Heat-induced Crosslinking Research Articles

Scalable phosphorylated cellulose production with improved environmental sustainability, crosslinkability and processability using 3D bioprinting for dye remediation

In the present work, phosphorylated cellulose (PC) gel has been produced following an environmentally benign approach using agro-based chemicals with improved yield. The PC gels produced were transparent, negatively charged with high consistency, charge content (1133.33 mmol/kg), degree of substitution (DS) of 0.183 and increased yield (>87 %). The XPS and EDS analysis confirms the covalently bonded phosphate groups at weight percent of 9.42 % and 11.01 %, respectively. The life cycle assessment (LCA) shows that PC gel production via the phosphorylation route is an ecologically favourable strategy compared with traditional TEMPO oxidation, resulting in 1.67 times lower CO2 emission. The rheological studies of PC gels show shear-thinning behaviour with improved 3D printability followed by heat-induced crosslinking of phosphate groups. The mechanistic insights for the condensation of phosphate to form a phosphoric ester group during cross-linking were evaluated through 31P solid-state NMR and XPS studies. Interestingly, the 3D-printed structures showed high structural stability under both compression and tensile load in both dry and wet conditions, with high water absorption (5408.33 %) and swelling capacity of 700 %. The structures show improved methylene blue (MB) remediation capabilities with a maximum removal efficiency of 99 % for 10–200 mg/L and more than seven times reusability. This work provides a green, facile and energy-efficient strategy for fabricating PCs with easy processability through additive manufacturing techniques for producing value-added products, opening up new avenues for high-performance applications.

Robust and Flame-Retardant Zylon Aerogel Fibers for Wearable Thermal Insulation and Sensing in Harsh Environment.

The exceptional lightweight, highly porous, and insulating properties of aerogel fibers make them ideal for thermal insulation. However, current aerogel fibers face limitations due to their low resistance to harsh environments and a lack of intelligent responses. Herein, a universal strategy for creating polymer aerogel fibers using crosslinked nanofiber building blocks is proposed. This approach combines controlled proton absorption gelation spinning with a heat-induced crosslinking process. As a proof-of-concept, Zylon aerogel fibers that exhibited robust thermal stability (up to 650°C), high flame retardancy (limiting oxygen index of 54.2%), and extreme chemical resistance are designed and synthesized. These fibers possess high porosity (98.6%), high breaking strength (8.6MPa), and low thermal conductivity (0.036 W m-1 K-1 ). These aerogel fibers can be knotted or woven into textiles, utilized in harsh environments (-196-400°C), and demonstrate sensitive self-powered sensing capabilities. This method of developing aerogel fibers expands the applications of high-performance polymer fibers and holds great potential for future applications in wearable smart protective fabrics.

Triggered Crosslinking of Main-Chain Enediyne Polyurethanes via Bergman Cyclization.

Crosslinking chemistries occupy an important position in polymer modification with a particular importance when triggered in response to external stimuli. Enediyne (EDY) moieties are used as functional entities in this work, known to undergo a pericyclic Bergman cyclization (BC) to induce a triggered crosslinking of polyurethanes (PU) via the intermediately formed diradicals. Diamino-EDYs, where the distance between the enyne-moieties is known to be critical to induce a BC, are placed repetitively as main-chain structural elements in isophorone-based PUs to induce reinforcement upon heating, compression, or stretching. A 7-day compression under room temperature results in a ≈69% activation of the BC, together with the observation of an increase in tensile strength by 62% after 25 stretching cycles. The occurrence of BC is further proven by the decreased exothermic values in differential scanning calorimetry, together with characteristic peaks of the formed benzene moieties via IR spectroscopy. Purely heat-induced crosslinking contributes to 191% of the maximum tensile strength in comparison to the virgin PU. The BC herein forms an excellent crosslinking strategy, triggered by heat or force in PU materials.

Hot ceramic lithography of silica-based ceramic cores: The effect of process temperature on vat-photopolymierisation

An enhanced-temperature vat photo-polymerisation of ceramic-loaded mixtures, or so called hot ceramic lithography, has been presented in this paper, for 3D printing silica-based ceramic cores used in the investment casting of hot section parts in aero and industrial turbines. The pre and post-sintering properties of the 3D printed ceramic parts highly depend on the polymerisation degree of the base resin binders and the rheological behaviour of ceramic slurry, and hot lithography can play a significant role both. First, the paper aimed to better understand how different printing temperatures (25,35,45 and 55 °C) would affect the properties of the base binder mixtures before loading ceramic particles, by conducting tensile, DMA and FTIR analysis on binder-only prints. The impact of elevated printing temperature on the mechanical properties of ceramic loaded binder mixtures was further investigated. The highest mechanical properties for the base binder and the ceramic loaded mixtures were achieved when the process temperature was set closer to the binder mixtures' initial glass transition temperature (when the binder is cured at room-temperature). This printing temperature is yet lower than the level causing heat-induced cross-linking yet high to enhance the mobility of monomers and active species, when compared with conventional room-temperature vat photopolymerisation printing. The results indicate that printing temperature, if set as described, can significantly impact the mechanical property of parts and conversion rate of the base binders for a successful print process. The second part of this study investigates the impact of temperature on the rheological behaviour and peeling forces of highly loaded ceramic-loaded mixtures (61.2 vol% of SiO 2 -ZrSiO 4 ). The suspension showed shear thickening behaviour at 25 °C, hindering the replenishment of the slurry during printing, while a high transition from shear-thickening to shear-thinning behaviour, ideal for flawless ceramic 3D printing, was observed at an optimum process temperature of 35 °C. When print temperature increases to 45 °C and then 55 °C, the viscosity of the mixture at very low shear rate (when the material is static during printing) increases, hence affecting peeling forces. Comparing peeling force measurements and FTIR results showed the higher cross-linking density, and the reactivity of the base binder, leads to a stronger adhesion between the cured layers and the printer's transparent film, hence higher separation/peeling forces. To sum up, hot ceramic lithography at the right printing temperature can resolve many print and post print issues, such as deformation and cracking, leading to a stronger and more dimensionally-accurate ceramic components.

