Cross-linking Junctions Research Articles

Construction and characterization of alginate/calcium β-hydroxy-β-methylbutyrate hydrogels: Effect of M/G ratios and calcium ion concentration

Calcium β-hydroxy-β-methylbutyrate (CaHMB), a functional calcium salt, is used to maintain and improve muscle health. Here, a new hydrogel material prepared from alginate (ALG) with three M/G ratios (1:1, 2:1, and 1:2) and CaHMB (0–2 mg/mL) was investigated. CaHMB regulates the formation and properties of ALG hydrogels through chelation and hydrogen bonding. When the M/G ratio was 2:1, the anionic groups of CaHMB containing carboxyl and hydroxyl groups formed hydrogen bonds with the polysaccharide chains, hindering the capture of Ca2+ by the G-residue fragments of ALG, which in turn retarded the gelation process. The noncalcium cross-linked polysaccharide chain structure of ALG and the anionic group of CaHMB also affected the water distribution in the hydrogel, especially when M residue content ≥G residue content. Lower M/G ratios and higher CaHMB concentrations could increase the number of “egg box” crosslinking junctions of calcium alginate, and the microstructure was denser in the gel pores, resulting in a stronger gel strength and more free water bound in the gel matrix. This study provides a theoretical and methodological basis for the design of novel hydrogels by studying the crosslinking features of ALG/CaHMB.

Mechanical Performance of Copper-Nanocluster-Polymer Nanolattices.

A type of copper-nanocluster-polymer composites is reported and showcased that their 3D nanolattices exhibit a superior combination of high strength, toughness, deformability, resilience, and damage-tolerance. Notably, the strength and toughness of ultralight copper-nanocluster-polymer nanolattices in some cases surpass current best performers, including alumina, nickel, and other ceramic or metallic lattices at low densities. Additionally, copper-nanocluster-polymer nanolattices are super-resilient, crack-resistant, and one-step printed under ambient condition which can be easily integrated into sophisticated microsystems as highly effective internal protectors. The findings suggest that, unlike traditional nanocomposites, the laser-induced interface and the high fraction of ultrasmall Cu15 nanoclusters as crosslinking junctions contribute to the marked nonlinear elasticity of copper-nanocluster-polymer network, which synergizes with the lattice-topology effect and culminates in the exceptional mechanical performance.

Ligand-Mediated Mechanical Enhancement in Protein Complexes at Nano- and Macro-Scale.

Protein self-assembly plays a vital role in a myriad of biological functions and in the construction of biomaterials. Although the physical association underlying these assemblies offers high specificity, the advantage often compromises the overall durability of protein complexes. To address this challenge, we propose a novel strategy that reinforces the molecular self-assembly of protein complexes mediated by their ligand. Known for their robust noncovalent interactions with biotin, streptavidin (SAv) tetramers are examined to understand how the ligand influences the mechanical strength of protein complexes at the nanoscale and macroscale, employing atomic force microscopy-based single-molecule force spectroscopy, rheology, and bioerosion analysis. Our study reveals that biotin binding enhances the mechanical strength of individual SAv tetramers at the nanoscale. This enhancement translates into improved shear elasticity and reduced bioerosion rates when SAv tetramers are utilized as cross-linking junctions within hydrogel. This approach, which enhances the mechanical strength of protein-based materials without compromising specificity, is expected to open new avenues for advanced biotechnological applications, including self-assembled, robust biomimetic scaffolds and soft robotics.

Thermoreversible Gelation with Supramolecularly Polymerized Cross-Link Junctions.

