Cross-linking Reaction Time Research Articles

Cellulose Nanocrystals Crosslinked with Sulfosuccinic Acid as Sustainable Proton Exchange Membranes for Electrochemical Energy Applications.

Nanocellulose is a sustainable material which holds promise for many energy-related applications. Here, nanocrystalline cellulose is used to prepare proton exchange membranes (PEMs). Normally, this nanomaterial is highly dispersible in water, preventing its use as an ionomer in many electrochemical applications. To solve this, we utilized a sulfonic acid crosslinker to simultaneously improve the mechanical robustness, water-stability, and proton conductivity (by introducing -SO3−H+ functional groups). The optimization of the proportion of crosslinker used and the crosslinking reaction time resulted in enhanced proton conductivity up to 15 mS/cm (in the fully hydrated state, at 120 °C). Considering the many advantages, we believe that nanocellulose can act as a sustainable and low-cost alternative to conventional, ecologically problematic, perfluorosulfonic acid ionomers for applications in, e. fuel cells and electrolyzers.

Enhancing interface compatibility of UiO-66-NH2 and polyamide by incorporating dopamine into thin film nanocomposite membranes

The introduction of nanoparticles into nanofiltration (NF) membranes is an effective approach to improve the NF performance as the construction of water channels in the nanoparticles. However, the poor compatibility between polymer matrix and nanoparticles is a crucial issue that attracts numerous researchers. In this paper, dopamine (DA) molecule was introduced into the aqueous solution for preparing NF membranes by interfacial polymerization to build a bridge between polyamide matrix and nanoparticles (polydopamine coated UiO-66-NH2, U@PD) for its self-polymerization and high reactive activity of abundant of hydroxyl and amino groups. It was found that the addition of DA in aqueous reaction phase solution effectively reduced the membrane surface roughness, anchored more U@PD nanoparticles, improved the hydrophilicity of the membrane, and participated in the interfacial polymerization reaction to crosslink the nanoparticles and the polyamide matrix. The DA introduced TFN membrane achieved high water permeance (13.87 L·m−2·h−1·bar−1) and Na2SO4 rejection (97.96%) with DA concentration of 0.3 g/L, U@PD incorporating amount of 0.3 wt%, cross-linking reaction time of 45 min, and pH of 8.5. Additionally, the as-prepared membrane had an excellent selectivity of Cl−/SO42− (α = 45.85), which was higher than that of the membrane without DA crosslinking agent (α = 14.40). It is concluded that the addition of DA in the interfacial polymerization aqueous solution can effectively repair the defects between the nanoparticles and the polymer, thus provide a new idea for seawater desalination.

Preparation and Characterization of TPP-Chitosan Crosslinked Scaffolds for Tissue Engineering.

Scaffolds are three-dimensional porous structures that must have specific requirements to be applied in tissue engineering. Therefore, the study of factors affecting scaffold performance is of great importance. In this work, the optimal conditions for cross-linking preformed chitosan (CS) scaffolds by the tripolyphosphate polyanion (TPP) were investigated. The effect on scaffold physico-chemical properties of different concentrations of chitosan (1 and 2% w/v) and tripolyphosphate (1 and 2% w/v) as well as of cross-linking reaction times (2, 4, or 8 h) were studied. It was evidenced that a low CS concentration favored the formation of three-dimensional porous structures with a good pore interconnection while the use of more severe conditions in the cross-linking reaction (high TPP concentration and crosslinking reaction time) led to scaffolds with a suitable pore homogeneity, thermal stability, swelling behavior, and mechanical properties, but having a low pore interconnectivity. Preliminary biocompatibility tests showed a good osteoblasts’ viability when cultured on the scaffold obtained by CS 1%, TPP 1%, and an 8-h crosslinking time. These findings suggest how modulation of scaffold cross-linking conditions may permit to obtain chitosan scaffold with properly tuned morphological, mechanical and biological properties for application in the tissue regeneration field.

