Intrachain Hydrogen Bonds Research Articles

Impact of Sacrificial Hydrogen Bonds on the Structure and Properties of Rubber Materials: Insights from All-Atom Molecular Dynamics Simulations.

The inclusion of sacrificial hydrogen bonds is crucial for advancing high-performance rubber materials. However, the molecular mechanisms governing the impact of these bonds on material properties remain unclear, hindering progress in advanced rubber material research. This study employed all-atom molecular dynamics simulations to thoroughly investigate how hydrogen bonds affect the structure, dynamics, mechanics, and linear viscoelasticity of rubber materials. As the modified repeating unit ratio (β) increased, both interchain and intrachain hydrogen bond content rose, with interchain bonds playing a predominant role. This physical cross-linking network formed through interchain hydrogen bonds restricts molecular chain movement and relaxation and raises the glass transition temperature of rubber. Within a certain content of hydrogen bonds, the mechanical strength increases with increasing β. However, further increasing β leads to a subsequent decrease in the mechanical performance. Optimal mechanical properties were observed at β = 6%. On the other hand, a higher β value yields an elevated stress relaxation modulus and an extended stress relaxation plateau, signifying a more complex hydrogen-bond cross-linking network. Additionally, higher β increases the storage modulus, loss modulus, and complex viscosity while reducing the loss factor. In summary, this study successfully established the relationship between the structure and properties of natural rubber containing hydrogen bonds, providing a scientific foundation for the design of high-performance rubber materials.

Constructing Kosmotropic Salt‐Compatible PVA Hydrogels for Stable Zinc Anodes via Strong Hydrogen Bonds Preshielding Effect

AbstractPhysical hydrogels crosslinked by noncovalent interactions are promising in flexible zinc–metal batteries for their component controllability and environmental friendliness. However, the compatibility of PVA‐based electrolyte and kosmotropic salt remains a challenge, which shows an unclear mechanism. Here, a “good‐to‐poor” solvent substitution strategy is adopted to develop a PVA hydrogel electrolyte with good compatibility with kosmotropic salt (ZnSO4). Stretching the polymer conformation by preshielding the strong intrachain hydrogen bonds with good solvents and activating the polymer interchain interaction in situ with poor solvents will induce the formation of a homogeneous polymer network and the release of hydrated hydroxyl groups for fast ion transport. This multiscale microstructure improves the ionic conductivity and liquid retention of PVA hydrogels with kosmotropic salt. Additionally, this strong hydrogen bonds preshielded hydrogel electrolyte offers great battery performance, with 1300 h of stable cycling at 1 mA cm−2 with low voltage hysteresis; and an extended cycle life of 220 h at 68.4% zinc utilization. Importantly, the Zn//MnO2 pouch cell maintains 98.4% capacity after 100 cycles at 0.2 A g−1 and features strong environmental reliability. These concepts address the inherent barriers of kosmotropic salt‐based hydrogel electrolytes and advance their development in soft electronics.

Chirality-Regulated Clusteroluminescence in Polypeptides.

The low emission efficiency of clusteroluminogens restricts their practical applications in the fields of sensors and biological imaging. In this work, the clusteroluminescence of ordered/disordered polypeptides was observed, and the photoluminescence (PL) intensity of polypeptides can be modulated by the chirality of amino acid residues. Polyglutamates with different chiral compositions were synthesized, and the racemic polypeptides exhibited a significantly higher PL intensity than the enantiopure ones. This emission originates from the n-π* transition between C═O groups of polypeptides and is enhanced by clusterization of polypeptides. CD and Fourier transform infrared spectra demonstrated that the enantiopure and racemic polypeptides form α-helix and random coil structures, respectively. The disordered polypeptides can form more chain entanglements and interchain interactions because of their high flexibility, leading to more clusterizations and stronger PL intensity. The rigidity of ordered helical structures restrains the chain entanglements, and the formation of intrachain hydrogen bonds between amide groups of the backbone impairs the interchain interaction between polypeptides, resulting in lower PL intensity. The PL intensity of the polypeptides can also be manipulated by the addition of urea or trifluoroacetic acid. Our study not only elucidates the chirality/order-based structure-property relationship of clusteroluminescence in peptide-based polymers but also offers implications for the rational design of fluorescent peptides/proteins.