Structure and reversibility of cross-link junctions play pivotal roles in determining the nature of thermoreversible gelation and dynamic mechanical properties of the produced polymer networks. We attempt to theoretically explore new types of sol-gel transitions with mechanical sharpness by allowing cross-links to grow without upper bound. We consider thermoreversible gelation of the primary molecules R{Af} carrying the number f of low molecular weight functional groups (gelators) A. Gelators A are assumed to form supramolecular assemblies. Some examples are: telechelic polymers (f=2) carrying π-π stacking benzene derivatives at their both ends, and trifunctional star molecules (f=3) bearing multiple hydrogen-bonding gelators. The sol-gel transition of the primary molecules becomes sharper with the cooperativity parameter of the stepwise linear growth of the cross-links. There is a polymerization transition (crossover without singularity) of the junctions in the postgel region after the gel point is passed. If the gelator A tends to form supramolecular rings competitively with linear chains, there is another phase transition in the deep postgel region where the average molecular weight of the rings becomes infinite (Bose-Einstein condensation of rings). As a typical example of binary cross-links where gelators A and B form mixed junctions, we specifically consider metal-coordinated binding of ligands A by metal ions B. Two types of multi-nuclear supramolecular complexes are studied: (i) linear stacking (ladder) of the sandwich A2B units, and (ii) linear train of egg-box A4B units. To find the strategy towards experimental realization of supramolecular cross-links, the average molecular weight, the gel fraction, the average length of the cross-link junctions are numerically calculated for all of these models as functions of the functionality f, the concentration of the solute molecules, and the temperature. Potential candidates for the realization of these new types of thermoreversible gelation are discussed.

Gelation Time of Network-Forming Polymer Solutions with Reversible Cross-Link Junctions of Variable Multiplicity.

The gelation time tg necessary for a solution of functional (associating) molecules to reach its gel point after a temperature jump, or a sudden concentration change, is theoretically calculated on the basis of the kinetic equation for the stepwise cross-linking reaction as a function of the concentration, temperature, functionality f of the molecules, and multiplicity k of the cross-link junctions. It is shown that quite generally tg can be decomposed into the product of the relaxation time tR and a thermodynamic factor Q. They are functions of a single scaled concentration x≡λ(T)ϕ, where λ(T) is the association constant and ϕ is the concentration. Therefore, the superposition principle holds with λ(T) as a shift factor of the concentration. Additionally, they all depend on the rate constants of the cross-link reaction, and hence it is possible to estimate these microscopic parameters from macroscopic measurements of tg. The thermodynamic factor Q is shown to depend on the quench depth. It generates a singularity of logarithmic divergence as the temperature (concentration) approaches the equilibrium gel point, while the relaxation time tR changes continuously across it. Gelation time tg obeys a power law tg-1∼xn in the high concentration region, whose power index n is related to the multiplicity of the cross-links. The retardation effect on the gelation time due to the reversibility of the cross-linking is explicitly calculated for some specific models of cross-linking to find the rate-controlling steps in order for the minimization of the gelation time to be easier in the gel processing. For a micellar cross-linking covering a wide range of the multiplicity, as seen in hydrophobically-modified water-soluble polymers, tR is shown to obey a formula similar to the Aniansson-Wall law.

Effect of water on the accelerated sulfur vulcanization of natural rubber

Water present in natural rubber was found to play a role in forming carbon-carbon crosslinking junctions before forming carbon-sulfur crosslinking junctions when its effect on the accelerated sulfur vulcanization was investigated by analyzing low–molecular weight products through various instrumental analytical techniques. Samples were prepared by mixing deproteinized natural rubber (DPNR) with various moisture contents with stearic acid, ZnO, sulfur, and N-tert-butylbenzothiazole-2-sulfenamide, and they were vulcanized at 150 °C for various cure times. Low–molecular weight products such as 2,2′-dibenzothiazole disulfide (MBTS), 2-mercaptobenzothiazole (MBT), ZnMBT, Zn2+ and sulfur linked to the rubber were extracted from the vulcanized natural rubbers using benzene and dioxane, and subsequently measured by high-performance liquid chromatography, UV–visible spectroscopy, inductively coupled plasma atomic emission spectroscopy, and combustion analysis. The water present in natural rubber was found to strongly affect the production and formation rate of the low–molecular weight products in the accelerated sulfur vulcanization of natural rubber. As the result it suggests that the water promotes the formation of the C-C linkages, and then it sequentially promotes the formation of the C-S linkages.