High-performance positively charged hollow fiber nanofiltration membranes fabricated via green approach towards polyethyleneimine layer assembly

In this study, high performance positively charged hyper-branched polyethyleneimine (PEI) nanofiltration (NF) layer was assembled successfully on negatively charged polyethersulfone/polyimide (PES/PI) hollow fiber ultrafiltration (UF) membranes under different conditions. In accordance with principles of green chemistry, glutaraldehyde (GA) as cross-linker and purely aqueous solutions were used as a less hazardous alternative compared to, e.g. trimesoyl chloride in an organic solvent. The effects of the surface roughness and charge of the substrate UF membranes, due to the presence of PI, and various fabrication conditions, such as pH of PEI aqueous solution, GA/PEI ratio and crosslinking reaction time, were investigated and discussed. Electron microscopy images revealed the successful assembly of the PEI NF layer at uniform coverage of the PES/PI UF membranes. It was found that the varied preparation conditions drastically affect the membrane surface hydrophilicity, surface zeta potential, permeation flux, and salt rejection. The membrane fabricated at optimum conditions had a molecular weight cut-off of ≤ 400 Da; steric hindrance and Donnan exclusion resulted to achieve salt rejections of 94.2% and 87.4% for MgCl2 and MgSO4, respectively. Moreover, fabricated membranes were tested through three cycles of six-hour filtrations and over 95% flux recovery after the filtration of salts via the backwashing process was recorded.

Protonic conductivity and thermal properties of cross-linked PVA/TiO2 nanocomposite polymer membranes

Nanocomposite polymer membranes based on PVA/TiO2 were prepared by a solution casting method. Glutaraldehyde solution (GA) was used as linking agent to improve the chemical, thermal and physical properties of the membranes. The degree of cross-linking was varied by changing the reaction time. The phase behavior of the membranes was examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). High resolution SEM micrographs show that the TiO2 nanoparticles are homogeneously dispersed, whilst the PVA crosslinks with the inorganic phase and fill in the gap between the nanoparticles. The ionic conductivity measurements were studied by impedance spectroscopy in the radio frequency range between 5 kHz to 5 MHz. Proton conductivity increases by several orders of magnitude with increasing cross-linking reaction time, reaching a maximum of 0.016 Scm−1 at 130 °C for the PVA/TiO2 composition of 1:12%, which was cross-linked for 42 h and then immersed in a 32 wt% KOH solution for 24 h. The ionic activation energy of the prepared membranes ranged from 0.038 KeV to 0.121 KeV. This result was carried out to obtain an estimation of the desorption time of water in the range from room temperature to the decomposition temperature around 500 °C.

Relationship between Cross-Linking Reaction Time and Anisotropic Mechanical Behavior of Enzyme Crystals

The application of protein particles as useful biocatalysts in the chemical or pharmaceutical industry is restricted due to their fragility and sensitivity to changes of environmental conditions. T...

Insect Cuticle-Mimetic Hydrogels with High Mechanical Properties Achieved via the Combination of Chitin Nanofiber and Gelatin.

By mimicking the natural sclerotization process of insect cuticles, a novel nanofiber-reinforced gelatin hydrogel was developed with improved mechanical properties, which was further strengthened through quinone cross-linking. Because quinone cross-linking reacts between amino groups by increasing the amino group content on the chitin crystalline surface through alkali treatment, surface-deacetylated chitin nanofibers (SD-ChNFs) were prepared to facilitate the cross-linking reaction between SD-ChNF and gelatin. This technique resulted in a tough hydrogel with a dark color. In comparison to a non-cross-linked version, the quinone-cross-linked SD-ChNF/gelatin hydrogel exhibited significantly improved tensile performance. Notably, by controlling the cross-linking reaction time from 6 to 48 h, the tensile strength of the quinone-cross-linked hydrogels can be modified and can reach as high as 2.96 MPa while displaying a variable brown color. Given the eco-friendly, biocompatible, and sustainable properties of chitin and gelatin, these bioinspired hydrogels provide potential applications in the agricultural and biomedical fields.

A facile preparation of positively charged composite nanofiltration membrane with high selectivity and permeability

An excellent positively charged nanofiltration (NF) membrane is of great significance for water softening. In this work, a novel positively charged composite PEI/TMA NF membrane was prepared by gradient cross-linking polyethyleneimine (PEI) with trimesic acid (TMA) on polyethersulfone ultrafiltration (PESU) membrane. The effects of concentration of PEI and TMA, the cross-linking reaction temperature and reaction time on membrane structure and performance were investigated in detail. The effects of PEI and TMA, reaction temperature and time on the membrane structure, composition and performance were investigated. The optimized preparation conditions of PEI/TMA NF membrane were fixed at 0.6% (m/v) PEI, 0.1% (m/v) TMA, 0.15% (m/v) SDS at 90 °C for 10 min. The as-prepared membrane exhibited a pure water flux of 19.1 L m−2 h−1 bar−1 and a rejection of MgCl2 93.0%, NaCl 38.2%, MgSO4 20.4%, and Na2SO4 8.8%. The functional layer is ultra-thin. Its high selectivity mainly results from Donnan effect. The long-term testing of our PEI/TMA NF membrane show a high stability. Compared with other positively charged membranes, the PEI/TMA NF membrane had unique advantages in the production process and a comprehensive performance.