Synthesis And Conformational Behaviors of Unnatural Peptides Alternating Chiral And Achiral α,α-Disubstituted α-Amino Acid Units.

The development of peptidomimetics to modulate the conformational profile of peptides has been extensively studied in the fields of biological and medicinal chemistry. However, large-scale synthesis of peptidomimetics with both an ordered sequence and a controlled secondary structure is highly challenging. In this paper, the framework of peptidomimetics has been designed to be alternating an achiral α,α-disubstituted α-amino acid unit and a chiral α-methylphenylalanine unit. The polymers were synthesized via our invented Ugi reaction-based polycondensation technique. The chiral higher-order structures of the alternating peptides were evaluated mainly through CD spectroscopy. The UV-vis and CD spectra of the polymers in three solvents were systematically measured at various temperatures. The anisotropic factors of CD (gCD ) values were calculated to know the chiroptical response. The results indicate the characteristic conformational behaviors. In a polar solvent, the hydrogen bonds between the N-H group of MePhe unit and the C = O of α,α-diphenylglycine unit outweigh the intraresidue hydrogen bonds in α,α-diphenylglycine unit, leading to the formation of a prevailing preferred-handed 310 -helical conformation. On the other hand, in a less polar solvent, the intrachain hydrogen bonds switch to intraresidue hydrogen bonds in α,α-diphenylglycine unit, which make the polymer adopting a prevailing extended planar C5 -conformation. This article is protected by copyright. All rights reserved.

Effects of the Degree of Deacetylation on the Single-Molecule Mechanics of Chitosans.

Chitosan is one of the most prevalent biomass materials, and its physicochemical and biological characteristics, such as solubility, crystallinity, flocculation ability, biodegradability, and amino-related chemical processes, are directly connected to the degree of deacetylation (DD). However, the specifics about the effects of the DD on the characteristics of chitosan are still unclear up to now. In this work, atomic force microscopy-based single-molecule force spectroscopy was used to study the role of the DD in the single-molecule mechanics of chitosan. Even though the DD varies largely (17% ≤ DD ≤ 95%), the experimental results demonstrate that the chitosans exhibit the same natural (in nonane) and backbone (in dimethyl sulfoxide (DMSO)) single-chain elasticity. This suggests that chitosans have the same intra-chain hydrogen bond (H-bond) state in nonane and to which these H-bonds can be eliminated in DMSO. However, when the experiments are carried out in ethylene glycol (EG) and water, the single-chain mechanics are increased with the increases of the DD. The energy consumed to stretch chitosans in water is larger than that in EG, indicating that amino can form a strong interaction with water and induce the formation of the binding water around the sugar rings. The strong interaction between water and amino may be the key factor for the well solubility and chemical activity of chitosan. The results of this work are anticipated to provide fresh light on the significant role played by the DD and water in the structures and functions of chitosan at the single molecular level.

Hexagonal crystalline Magnus' green salt analogues prepared from hydroxy-functionalised Pt and Pd complexes.

New Magnus' green salt (MGS) analogues, [M(dabdOH)2][MCl4]·2H2O (dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol; M = Pd (1) and M = Pt(2)), in which [M(dabdOH)2]2+ and [MCl4]2- are stacked alternately to form linear chains, were obtained as hexagonal plate crystals. The hexagonal shape and large crystal size are unprecedented features as MGS analogues. An unusual trigonal grade separation of chain complexes has been revealed by the structural analysis. 1 and 2 exhibited remarkable yellow and pink colours, respectively, which are derived from weak M⋯M interactions. The dabdOH ligand, which has an additional hydrogen donor group (hydroxy group), produces a multiple-hydrogen-bond network. The combination of intrachain and interchain hydrogen bonds gives a two-dimensional (2D) hydrogen-bond sheet, and each 2D sheet is indirectly connected by hydrogen bonds via lattice water molecules. The OH-functionalised ligand greatly increases the hydrophilicity of the MGS analogues and yields the largest single crystals of all MGS analogues reported so far. The trigonal grade-separated chain structure is likely due to the geometric matching between the periodicity of chains and the short axis width of the chain. This strategy opens up new insight for preparing large crystals of MGS analogues and for constructing trigonal grade-separated nanowires in molecular crystals.