Poly(amidoamine) dendrimer-induced 3D crosslinked network constructed on polyphenylene sulfide nonwoven as a battery separator: Effect of generation number on cell performance

High power lithium-ion batteries put forward challenges for separator in terms of meeting high current density, safety as well as superior cycling stability. In this study, Poly(amidoamine) (PAMAM) dendrimer-introduced three-dimensional (3D) crosslinked coating layer is built on polyphenylene sulfide (PPS) nonwoven substrate to address above issues and the generation number effect are discussed thoroughly. Due to unique advantages of PAMAM dendrimer including its hydrophilicity to electrolyte, embedding effect of internal cavities, and spherical steric structure, the porosity, ionic conductivity, interfacial compatible as well as lithium-ion transference number are improved for the crosslinked composite separator especially at higher generation number, which endow battery with superior discharge capacity of 162.5 mAh g−1 at 0.2 C and outstanding C-rate capabilities. Moreover, through the adjustment of generation number, less crosslinking junctions but more crosslinking points at one junction can be achieved in PAMAM-induced 3D crosslinked structures, thus leading to the improvement of all mechanical properties concurrently. This behavior can shift the oxidative decomposition potential to 5.36 V and give superior cell performance retention (98.6%) even after 100 cycles for battery using the fourth generation PAMAM. Therefore, this study will provide a new route for separator when pursuing higher rate performance and cyclic stability.

κ-Carrageenan Associated with Fructose/Glycerol/Water LTTM: Toward Natural Thermosensitive Physical Gels

κ-Carrageenan was combined to a natural low-transition temperature mixture composed of fructose, glycerol, and water in a 1:1:5 molar ratio (FGW) to generate fully biobased physical gels through a simple method. The resulting gels can be easily prepared in various shapes. As for their hydrogel analogues, the rheological characterization of FGW-based samples evidenced thermosensitive gelation, attributed to the formation of aggregated H-bonded helices composed of κ-carrageenan chains. FGW-based gels exhibit a gel–sol transition temperature (Tg/s) higher than the one of hydrogels with recoverable and reversible viscoelastic properties upon successive heating/cooling cycles. SAXS analysis revealed a more extended conformation of chains in FGW that leads to more physical cross-linking junction zones within the materials. Such peculiar spatial internal organization provides a mechanical reinforcement demonstrated by compression tests. In conclusion, the use of the FGW solvent allows for designing κ-carrageenan physical gels with enhanced thermal and mechanical properties.

Dynamics of dual-junction-functionality associative polymer networks with ion and nanoparticle metal-coordinate cross-link junctions

We provide a canonical introduction to dual-junction-functionality associative polymer networks, which combine high and low functionality (f) dynamic cross-link junctions to impart load-bearing, dissipation, and self-repairing ability to the network. This unique type of network configuration offers an alternative to traditional dual-junction networks consisting of covalent and reversible cross-links. The high-f junctions can provide load-bearing abilities similar to a covalent cross-link while retaining the ability to self-repair and concurrently confer stimuli-responsive properties arising from the high-f junction species. We demonstrate the mechanical properties of this design motif using metal-coordinating polymer hydrogel networks, which are dynamically cross-linked by different ratios of metal nanoparticle (high-f) and metal ion (low-f) cross-link junctions. We also demonstrate the spontaneous self-assembly of nanoparticle-cross-linked polymers into anisotropic sheets, which may be generalizable for designing dual-junction-functionality associative networks with low volume fraction percolated high-f networks.

Song et al.: Dynamics of dual-junction-functionality associative polymer networks with ion and nanoparticle metal-coordinate cross-link junctions

Song <i>et al.</i>: Dynamics of dual-junction-functionality associative polymer networks with ion and nanoparticle metal-coordinate cross-link junctions

Mechanically mutable polymer enabled by light.

Human skin is a remarkable example of a biological material that displays unique mechanical characters of both soft elasticity and stretchability. However, mimicking these features has been absent in photoresponsive soft matters. Here, we present one synthetic ABA-type triblock copolymer consisting of polystyrene as end blocks and one photoresponsive azopolymer as the middle block, which is stiffness at room temperature and shows a phototunable transition to soft elastics athermally. We have synthesized an elastics we term “photoinduced soft elastomer,” where the photo-evocable soft midblock of azopolymer and the glassy polystyrene domains act as elastic matrix and physical cross-linking junctions, respectively. On the basis of the photoswitchable transformation between stiffness and elasticity at room temperature, we demonstrated precise control over nanopatterns on nonplanar substrates especially adaptable in the human skin and fabrication of packaged perovskite solar cells, enabling the simple, human-friendly, and controllable approach to be promising for mechanically adaptable soft photonic and electronic packaging applications.

