Chemical Cross-linking Structure Research Articles

In situ crosslinking of silica aerogel with reactive carbon fiber for high mechanical property, thermal insulation and superhydrophobicity

In this study, a novel silicon hydroxyl groups decorated long carbon chain grafted carbon fiber was synthesized, and a facile fabrication method was developed to produce reactive carbon fiber-silica aerogel composite with chemical crosslinking structure. Scanning electron microscopy images showed that silica aerogel matrix is wrapped on the surface of reactive carbon fibers. The reactive carbon fibers facilitated obstacle in shrinkage, and aerogel composites exhibited microporous behavior with average diameter of about 1.8 nm. Fourier-transform infrared and X-ray photoelectron spectroscopy confirmed the strong interfacial interaction between reactive carbon fibers and silica matrix. The modification mechanism of silicon hydroxyl groups decorated long carbon chain grafted carbon fiber, and the chemical crosslinking in aerogel composite were proposed. The reactive carbon fiber-silica aerogel composites synthesized in this work had been proved to have low density, excellent mechanical property, good tailoring performance, improved thermal stability, low thermal conductivity, high flame retardancy, high thermal insulation property, and superhydrophilicity. Hence, this work provided a theoretical basis for improving the performance and expanding the application of silica aerogel.

Conductive and Enhanced Mechanical Strength of Mo2Ti2C3 MXene-Based Hydrogel Promotes Neurogenesis and Bone Regeneration in Bone Defect Repair.

Bone defects are common with increasing high-energy fractures, tumor bone invasion, and implantation revision surgery. Bone is an electroactive tissue that has electromechanical interaction with collogen, osteoblasts, and osteoclasts. Hydrogel provides morphological plasticity and extracellular matrix (ECM) 3D structures for cell survival, and is widely used as a bone engineering material. However, the hydrogels have poor mechanical intensity and lack of cell adhesion, slow gelation time, and limited conductivity. MXenes are novel nanomaterials with hydrophilic groups that sense cell electrophysiology and improve hydrogel electric conductivity. Herein, gelatin had multiple active groups (NH2, OH, and COOH) and an accelerated gelation time. Acrylamide has Schiff base bonds to cross-link with gelatin and absorb metal ions. Deacetylated chitosan improved cell adhesion and active groups to connect MXene and acrylamide. We constructed Mo2Ti2C3 MXene hydrogel with improved elastic modulus and viscosity, chemical cross-linking structure, electric conductivity, and good compatibility. Mo2Ti2C3 MXene hydrogel exhibits outstanding osteogenesis in vitro. Mo2Ti2C3 MXene hydrogel promotes osteogenesis via alkaline phosphatase (ALP) and alizarin red S (ARS) staining, improving osteogenic marker genes and protein expressions in vitro. Mo2Ti2C3 MXene hydrogel aids new bone formation in the in vivo calvarial bone defect model via micro-CT and histology. Mo2Ti2C3 MXene hydrogel facilitates neurogenesis factors nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) expression, and aids newly born neuron marker Tuj-1 and sensory neuron marker serotonin (5-HT) and osteogenesis pathway proteins, runt-related transcription factor 2 (Runx2), osteocalcin (OCN), SMAD family member 4 (SMAD4), and bone morphogenetic protein-2 (BMP2) in the bone defect repair process. Mo2Ti2C3 MXene hydrogel promotes osteogenesis and neurogenesis, which extends its biomedical application in bone defect reconstruction.

Flexible and crosslinking electrospun porous carbon nanofiber membranes as freestanding binder-free anodes for lithium-ion batteries

Electrospinning carbon nanofibers with one-dimensional nanostructures have enormous potential for electronic applications owing to their flexibility, electrical conductivity, high surface area and attractive structure. In this paper, porous nitrogen-doped carbon nanofiber membranes with chemical crosslinking structure (PNCNMs-CC) were prepared for lithium-ion batteries anodes by crosslinking treatment and carbonization process using polyimide as precursor, melamine as nitrogen source and ethyl orthosilicate as pore-forming agent. The heightened nitrogen content synergizes with the augmented specific surface area, engendering a striking electrochemical enhancement. Rich pore structure provides more active sites. When utilized as a binder-free, current collector-free anode, PNCNMs-CC shows an excellent rate performance and long cycle stability. The first discharge specific capacity of PNCNMs-CC delivered 710 mAh g−1 at 50 mA g−1. After 1000 cycles at a high current density of 1 A g−1, PNCNMs-CC shows 88 % capacity retention, demonstrating a promising candidate as the flexible anodes for high-performance energy storage devices.