A Molecular Dynamics Simulation Study of Amine-Carboxyl Ionic Interactions and Their Distribution in a Polysiloxanes Network

The ionic crosslinking mechanism of polysiloxanes networks based on amine/carboxyl interactions was investigated by molecular dynamics and tensile performance experiments. The “one-chain-crosslinking” mechanism between crosslinkers and polymers was investigated based on the corresponding simulations. The experimental results showed that the tensile strength and elongation at break of amine-functional polysiloxanes (AFPs)/carboxyl-functional polysiloxanes (CFPs) (1.54 MPa@264%, respectively) were higher than that of amine and carboxyl-functional polysiloxanes (ACFPs) (0.66 MPa@45%). The simulation results indicated that the number of intrachain and interchain hydrogen bonds played an important role for improving the mechanical performance of the polymer networks. In addition, it was found that the orientation and entanglement of the polymer chains had a direct correlation with hydrogen bonding in the equilibrium state and during the stretching process. The mean square radius of gyration (R g2), the mean square end-to-end distance (R ee2), the orientation parameter, the entanglement parameter, and the mean square displacement (MSD) of the chains in the equilibrium state were calculated. The molecular dynamic simulations were verified by comparing the simulated results with the tensile and rheological experimental results of the AFPs/CFPs and ACFPs elastomers.

Effect of natural deep eutectic solvents of non-eutectic compositions on enzyme stability

Natural deep eutectic solvents (NADES) represent a green alternative to common organic solvents in the biochemical industry due to their benign behavior and tailorable properties, in particular as media for enzymatic reactions. However, to fully exploit their potential in enzymatic reactions, there is a need for a more fundamental understanding of how these neoteric solvents influence the course of these reactions. Thus, the aim of this study is to investigate the influence of NADES with various molar compositions on the stability and structure of enzymes, applying molecular dynamics simulations. This can help to better understand the effect of individual compounds of NADES, in addition to eutectic mixtures. More specifically, we simulate the behavior of Candida antarctica lipase B (CALB) enzyme in NADES composed of choline chloride with either urea, ethylene glycol or glycerol. Hereto, we monitor the NADES microstructure, the general stability of the enzyme and changes in the structure of its active sites and surface residues. Our simulations show that none of the studied NADES systems significantly disrupt the microstructure of the solvent or the stability of the CALB enzyme within the time scales of the simulations. The enzyme preserves its initial structure, size and intra-chain hydrogen bonds in all investigated compositions and, for the first time reported, also in NADES with increased hydrogen bond donating compound ratios. As the main novelty, our results indicate that, in addition to the composition, the molar ratio can be an additional variable to fine-tune the physicochemical properties of NADES without altering the enzyme characteristics. These findings could facilitate the development and application of task-tailored NADES media for biocatalytic processes.

Molecular Dynamics Study of Cellulose Nanofiber Alignment under an Electric Field.

The alignment of cellulose by an electric field is an interesting subject for cellulose material processing and its applications. This paper reports an atomistic molecular dynamics simulation of the crystalline cellulose nanofiber (CNF) model in varying electric field directions and strengths. GROMACS software was used to study crystalline cellulose 1β consisting of 18 chains in an aqueous environment at room temperature, and an electric field was applied along the cellulose chain direction and the perpendicular direction with varying field strength. The root-mean-square displacement, radius of gyration, end-to-end length, and hydrogen bond population of the crystalline CNF model were analyzed to determine the effects of the applied electric field on the structure of the CNF model. The results suggest that the nanosecond electric field can induce the orientation of the CNF along the applied electric field direction. The alignment rate and ability to maintain the alignment depend on the electric field strength. Analysis of the radius of gyration, end-to-end length, and bond lengths for intrachain and interchain hydrogen bonds revealed no significant effect on the cellulose structure. Cellulose alignment in an electric field has the potential to broaden the design of electric field-induced processing techniques for cellulose filaments, thin films, and electro-active cellulose composites.