Crystalline elastomeric block copolymer/ionic liquid membranes with enhanced mechanical strength and gas separation properties

We report a high-performance, three-dimensional (3D) interconnected crystalline elastomeric membrane with enhanced mechanical strength and CO2 separation properties. The membranes were prepared using an amphiphilic crystalline block copolymer, poly(ethylene glycol) monomethyl ether-block-poly(acrylonitrile) (mPEG-b-PAN), synthesized via one-pot free-radical polymerization under environmental-friendly, mild conditions (in water at 50 °C). The strong interaction of the mPEG-b-PAN block copolymer with ionic liquid (IL) resulted in a microphase-separated, interconnected elastomeric structure in which the PAN crystallites acted as crosslinking junctions. Exceptionally high IL loadings of up to 300 wt % were efficiently confined in the crystalline network while maintaining outstanding mechanical properties. The mPEG-b-PAN/IL membrane with 250 wt % IL loading achieved the highest separation performance with a CO2 permeability of 456.4 barrer, CO2/N2 selectivity of 61.4, CO2/CH4 of 18.1 and CO2/H2 of 12.3. The CO2/N2 separation performance exceeds the Robeson upper bound (2008) and approached the upper bound (2019). Both the separation properties and mechanical strength of the mPEG-b-PAN/IL elastomeric membranes outperformed those of homopolymer PAN (H-PAN) and other membranes with ILs. Interestingly, the mPEG-b-PAN/IL membranes were effortlessly prepared regardless of the surface properties of the substrate and IL loading, in contrast to H-PAN/IL, indicating that the amphiphilic property of the polymer plays a pivotal role in turning the interface.

Hydrolyzable Poly(β-thioether ester ketal) Thermosets via Acyclic Ketal Monomers.

Hydrolytically degradable poly(β-thioether ester ketal) thermosets are synthesized via radical-mediated thiol-ene photopolymerization using three novel dialkene acyclic ketal monomers and a mercaptopropionate based tetrafunctional thiol. For all thermoset compositions investigated, degradation behavior is highly tunable based on the structure of the incorporated ketal and pH. Complete degradation of the thermosets is observed upon exposure to acidic and neutral pH, and under high humidity conditions. Polymer networks composed of cross-link junctions based on acyclic dimethyl ketals degrade the quickest, whereas networks containing acyclic cyclohexyl ketals undergo hydrolytic degradation on a longer timescale. Thermomechanical analysis reveals low glass transition temperatures and moduli typical of thioether-based thermosets.

Study on Effect of Accelerator on Vulcanization of Natural Rubber through Rubber-state NMR Spectroscopy

Effect of accelerators on the vulcanization of natural rubber was investigated by analyzing structure of the crosslinking junctions through rubber state NMR spectroscopy. The accelerated sulfur vulcanization of natural rubber was carried out with one of the three accelerators, i.e., Zinc diethyldithiocarbamate (ZDEC), Zinc dibutyldithiocarbamate (ZDBC) and Zinc salt of 2-mercaptobenzothiazole (ZMBT), in the presence of sulfur, ZnO and stearic acid. Structure of the crosslinking junctions of the vulcanized natural rubbers was determined through rubber-state NMR spectroscopy with bilevel complete decoupling (BCM), distortionless enhancement by polarization transfer (DEPT) and attached proton test (ATP). The mechanism of the vulcanization of natural rubber was demonstratively analyzed on the basis of the structure of the resulting crosslinking junctions. It was classified into two schemes: that is, radical mechanism and ionic mechanism followed by radical mechanism, depending on the sort of accelerators used in the present study.

Thermoreversible Gelation with Two-Component Mixed Cross-Link Junctions of Variable Multiplicity in Ternary Polymer Solutions.

