydrogen bonding between oxygen of acetate ion and methyl-hydrogen of TMAO. Neutral solute (Cl0) prefers to interact with methyl-carbon of TMAO with increasing of TMAO concentration in the solution. Ion (ammonium/acetate)-urea hydrogen bond lifetimes have negligible effect when TMAO is added to the solution. TMAO mainly strengthens the ion-water and water-water hydrogen bond lifetimes, and also forms stronger hydrogen bonds with water molecules. We have also verified the effects of change of L-J combination rule along with another TMAO model (Shea model) in case of a NH4+and CH3COO−, as these ions resemble the zwitter-ionic form of amino acids. It was found that at intermediate concentration of TMAO, Shea model does not show appreciable difference with Kast (AM), whereas at higher concentration, Shea model shows higher preference to interact with ammonium ion.

Terminally and randomly functionalized polyisoprene lead to distinct aggregation behaviors of polar groups

Abstract Non-covalent interactions imposed by polar groups affect the properties of polyisoprene in various ways. But traditional modification method only led to randomly functionalized product which ignored the effects of sequence and location of polar groups. In this work, by using a novel polar monomer hydroxyl-myrcene (HMY), hydroxyl group terminally (BC–OH) and randomly functionalized (RC-OH) polyisoprene copolymers were successfully obtained. Due to the different hydroxyl group sequences, BC-OH and RC-OH shows different morphologies and rheological properties. Furthermore, polyhedral oligomeric silsesquioxanes (POSS) with urea bond was used as nanofiller to attach on BC-OH and RC-OH and generating BC-POSS and RC-POSS, respectively. The assembly of POSS and the formation of urea hydrogen bond will occur collaboratively. As a result, BC-POSS and RC-POSS showed distinct morphological properties and thermal stabilities. It is found that ordered urea bonds were formed in RC-POSS, while less ordered urea bonds were formed in BC-POSS. Overall, these results suggest that the tailoring of polar group sequence in polyisoprene copolymers could be a promising avenue for tuning polyisoprene performance via tuning aggregation behaviors of the polar groups.

Self-healing polymers with tunable mechanical strengths via combined hydrogen bonding and zinc-imidazole interactions

The development of self-healing polymers with tunable mechanical performances is an enormous challenge. Herein, self-healing polymers with tunable mechanical strengths via combined hydrogen bonding and Zn(II)-imidazole interactions were successfully synthesized. This synthesis was accomplished by introducing urea hydrogen bonding and Zn(II)-imidazole interactions into self-healing polymers polymerized from a new bifunctional monomer of 2-(3-(3-imidazolylpropyl)ureido)ethyl acrylate (IUA). The dual dynamic effects of urea hydrogen bonding and Zn(II)-imidazole interactions can simultaneously endow polymers with better self-healing capacities and mechanical properties. The synthesized polymers with urea hydrogen bonding and Zn(II)-imidazole interactions motifs exhibited over 90% self-healing efficiency under mild conditions. The mechanical strengths of the polymers can be flexibly tuned from 35.0 kPa to 4.41 MPa by varying the molar ratio of imidazole/Zn(II). When the polymers are damaged, the urea hydrogen bonds and Zn(II)-imidazole coordination can contribute to the reconstruction of the broken polymer networks and thus provide self-healing abilities. The interactions of hydrogen bonding and Zn(II)-imidazole coordination were confirmed by in-situ FT-IR spectra of the self-healing polymers at different temperatures.

Function-Oriented: The Construction of Lanthanide MOF Luminescent Sensors Containing Dual-Function Urea Hydrogen-Bond Sites for Efficient Detection of Picric Acid.

Two novel lanthanide metal-organic framework (Ln-MOF) luminescent sensors for the detection of picric acid have been successfully assembled. Following a function-oriented strategy, urea hydrogen-bonding functional sites were introduced into two MOF frameworks. A structural analysis indicated that the two MOFs have the exact same structure, namely 2D layers with diamond-shaped holes that are accumulated into a 3D framework through the hydrogen-bonding interactions between urea and carboxylate groups. Interestingly, only half of the urea units are involved in supporting the MOF framework through N-H⋅⋅⋅O hydrogen-bonding interactions, whereas the other half are located in the pore channel and act as empty recognition sites. Abundant N-H urea bonds are present in the inner walls of three types of interpenetrating 1D channels. Luminescence studies revealed that the two Ln-MOFs exhibit high sensitivity, good selectivity, and a fast luminescence quenching response towards picric acid. In particular, the two Ln-MOFs can be simply and quickly regenerated, and exhibit excellent recyclability. In summary, we have successfully used a function-oriented strategy to achieve multiple functions in a ligand to construct lanthanide MOF luminescent sensors for the detection of picric acid, thereby providing a potential strategy for the future development of MOF luminescent sensors with a specific target.

NO2 Solvation Structure in Choline Chloride Deep Eutectic Solvents-The Role of the Hydrogen Bond Donor.

Deep eutectic solvents (DESs) are promising candidates as alternate media for industrial gas sequestration processes, such as denitrification via NO2 adsorption. Here, quantum chemical methods are employed to characterize the NO2 solvation structure and adsorption mechanism in choline chloride DES with urea, methylurea, and thiourea hydrogen bond donors. Our results show that the solvation structure of NO2 in bulk choline chloride DES is determined by the type of the hydrogen bond donor present. Changing the structure of the hydrogen bond donor not only changes its NO2 coordination mechanism but also changes that between NO2 and the choline and chloride ions in the DES. By using an energy decomposition analysis scheme, we show that the principle forces stabilizing NO2 in these DES are dispersion and polarization interactions and, consequently, that NO2 adsorption is most favorable in the choline chloride-thiourea DES. These results highlight a potential route for the optimization of choline chloride DES for denitrification, by modulating the hydrogen bond donor structure.