A Comparison between the Lower Critical Solution Temperature Behavior of Polymers and Biomacromolecules

All-atom molecular dynamics (MD) simulations are employed to compare the lower critical solution temperature (LCST) behaviors of poly(N-isopropylacrylamide) (PNIPAM) and elastin-like polypeptides (ELPs) with the canonical Val-Pro-Gly-Val-Gly ((VPGVG)n) sequence over a range of temperatures from 280 K to 380 K. Our simulations suggest that the structure of proximal water dictates the conformation of both the (VPGVG)n ELPs and PNIPAM chains. Specifically, the LCST transition in ELPs can be attributed to a combination of thermal disruption of the network of the proximal water near both hydrophilic and hydrophobic groups in the backbone and side-chain of (VPGVG)n, resulting in a reduction in solvent accessible surface area (SASA). This is accompanied with an increase in the secondary structure above its LCST. In the case of PNIPAM, the LCST transition is a result of a combination of a reduction in the hydrophobic SASA primarily due to the contributions of isopropyl side-chain and less to the backbone and the formation of intra-chain hydrogen bonds between the amide groups on the side-chain above its LCST.

Role of Functional Groups in the Monomer Molecule on the Radical Polymerization in the Presence of Graphene Oxide. Polymerization of Hydroxyethyl Acrylate under Isothermal and Non-Isothermal Conditions.

Functional groups in a monomer molecule usually play an important role during polymerization by enhancing or decreasing the reaction rate due to the possible formation of side bonds. The situation becomes more complicated when polymerization takes place in the presence of graphene oxide since it also includes functional groups in its surface. Aiming to explore the role of functional groups on polymerization rate, the in situ bulk radical polymerization of hydroxyethyl acrylate (HEA) in the presence or not of graphene oxide was investigated. Differential scanning calorimetry was used to continuously record the reaction rate under both isothermal and non-isothermal conditions. Simple kinetic models and isoconversional analysis were used to estimate the variation of the overall activation energy with the monomer conversion. It was found that during isothermal experiments, the formation of both inter- and intra-chain hydrogen bonds between the monomer and polymer molecules results in slower polymerization of neat HEA with higher overall activation energy compared to that estimated in the presence of GO. The presence of GO results in a dissociation of hydrogen bonds between monomer and polymer molecules and, thus, to higher reaction rates. Isoconversional methods employed during non-isothermal experiments revealed that the presence of GO results in higher overall activation energy due to the reaction of more functional groups on the surface of GO with the hydroxyl and carbonyl groups of the monomer and polymer molecules, together with the reaction of primary initiator radicals with the surface hydroxyl groups in GO.

Influence of Temperature on the Properties of Cellulose Iβ based on Molecular Dynamics Simulations

Natural plants, such as cotton and linen, are rich in cellulose Iβ. The properties of cellulose Iβ under different temperatures was studied using molecular dynamics simulations. Firstly, the crystal of cellulose Iβ was built. To verify the model, the X-ray fibre diffraction and thermal expansion coefficients were calculated, which were found to agree with experimental results. Then the Mulliken population of the bonds were computed and the movement of the centre chain and hydrogen bonds studied over the range 300-550 K using a PCFF force field. The results of the Mulliken population reveal the three steps of pyrolysis. The higher the temperature is, the more intensely the movement of the centre chain is. However, the impact of temperature on the movement of the centre chain is not obvious. From 300 K to 550 K, the total number of hydrogen bonds decreased by only 20%. Moreocer, the rupture of intrachain hydrogen bonds and the formation of interchain hydrogen bonds at 400 K ~ 450 K temperature occurred.