Theoretical scheme is developed to study thermoreversible gelation interfering with liquid–liquid phase separation in mixtures of reactive f-functional molecules R{A} and g-functional ones R{B} dissolved in a common solvent. Formed polymer networks are assumed to include multiple cross-link junctions containing arbitrary numbers and of functional groups A and B of each species. Sol-gel transition lines and spinodal lines are drawn on the ternary phase plane for some important models of multiple cross-link junctions with specified microscopic structure. It is shown that, if the cross-link structure satisfies a certain simple condition, there appears a special molar ratio of the two functional groups at which gelation takes place with a lowest concentration of the solute molecules, as has been often observed in the experiments. This optimal gelation concentration depends on f and g (functionality) of the solute molecules and the numbers and (multiplicity) of the functional groups in a cross-link junction. For cross-links which allow variable multiplicity, special attention is paid on the perfectly immiscible cross-links leading to interpenetrating polymer networks, and also on perfectly miscible cross-links leading to reentrant sol-gel-sol transition. Results are compared with recent observations on ion-binding polymer solutions, polymer solutions forming recognizable biomolecular complexes, polymer/surfactant mixtures, hydrogen-bonding polymers, and hydrophobically-modified amphiphilic water-soluble polymers.

Modulation of Properties through Covalent Bond Induced Formation of Strong Ion Pairing between Polyelectrolytes in Injectable Conetwork Hydrogels.

In situ simultaneous formation of both covalent linkages and ion pair is challenging yet necessary to control the biological properties of a hydrogel. We report that the generation of covalent linkages (+N-C) facilitates the simultaneous formation of ion pairs between polyelectrolytes (PEs) in a hydrogel network. Co-injection of tertiary amine functional macromolecules and reactive poly(ethylene glycol) (PEG) containing negatively charged PE leads to the formation of hydrogel conetworks consisting of covalent junctions and ion pairs. Our design is based on the gradual appearance of +N-C junctions followed by formation of ion pairs. This strategy provides an easy access to hydrogel networks bearing a predetermined proportion of ion pair and covalent cross-linking junction. The proportion of ion pair could be varied by introducing a precalculated proportion of mono- and difunctional reactive PEG in the hydrogel system. The topology of the prepolymer and the hydrogel could be modulated (graft) during hydrogel formation. This approach is applicable to obtain covalent/ionic, covalent bond induced purely ionic, and purely covalent hydrogels of several macromolecular entities. The effect of ion pairing in the hydrogels is strongly reflected in the modulus, strain bearing, degradation, free volume, swelling, and drug release properties. The hydrogels exhibit microscopic recovery of modulus after application of high amplitude strain depending on the prepolymer concentration (chain entanglement) and nature of hydrogel network. The hydrogels are hemocompatible, and the covalent/ionic hydrogels show a slower release of methotrexate than that of the purely covalent hydrogel. This work provides an understanding for the in situ construction and manipulation of biological properties of hydrogels through the covalent bond induced formation of a strong ion pair.

Quantitative analysis of crosslinking junctions of vulcanized natural rubber through rubber-state NMR spectroscopy

Rubber-state nuclear magnetic resonance (NMR) spectroscopy, a novel analytical method, was applied to quantitative analyses of crosslinking junctions and uncrosslinked pendant sulfidic groups for natural rubber (NR) vulcanizates. NR was vulcanized with two formulations: i.e., one including sulfur, ZnO, stearic acid and N-tert-butylbenzothiazole-2-sulfenamide (TBBS) as an accelerator and the other including sulfur, ZnO and stearic acid without the accelerator. The resulting NR vulcanizates were characterized by swelling methods and rubber-state NMR spectroscopy to determine crosslink density, number of sulfur atoms at crosslinking junctions and structure of crosslinking junctions, respectively. Small signals appearing in rubber-state 13C and 1H NMR spectra for the NR vulcanizates were assigned to crosslinking junctions, uncrosslinked pendant sulfidic groups and their neighboring groups by two-dimensional NMR spectroscopy. Based on the assignments, a proportion of the amount of CH linking to sulfur as the crosslinking junctions to total amount of CH linking to sulfur were estimated from intensities of the signals for the NR vulcanizates.