Development of a Direct Compression Excipient from Epichlorohydrin-Crosslinked Carboxymethyl Rice Starch with Sodium Silicate Using a Coprocessing Technique

A new, free-flowing tablet disintegrant based on a coprocessed, cross-linked carboxymethyl rice starch (CXO) is developed using a combination of chemical and physical modifications. Cross-linked carboxymethyl starches are synthesized from an etherification between native rice starch and monochloroacetic acid and a cross-linking reaction with epichlorohydrin with various ratios of the etherification reaction time to the cross-linking reaction time (1:1, 1:0.67, and 1:0.33). The modified starches are then coprecipitated with 0–25% sodium silicate solution in methanolic solvent to obtain the CXOs. The physicochemical properties of the CXOs are characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffractometry, and differential scanning calorimetry. The solubility, swellability, moisture sorption capacity, and flowability parameters are also investigated and compared with those of native starch. A 10% sodium silicate solution is found to be best suited for coprocessing, yielding a modified starch (CXO-12) with excellent flowability and compressibility. The lubricant sensitivity ratio of CXO (0.13) is 4–6 times lower than those of sodium starch glycolate (SSG) and croscarmellose sodium (CCS), and its dilution potential (11.2% w/w) is the best among the three excipients. The disintegration times of the model drug tablets containing CXO (16.83 ± 3.19 s) are not significantly different from those containing SSG (14.00 ± 2.45 s) or CCS (14.00 ± 2.83 s). A dissolution study showed that the drug release is greater than 80% after 15 min in the medium. These results suggest that CXO could be applied as a free-flowing super disintegrant for direct compression processes.

Preparation of Zirconium Pillared Montmorillonite Composites

Zr-pillared montmorillonite (Zr-pmt) was obtained from which ZrOCl2.8H2O cross-linked Na-mt through Sol-gel means. Several important parameters of the preparation of Zr-pmt, such as the initial concentration of montmorillonite (mt), Zr/mt, aging time of the pillaring agent and cross-linking reaction time were investigated in detail to evaluate performances of Zr-pmt. Under the conditions of the montmorillonite initial concentration of 4%, Zr/mt of 7.5 mmol/g, aging time of the pillaring agent for 3 h and cross-linking reaction time of 6 h, the maximum basal spacing (d001) of Zr-pmt is 2.06 nm. Zr-pmt material is expected to be used for adsorption of wastewater.

Sucrose hydrolysis by invertase immobilized on Duolite A-568 employing a packed-bed reactor

ABSTRACTThe conversion of sucrose to a highly concentrated commercial syrup by immobilized invertase by combining the processes of adsorption and cross-linking using Duolite A-568 as the carrier was studied. Central Composite Design (CCD) was used to assess the effect of glutaraldehyde concentration and cross-linking reaction time on immobilized enzyme activity throughout the hydrolysis of sucrose in a batch reactor. Cross-linking optimization allowed us to find the optimum conditions for activity with a glutaraldehyde concentration of 0.6 g · L−1 and a cross-linking time of 6 h. The temperature and pH that maximized the activity of the immobilized biocatalyst in the cross-linking process were 50°C and 4.0, respectively. Cross-linking allows the biocatalyst to be active at higher temperatures and lower pH. High-sucrose conversions to invert sugar using a continuous fixed-bed reactor were obtained. The immobilized biocatalyst also demonstrated greater thermal stability at low temperatures.

The effects of thermally reversible agents on PVC stability properties

One kind of thermally reversible cross-linking agents for improving PVC thermally stability was synthesized. The chemical structure and thermally reversible characteristics of cross-linking agents were investigated by FTIR and DSC analysis, respectively. FTIR results confirmed that the cyclopentadienyl barium mercaptides ((CPD-C2H4S)2Ba) were successfully synthesized. DSC results showed it has thermally reversible characteristics and the depolymerization temperature was between 170 °C and 205 °C. The effects of cross-linking reaction time on gel content of Poly(vinyl chloride) compounds was evaluated. The gel content value arrived at 42% after being cross-linked for 25 min at 180 C. The static thermally stability measurement proved that the thermally stability of PVC compounds was improved.