Intrinsic kink deformation in nanocellulose

Sharp bends can be widely observed in isolated cellulose nanofibrils (CNFs) after mechanical treatment, referred to as kink dislocations that are previously found in wood cell walls under compression. The non-Gaussian distribution of kink angle implies some inherent deformation behaviors of cellulose nanocrystals (CNCs) hidden in the formation of kink dislocations in CNFs. We herein perform molecular dynamics simulations to investigate the kink deformation of nanocellulose. It is interesting to find an intrinsic deformation mode of Iβ CNCs under uniaxial compression, in which the metastable structure of kinked CNCs turns out to be the triclinic Iα phase with twin boundaries originated from interlayer dislocation-induced allomorphic transition. An intrinsic kink angle (~60°) is defined based on geometric traits of stable kinked CNCs. Moreover, the weakened intrachain hydrogen bonds in twin boundaries lead to exposed glycosidic bonds and damaged hydrogen-bonding networks, which would act as the origin of kink defects in nanocellulose.

Hofmeister effect on the viscosity properties of gelatin in dilute solutions

The effect of various Hofmeister anions on the molecular conformation of gelatin in dilute solutions was investigated by viscosity, optical rotation and dynamic light scattering (DLS). The results showed that the intrinsic viscosity of gelatin decreased in the presence of the kosmotropic anions such as Citrate3−, SO42−, H2PO4− and MeCOO−, whereas it was increased with the addition of chaotropes such as Cl− and KSCN−. Furthermore, the intrinsic viscosity of gelatin was directly correlated to the hydration entropy of kosmotropic anions, suggesting that the decrease of the intrinsic viscosity was attributed to the strong hydration effect of kosmotropes. The strong dehydration of gelatin facilitated the folding of the polymer chains into helix bundles, validated by the results of optical rotation. On the contrary, the chaotropic anions could interact directly with polypeptide backbones, and the intrachain hydrogen bonds were destroyed. As a result, the polymer chains expanded, which was confirmed by DLS data, and the intrinsic viscosity was increased. These observations indicate that the molecular conformation of gelatin can be modulated by Hofmeister anions.

Understanding the dissolution of softwood lignin in ionic liquid and water mixed solvents

Lignin is the most abundant heterogeneous aromatic polymer on earth to produce a large number of value-added chemicals. Besides, the separation of lignin from the lignocellulosic biomass is essential for cellulosic biofuel production. For the first time, we report a cosolvent-based approach to understand the dissolution of lignin with 61 guaiacyl subunits at the molecular level. Atomistic molecular dynamics simulations of the lignin were performed in 0%, 20%, 50%, 80%, and 100% 1-Ethyl-3-Methylimidazolium Acetate (EmimOAc) systems. The lignin structure was significantly destabilized in both 50%, and 80% EmimOAc cosolvents, and pure EmimOAc systems leading to the breakdown of intrachain hydrogen bonds. Lignin-OAc and lignin-water hydrogen bonds were formed with increasing EmimOAc concentration, signifying the dissolution process. The OAc anions mostly solvated the alkyl chains and hydroxy groups of lignin. Besides, the imidazolium head of Emim cations contributed to solvation of methoxy groups and hydroxy groups, whereas ethyl tail interacted with the benzene ring of guaiacyl subunits. Effective dissolution was obtained in both the 50% and 80% EmimOAc cosolvent systems. Overall, our study presents a molecular view of the lignin dissolution focusing on the role of both cation and anion, which will help to design efficient cosolvent-based methods for lignin dissolution.

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

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.