A Novel Triple Crosslinking Strategy on Carbon Nanofiber Membranes as Flexible Electrodes for Lithium-Ion Batteries.

In order to solve the problem of low electrical conductivity of carbon nanofiber membranes, a novel triple crosslinking strategy, including pre-rolling, solvent and chemical imidization crosslinking, was proposed to prepare carbon nanofiber membranes with a chemical crosslinking structure (CNMs-CC) derived from electrospinning polyimide nanofiber membranes. The physical-chemical characteristics of CNMs-CC as freestanding anodes for lithium-ion batteries were investigated in detail, along with carbon nanofiber membranes without a crosslinking structure (CNMs) and carbon nanofiber membranes with a physical crosslinking structure (CNMs-PC) as references. Further investigation demonstrates that CNMs-CC exhibits excellent rate performance and long cycle stability, compared with CNMs and CNMs-PC. At 50 mA g−1, CNMs-CC delivers a reversible specific capacity of 495 mAh g−1. In particular, the specific capacity of CNMs-CC is still as high as 290.87 mAh g−1 and maintains 201.38 mAh g−1 after 1000 cycles at a high current density of 1 A g−1. The excellent electrochemical performance of the CNMs-CC is attributed to the unique crosslinking structure derived from the novel triple crosslinking strategy, which imparts fast electron transfer and ion diffusion kinetics, as well as a stable structure that withstands repeated impacts of ions during charging and discharging process. Therefore, CNMs-CC shows great potential to be the freestanding electrodes applied in the field of flexible lithium-ion batteries and supercapacitors owing to the optimized structure strategy and improved properties.

Iron polyphthalocyanine-derived ternary-balanced Fe3O4/Fe3N/Fe-N-C@PC as a high-performance electrocatalyst for the oxygen reduction reaction

The oxygen reduction reaction (ORR) is the cornerstone reaction of the cathode in metal-air batteries; however, slow kinetics requires high-performance catalysts to promote the reaction. Polyphthalocyanine (PPc) has a typical chemical cross-linking structure and uniformly dispersed metal active sites, but its poor activity and conductivity limit its applications as an ORR catalyst. Herein, a manageable and convenient strategy is proposed to synthesize ternary ORR catalysts through the low-temperature pyrolysis of FePPc. The optimal catalyst, Fe3O4/Fe3N/Fe-N-C@PC-2.5, exhibits excellent ORR activity in alkaline solution with a half-wave potential of 0.90 V, which is significantly higher than that of commercial 20% Pt/C (0.84 V). Electrochemical tests and extended X-ray absorption fine structure spectroscopy reveal that the superior ORR activity of Fe3O4/Fe3N/Fe-N-C@PC-2.5 could be ascribed to the balance of its ternary components (i.e., Fe3O4, Fe3N, and Fe-N4 species). A Zn-air battery incorporating Fe3O4/Fe3N/Fe-N-C@PC-2.5 as an air cathodic catalyst delivers a high open-circuit voltage and peak power density. During galvanostatic discharge, the battery demonstrates a specific capacity of 815.7 mA h g−1. The facile strategy of using PPc to develop high performance composite electrocatalysts may be expanded to develop new types of catalysts in the energy field.

NH2-UiO-66 coated fibers to balance the excellent proton conduction efficiency and significant dimensional stability of proton exchange membrane

The proton exchange membrane (PEM) with high sulfonation degree (DS) exhibits high proton conductivity but poor dimensional stability and mechanical properties. In order to improve dimensional stability and strength of PEM as well as maintaining excellent proton conductivity, we proposed to modify pre-oxidized polyacrylonitrile nanofibers (PPNF) through amino functionalization with NH2-UiO-66 (NU6). Compared with primary sulfonated poly (ether ether ketone) (SPEEK), the dimensional stability of 1.3% NU6 @PPNF-SPEEK composite membrane increase 30% due to the chemical crosslinking structure of PPNF. The acid-enriched layers, which provide an efficient proton transport channel, form along the fibers because of interaction between NU6 and -SO3H, since then composite membranes exhibit higher proton conductivity. Most importantly, this study firstly revealed the mechanism of formation process of sulfuric acid-rich layer through FTIR and EDS, which could provide a strategy for constructing and adjusting the proton transport channel efficiently. In summary, NU6@PPNF-SPEEK with excellent proton conductivity, outstanding dimensional stability and great mechanical properties in hydrated condition has been successfully fabricated.

Thermally Induced Switch of Coupling Reaction Using the Morphological Change of a Thermoresponsive Polymer on a Reactive Heteroarmed Nanoparticle.