Optimizing immobilization of avidin on surface-modified magnetic nanoparticles: characterization and application of protein-immobilized nanoparticles.

A simple optimization method of immobilization of avidin on magnetic nanoparticles (MNPs)' surface was proposed in this study. The avidin-immobilized MNPs were then developed and used to immobilize a model enzyme [Horseradish peroxidase (HRP)]. The loading capacity (LC) and activity of avidin-immobilized MNPs were optimized through selecting the most appropriate nanoparticle's size and shape, glutaraldehyde concentration, cross-linking reaction time, ultrasonic processing time, and initial concentration of avidin. The LC under optimized conditions was 63.37±1.29mg avidin/g MNPs, and the immobilized protein was still able to maintain its high biological activity of 10.86±0.13 U/mg (biotin-binding activity of nature avidin was 14.1U/mg) and better thermal stability compared to free avidin. A highly reusable, stable, and easily recovered immobilized HRP was obtained using MNPs as carriers. The immobilized HRP was reused repeatedly more than 9 times and retained more than 65% of its original activity.

Chlorine resistant binary complexed NaAlg/PVA composite membrane for nanofiltration

A composite nanofiltration (NF) membrane was prepared by coating a thin layer of sodium alginate (NaAlg)-polyvinyl alcohol (PVA) blend on the polysulfone support, then cross linked in two steps with calcium chloride and glutaraldehyde respectively. Structural and morphological characteristics of the NF composite membranes were determined by FT-IR and SEM. The performance of the membrane was evaluated in nanofiltration studies, and the effect on performance of experimental parameters including the NaAlg/PVA blend ratio, cross linking reaction time, membrane thickness and operating time was investigated. The permeance properties of the membrane were determined using 2000ppm solutions of NaCl and MgSO4. A water flux of 80L/m2.h with 46% salt rejection was observed for the monovalent salt solution. However, a decrease in flux to 68L/m2h with an increase in salt rejection to 80% was noticed for the divalent ions. The chlorine tolerance of the composite membrane was tested by exposing it to the NaOCl solution for a different time span. A slight change in water flux and salt rejection was observed, revealing a good stability of the membrane in the chlorine retaining environment. Hence, the prepared binary cross-linked NaAlg/PVA composite membrane can successfully be utilized for the purpose of nanofiltration and/or desalination.

Preparation of PVA Nanofiltration Membrane with Good Solvent Resistance

Polyvinylalcohol (PVA) with good solvent resistance was used in preparation of nanofiltration membrane. The preparation parameter such as PVA concentration, cross-linking agent concentration and reaction time are studied in detail. When the composite membrane was prepared under optimized conditions and tested at 0.4 MPa and 20 °C, the flux of the NF membrane was about 15 L/m2h and the Na2SO4 rejection of it was about 53.3%.

Template-Free Uniform-Sized Hollow Hydrogel Capsules with Controlled Shell Permeation and Optical Responsiveness

This study has established a robust and straightforward method for the fabrication of uniform poly(vinylamine) hydrogel capsules without using templates that combines the dispersion polymerization and the sequential hydrolysis/cross-linking. The particle sizes are determined by the degree of cross-linking as well as by the cross-linking reaction time, while the shell thickness is independent of these variables. Diffusion-limited reactions occur at the periphery of the particles, leading to the formation of hydrogel shells with a constant thickness. The treatment of the surfaces of hollow hydrogel capsules with oppositely charged biopolymers limits the permeability through the shell of species even with low molecular weights less than 400 g/mol. Furthermore, we demonstrated that the hydrogel shell phase decorated with Au nanoparticles can be optically ruptured by exposure to laser pulse, a feature that has potential uses in optically responsive drug delivery.