Interfacial self-assembly of polysaccharide rods and platelets bridging over capillary lengths

HypothesisGeneration of long-range ordering of colloidal particles through anisotropic interactions is of growing interest in material designing. At submicron-scale, routine works use synthetic spheres or rods but the knowledge pertaining to assembly of binary combination of particles is severely restricted. Improved understanding of the fundamental aspects that drive self-assembly, can lead to robust strategies for fabrication of topographically oriented films. ExperimentThe fluidical geometry of a liquid crystalline (LC) solution of polysaccharide consisting of micron-sized rod and platelet units was explored. The solutions, characterized for their rheological behavior, were evaporated from a rectangular cavity. The assembly and orientation of the units was monitored by polarizing microscopy and the interparticle capillary forces approximated mathematically. FindingsThe units deposited into an uninterrupted membrane upon interfacial evaporation, forming a bridge along the 8 mm gap, linking the substrates. The membrane, composed of a lamellar structure, was uniaxially oriented along the direction of the gap. The rheological estimations corroborated an extremely high value of viscosity with the presence of crosslinking junctions in this solution when compared to a solution with only rod units, capable of bridging a maximum of 1 mm. It has been demonstrated for the very first time that the presence of platelet-units contributes lateral capillary interactions and assist rod-units towards a wider, self-assembled structure.

Engineered self-healable elastomer with giant strength and toughness via phase regulation and mechano-responsive self-reinforcing

Acquiring simultaneous high strength and toughness, and efficient self-healability of polyurethane (PU) elastomer is of great challenge due to their exclusive dependence on hard segments. Herein, a transparent, puncture-resistant, notch-insensitive, resilient and self-healable elastomer (PDO-IP2.5) with giant tensile strength (29.5 ± 0.9 MPa), excellent toughness (124.9 ± 2.2 MJ/m3), outstanding fracture energy (83.9 kJ/m2), high puncture energy (625 mJ) and balanced self-healing efficiencies (SEs calculated in tensile strength, elongation at break and toughness > 95%) and fast physical wound re-mending (~60 s upon hot airflow) is successfully developed through deliberate phase regulation, i.e. rational design of asymmetric and bulky hard segments and optimization of hard segments content. The resulting loosely-packed hard domains play a pivotal role in conferring self-healability and generating stress-induced crystallization (SIC) of polytetramethylene ether glycol (PTMEG) chains during large deformation, which not only facilitates the chain mobility for heat-responsive dynamic exchange of hydrogen bonds and oxime-carbamate but also dissipate external force during stretching to help the SIC evolution of PTMEG chains. The SIC domains act as physical crosslinking junctions and reinforcing phase, resulting in remarkable strengthening and toughening. This strategy could enlighten researchers in academia and industry to develop strong and tough elastic materials. In the meantime, the unique comprehensive performance of the as-synthesized elastomer confers it with potential practical use in engineer-grade fields of puncture-resistant tire sealant and durable protective coatings.

Robust superabsorbent p(MAPTAC) hydrogels with long physical cross-link junctions: synthesis, characterization and their performance for phosphate removal from wastewater

Abstract Ionic hydrogels with great water absorption capacity generally display poor mechanical strength that limits their use and narrows down their application areas. In this study, the new ionic hydrogel composed of poly (3-methacrylamido propyl trimethyl ammonium chloride) crosslinked with N, N-methylenebisacrylamide and sulphate ions was synthesized to obtain the hydrogel formulation which exhibits both huge swelling capacity and high mechanical stability, simultaneously. The successively synthesized gels with this strategy achieved a swelling capacity of 270 g/g and a modulus increased up to 20.43 kPa, indicating that they have a great potential to use in applications in which the both properties are required. The gels carrying a great number of cationic sites were also found to have a high affinity to phosphate ions, attaining an sorption value of 370 mg/g gel and to exhibit pseudo-second-order kinetic and Langmuir sorption isotherm models. The obtained results revealed that the new pMAPTAC gels have good potential for both phosphate sorption and high water uptake capacity without losing structural integrity owing to their enhanced mechanical strength.