Control of the cross-linking reaction is imperative when developing a sophisticated in situ forming hydrogel in the body. In this study, a heteroarmed thermoresponsive (TR) nanoparticle was designed to investigate the mechanism of controlling reactivity of the functional groups introduced into the nanoparticles. The coupling reaction was suppressed/proceeded by utilizing temperature-induced morphological changes of the TR polymer. The heteroarmed TR nanoparticle was prepared by the coassembly of amphiphilic block copolymers possessing both a TR segment and hydrophilic segment with reactive functional groups of succinimide. The longer TR chain on the nanoparticle covered the succinimide group and suppressed the reaction with the primary amine on the external nanoparticle. In contrast, the coupling reaction was promoted at a high temperature to create the chemical cross-linking structure between the nanoparticles because of the exposure of the succinimide group on the surface of the particle as a consequence of the morphological change of the TR polymer. In addition, the thermally controlled chemical reaction modulated initiation of the gelation using a highly concentrated nanoparticle solution. The heteroarmed TR nanoparticle offers great practical advantages for clinical uses, such as embolization agents, through precise control of the reaction.

Semi-interpenetrating network anion exchange membranes based on quaternized polyvinyl alcohol/poly(diallyldimethylammonium chloride)

The semi-interpenetrating network anion exchange membranes (AEMs) based on quaternized polyvinyl alcohol (QPVA) and poly(diallyldimethylammonium chloride) (PDDA) are synthesized. The chemical cross-linking structure is formed between hydroxyl groups of QPVA and aldehyde groups of glutaraldehyde (GA), which makes PDDA more stable embed in the QPVA matrix and also improves the mechanical properties and dimensional stability of AEMs. Due to the phase separation phenomenon of AEMs swelling in water, a microporous structure may be formed in the membrane, which reduces the transmission resistance of hydroxide ions and provides a larger space for the transfer of hydroxide ions, thus improving the conductivity. The ring structure of PDDA is introduced as a cationic group to transfer hydroxide ions, and shields the nucleophilic attack of the hydroxide ions through the steric hindrance effect, which improves alkaline stability. The hydroxide conductivity of semi-interpenetrating network membrane (QPVA/PDDA0.5-GA) is 36.5 mS cm−1 at 60 °C. And the membrane of QPVA/PDDA0.5-GA exhibits excellent mechanical property with maximum tensile strength of 19.6 MPa. After immersing into hot 3 mol L−1 NaOH solutions at 60 °C for 300 h, the OH− conductivity remains 78% of its initial value. The semi-interpenetrating network AEMs with microporous structure exhibit good ionic conductivity, mechanical strength and alkaline durability.

Synthesis of photocurable cellulose acetate butyrate resin for continuous liquid interface production of three-dimensional objects with excellent mechanical and chemical-resistant properties

Three-dimensional (3D) printing parts with excellent resolution and high performance are of great significance for scientific and engineering applications. In this study, a novel photocurable cellulose acetate butyrate (PC-CAB) resin was synthesized for continuous liquid interface production (CLIP) to construct 3D objects with high resolution, tailored mechanical properties, excellent chemical resistance and thermal stability. Particularly, the tensile and flexural strength of the CLIP 3D printed specimen could reach 44.67 and 64.53 MPa, respectively. Their solvent resistance against various organic solvents and strong acidic/basic solutions was evaluated. As expected, the 3D prints could well maintain their structural integrity and exhibited very low swelling ratios owing to the photo-induced chemical crosslinking structure. Notably, even after immersion in methylene chloride or 1.0 M acid/alkali for 3 h, the 3D prints still showed excellent mechanical and thermal properties. Further study demonstrated that when PC-CAB in the CLIP ink was optimized to 20 wt% while the photoinitiator (PI) was 0.5 wt%, complex-structured 3D printed objects with high surface quality could be obtained under specific printing parameters.

Toward Super-Tough Poly(l-lactide) via Constructing Pseudo-Cross-link Network in Toughening Phase Anchored by Stereocomplex Crystallites at the Interface

We demonstrated a novel strategy to toughen poly(l-lactide) (PLLA) by constructing pseudo-cross-link networks based on chain entanglements of long-chain branched structure in the toughening phase, which were anchored by stereocomplex (SC) crystallites at the interface. The formation of pseudo-cross-link network was achieved by simple blending of the copolymer of long-chain branched polycaprolactone and poly(d-lactide) (LB-PCL- b-DLA) with PLLA without introducing any chemical cross-linking structure or nonbiodegradable component. The microscopic morphology analysis suggests that the interface-formed SC crystallites not only enhanced the interfacial interaction between LB-PCL and PLLA but also obviously increased the matrix crystallization rate. Different from those blends without SC crystallites or long-chain branched structures, nano-microgels were observed in chloroform solution of the PLLA/LB-PCL- b-DLA blend, suggesting the formation of pseudo-cross-link network. The pseudo-cross-link network in LB-PCL toughening phase endows PLLA a significantly improved impact toughness (49.5 kJ/m2), which is almost 13 times than that of neat PLLA. Moreover, matrix crystallinity and spherulite size of the PLLA matrix also play significant roles in toughening. Only sufficiently crystallized PLLA with proper spherulite size can effectively trigger the matrix shear yielding, meanwhile, facilitate the energy dissipating.