Performance of Water Absorption and Retention Material and Chemical Synthesis of Calcium Lignin Sulphonate with Acrylamide and Acrylic Acid

By graft copolymerization of calcium lignin sulphonate with acrylic acid and acrylamide,high water absorption and retention material was prepared. Effect of process parameters on graft copolymerization and sample properties was investigated by means of the single-factor method, and the microstructure of fracture surface of the water absorbent resin was analyzed by SEM. The result shows that: when mass percents of acrylic, acrylamide, calcium lignin sulphonate and crosslinking agent are 65.5%, 19.5%, 15%and 0.05%, respectively, and neutralization degree is 60% with 2h of initiation reaction time and 1.5h of crosslinking reaction time. The water absorption mean speed of material is up to 6g•g-1•h-1 and the highest water retention rate is up to 96.32%.The water absorption rate of material is up to 820g/g, and 0.9wt% NaCl solution absorption rate is up to 83g/g. The fracture surface of the water absorbent resin shows convexo-concave morphology which has lots of micropores.

Preparation of High Water Absorption and Retention Material by Graft Copolymerization of Calcium Lignin Sulphonate with Acrylic Acid and Acrylamide

High water absorption material was prepared by graft copolymerization of calcium lignin sulphonate with acrylic acid and acrylamide. Effect of process parameters on graft copolymerization and sample properties was investigated by means of the single-factor method, and the microstructure of fracture surface of the water absorbent resin was analyzed by SEM. The result shows that: when mass percents of acrylic, acrylamide, calcium lignin sulphonate, initiator and crosslinking agent are 65.5%, 19.5%, 15%, 0.09%, and 0.05%, respectively, and neutralization degree is 60% with 2h of initiation reaction time and 1.5h of crosslinking reaction time. The water absorption rate of material is up to 1010g/g, and 0.9wt% NaCl solution absorption rate is up to 110g/g.

Quantitatively controlled in situ formation of hydrogel membranes in microchannels for generation of stable chemical gradients

The in situ formation of membranes in microfluidic channels has been given attention because of their great potential in the separation of components, cell culture support for tissue engineering, and molecular transport for generation of chemical gradients. Among these, the porous membranes in microchannels are vigorously applied to generate stable chemical gradients for chemotaxis-dependent cell migration assays. Previous work on the in situ fabrication of membranes for generating the chemical gradient, however, has had several disadvantages, such as fluid leaking, uncontrollable membrane thickness, need of extra equipment, and difficulty in realizing stable interfacial layers. In this paper, we report a novel technique for the in situ formation of membranes within microchannels using enzymatically crosslinkable hydrogels and microfluidic techniques. The thickness of the membrane can be controlled quantitatively by adjusting the crosslinking reaction time and velocity of the microfluidics. By using these techniques, parallel dual hydrogel membranes were prepared within microchannels and these were used for the generation of stable concentration gradients. Moreover, the migration of Salmonella typhimurium was monitored to validate the efficacy of the chemical gradients. These results suggest that our in situ membrane system can be used as a simple platform to understand many cellular activities, including cell adhesion and migration directed by chemotaxis or complex diffusions from biological fluids in three-dimensional microstructures.

EVALUATION OF CROSS-LINKING TIME FOR POROUS GELATIN HYDROGELS ON CELL SHEET DELIVERY PERFORMANCE

To overcome the drawbacks posed by surgical manipulation of bioengineered corneal endothelial cell (CEC) sheets, a simple stirring process combined with freeze-drying method was recently developed for the production of cross-linked porous gelatin hydrogels that can provide the support structure and improve the aqueous humor circulation. In this study, we further evaluated the influence of cross-linking time (0–48 h) on the delivery performance of carbodiimide modified gelatin carriers. It was found that smaller pore size, lower porosity, and larger superficial area were associated with increasing extent of cross-linking of the carrier discs. Although the hydrogels treated for short reaction time (i.e., below 6 h) had low resistance to initial nutrient permeation, these materials exhibited rapid swelling, implying a potential anterior segment tissue squeezing effect for use as intraocular implants. In addition, the delivery carriers with limited extent of cross-linking degraded too fast to be effective for retention of cell sheet grafts at the site of injury. By contrast, the gelatin samples with cross-linking degrees greater than 50% showed slower degradation rates and smaller porous structure, thereby possibly causing a significant inhibition of CEC proliferation. Cell sheet transfer studies demonstrated that the carrier discs with a high cross-linking degree (59.4 ± 1.3%) were more difficult to achieve stable cell attachment than their counterparts with a low cross-linking degree (48.3 ± 1.5%). Our findings suggest that among the cross-linked porous samples studied, 12 h is the best cross-linking reaction time for preparation of cell sheet carriers with suitable delivery performance.