Efficient and thermally stable BHJ solar cells based on a soluble hydroxy-functionalized regioregular polydodecylthiophene

A new regioregular polythiophene derivative, called poly[3-(12-hydroxydodecyl)thiophene] (PT12OH), was synthesized by post-functionalizing its ω-brominated precursor poly[3-(12-bromododecyl)thiophene] (PT12Br) prepared using the Grignard metathesis route. Thanks to the optimal balance between hydrophilic and hydrophobic groups within its structure, PT12OH was highly soluble and easily filmable from common organic solvents allowing for its complete characterization. It also showed enhanced thermal properties, crystallinity, and self-assembling capabilities by the formation of strong inter- and intrachain hydrogen bonds. Bulk heterojunction photovoltaic cells with PT12OH and PC61BM showed a PCE of 4.83% and a remarkable over-time stability, offering good photoconversion efficiency even after 120 h of accelerated aging. Indeed, the PCE decrease was 34% for the hydroxylated polymer and 65% for its brominated precursor. It should also be pointed out that the enhanced thermal stability of PT12OH was achieved without resorting to any complex post-annealing photochemical, thermal, or chemical treatment and was thus directly ascribable to the polymer chemical structure. The simple and effective synthetic procedure, photovoltaic efficiency, and enhanced stability revealed the potential of PT12OH for large-scale organic solar cell applications.

Mechanically Stable Dynamic Urea Bond‐Based Crosslinked Polymer Blend with Tunable Self‐Healable and Physical Properties

Front Cover: In article number 2000527 by Faai Zhang and co-workers, a crosslinked polymer blend based on a urethane‒arcylate system with a combination of reversibly noncovalent intrachain and interchain hydrogen bonds and dynamically covalent urea bonds is developed through a facile in situ photoinduced copolymerization. The materials exhibit rapid self-healing at mild conditions (70 °C, 30 min), extreme toughness (≈34.76 MJ m−3), high tensile strength, superior stretchability, long-term stability, recyclability, and weldability.

Mechanically Stable Dynamic Urea Bond‐Based Crosslinked Polymer Blend with Tunable Self‐Healable and Physical Properties

AbstractPolymer networks crosslinked by reversible noncovalent crosslinks have been applied in self‐healing and recyclable sustainable materials but result in limited mechanical strength. Herein, a crosslinked polymer blend that is based on a urethane–arcylate system with a combination of reversibly noncovalent intrachain and interchain hydrogen bonds and dynamically covalent urea bonds is developed through facile in situ photo‐induced copolymerization. An essential step is the introduction of a flexibly dynamic crosslinker bearing robustly hindered urea bonds and urethane–urea structures into the network, which endows the dynamic network with a synergy of mechanical robustness and desirable self‐healing ability. The dynamic networks exhibit rapid self‐healing at mild conditions (70 °C, 30 min), extreme toughness (≈34.76 MJ m−3), high tensile strength (≈7.78 MPa), superior stretchability (≈932%), long‐term stability, recyclability, and weldability. More importantly, the mechanical and self‐healing properties of the resultant materials can be fine‐tuned by adjusting the dynamic crosslinker content. These superior properties are attributed to the dynamic reversibility of hydrogen bonds and urea bonds as monitored by rheological tests. The extremely facile fabrication approach and superior properties of the resulting self‐healing polymers can find applications in sustainable smart materials and self‐healing conductive sensors.

Temperature-Dependent Blue Shifting of O-H Stretching Frequencies in Crystalline Cellulose Explained.

Increasing the temperature of a chemical system generally causes covalent bonds to lengthen and weaken, often the first step in initiating chemical reactions. However, for some hydrogen-bonded systems, infrared (IR) spectroscopy measurements reveal that covalent O-H bonds actually strengthen and therefore shorten when heated. In 1957, Finch and Lippincott proposed a simple one-dimensional (1D) model to explain this effect, in which thermal excitation of intermolecular stretching modes leads to lengthening and weakening of intermolecular O-H···O hydrogen bonds, thereby indirectly strengthening the associated covalent O-H bonds. Taking cellulose (an infinitely repeating polymer of d-glucose) as an example, we use molecular dynamics modeling to show that the same mechanism is responsible for temperature-dependent blue shifting of O-H stretching bands in IR spectra of carbohydrate biopolymers, except that interchain hydrogen bonds are weakened by thermal excitation of chain-separation modes, while intrachain hydrogen bonds are weakened by thermally induced changes in ring puckering and orientation of ring substituents but not reorientation of glucose units relative to one another or overall twisting of the cellulose chains.