Super toughened immiscible poly(l-lactide)/poly(ethylene vinyl acetate) (PLLA/EVA) blend achieved by in situ cross-linking reaction and carbon nanotubes

Toughening modification is always one of the main subjects of structural materials. A small quantity of dicumyl peroxide (DCP) and carbon nanotubes (CNTs) were simultaneously introduced into an immiscible poly(l-lactide)/poly(ethylene vinyl acetate) (PLLA/EVA) blend through melt compounding processing. The presence of chemical cross-linking structure in the polymer components and the physical network of CNTs were demonstrated through a series of direct and indirect characterization methods. The results demonstrated that DCP and CNTs exhibited synergistic toughening effects and the dynamically vulcanized blend composites exhibited excellent impact toughness. This work provides an alternative way to improve the impact toughness of an immiscible polymer blends.

Effect of physical and chemical crosslinking structure on fatigue behavior of styrene butadiene elastomer

ABSTRACTThe fatigue behavior of two styrene butadiene rubbers, thermoplastic elastomer (SBS) with physical crosslinking structure and vulcanized styrene butadiene rubber (SBR) with chemical crosslinking structure, was investigated. The fatigue lives of SBS and SBR were greatly affected by the fatigue frequency, amplitude, and temperature. The fatigue lives of both SBS and SBR decreased with the increasing fatigue frequency and amplitude. However, the effect of fatigue temperature on the fatigue life of SBS was different from that of SBR. With the increasing fatigue temperature, the fatigue life of SBS first decreased and then increased, however, the fatigue life of SBR showed continuous decreasing trend. The tensile strength of SBS first increased and then decreased with the increase of fatigue time, but the tensile strength of SBR was not significantly influenced by the fatigue time. The fatigue amplitude had greater effect on the tensile strength of SBS than that of SBR. The permanent deformation of both SBS and SBR increased with temperature; however, the parameter for the former rubber was higher than that of the latter. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40917.

Synthesis of chitosan networks: Swelling, drug release, and magnetically assisted BSA separation using Fe3O4 nanoparticles

Chitosan (CS) nanohydrogel networks were prepared by reaction with glyceroldiglycidylether (GDE) and poly(dimethylsiloxane) (PDMS), as crosslinking agents in an emulsion system. The nanogel content increased with increasing the amount of crosslinkers and reached to a maximum of 90% with GDE. The nanogels structure was characterized by FT-IR, AFM, DSC, and TGA. The average size for CS-GDE and CS-PDMS particles were 59nm and 180nm, respectively. The swelling behavior of nanohydrogels was observed to be dependent on pH, temperature, degree of crosslinking, and on the chemical structure of crosslinker. The equilibrium water content of CS-GDE nanohydrogels reached to a maximum of 600% at neutral pH, and decreased at high and low pH and low temperature. These nanohydrogels were tested for sodium diclofenac (SDF) loading and releasing efficiency. The covalent conjugation of bovine serum albumin (BSA) and magnetic Fe3O4 nanoparticles on the hydrogels were found to hold a potential application in magnetically assisted bioseparation.

Effect of Modification of Coal Powder Using Silane Coupling Agent on Mechanical Properties of SBS

Coupling agent γ-methacryloxypropyltrimethoxysilane（KH-570） was used as modifier to improve the superficial capacity of coal powder. Modified coal powder/thermoplastic butadiene-styrene rubber (SBS) composites were prepared by mixing procedure. The modified and unmodified coal powder and mechanical properties of composites were characterized and analyzed by Fourier Transform Infrared Spectroscopy (FTIR), thermogravimetric analysis (TGA), contact angle measuring instrument(CAMI), sedimentation test, rubber process analyzer (RPA) and scanning electron microscopy (SEM). Results showed that KH-570 can form chemical union with coal powder. The agglomeration of coal powder particles was effectively restricted after surface modification. The modified coal powder particles can be dispersed equally in rubber and form physical and chemical crosslinking structure with rubber.