Thermal treatment of dry zein to improve rheological properties in gluten-free dough

Zein is a gluten-free storage protein from corn that provides extensibility to starch-based doughs. However, zein does not confer a strong elastic behavior, a fundamental feature to produce high quality bread. Previous research showed that extrusion above 160 °C can improve zein dough viscoelasticity through the generation of a high molecular weight fraction. To determine whether thermal treatments alone could similarly improve zein function in doughs, dry zein was heated in a vacuum oven at temperatures of 160–200 °C. Gel electrophoresis of treated samples showed that thermal treatment promoted development of higher molecular weight zein fractions, which were resistant to dissolution in reducing conditions, presence of surfactant, and urea. Infrared spectroscopy also revealed that thermal treatments increased β-sheet content and decreased subsequent rearrangement following hydration. Zein hydrated with equal parts of water displayed increased elasticity when the zein was thermally treated at 160–190 °C. Doughs prepared with rice starch and zein treated at 160 °C exhibited improved strain-hardening behavior at higher extensional strain when compared to doughs prepared with unheated zein; no improvements were observed for higher temperature treatments. Electron microscopy revealed formation of discrete protein bodies among doughs prepared with zein treated at high temperatures, unlike fibrous strands observed for lower temperature treatment. Controlling heat-induced cross-linking and structural rearrangement of zein enhances its function in gluten-free doughs.

Needleless electrospinning of poly(acrylic acid) superabsorbent: Fabrication, characterization and swelling behavior

In this work, nanofibrous hydrogel has been fabricated from needleless electrospinning of Poly(acrylic acid) (PAA) in an aqueous solution with different concentrations. First, all solution samples were characterized for pH, surface tension, conductivity and viscosity. Next, electrospinnability of the PAA-water dope solution was investigated using the needleless electrospinning technique under constant conditions. Results indicated that the PAA-water solution was not electrospinnable. Therefore, the neutralization of carboxylic groups in the PAA chemical structure using the NaOH solution was investigated to enhance the PAA electrospinnability. Morphology observation revealed that the fiber diameters ranged from 40 to 250 nm and increased with increasing the solution concentrations. Increasing the neutralization degree (10%, 15% and 20% with 50 wt% NaOH solution) led to increase the dope viscosity and conductivity. The resultant nanofibers could be rendered water-insoluble by incorporating 1,4-butanediol diglycidyl-ether in the PAA-water dope solution, then heat-induced crosslinking was performed using a microwave at different curing times (1–5 min) and temperatures (45–105 °C). The nanofibrous hydrogel mat was then characterized by FTIR. The resulting nanofibrous hydrogel showed remarkable water absorption capacity up to 17,000% and 51,000% (within 15 min) in the standard saline solution and distilled water, respectively.

Waterborne Methacrylate-Based Vitrimers.

We demonstrate waterborne, unimolecularly dissolved vitrimer prepolymer systems that can be transferred into a vitrimer material using catalytic transesterification. The one-component prepolymer system can be processed via film casting and subsequent heat-induced cross-linking. A variation of the density of side chain hydroxy groups over ester and amide groups in the methacrylate/methacrylamide backbone, as well as of the Lewis acid catalyst loading, allow control of the extent of cross-linking and exchange rates. The increase of the amount of both catalyst and hydroxy groups leads to an acceleration of the relaxation times and a decrease of the activation energy of the transesterification reactions. The system features elastomeric properties, and the tensile properties are maintained after two recycling steps. Thus far, vitrimers have been limited largely to hydrophobic polymers; this system is a step forward toward waterborne, one-component materials, and we demonstrate its use in waterborne bioinspired nanocomposites.

Stable pH responsive layer-by-layer assemblies of partially hydrolysed poly(2-ethyl-2-oxazoline) and poly(acrylic acid) for effective prevention of protein, cell and bacteria surface attachment

Polyoxazolines have received increasing attention as low-fouling materials with good stability and ease of functional group incorporation. We investigated layer-by-layer (LbL) assembly of poly(2-ethyl-2-oxazoline) (PEOX) with poly(acrylic acid) (PAA) to incorporate PEOX into thin conformal coatings with controllable thicknesses ranging from the nano- to micron range. Partial hydrolysis of PEOX (to form PEOX-I) was used to introduce secondary amine groups that enable post-assembly multilayer stabilization by heat-induced crosslinking. While as-assembled multilayers dissolve in aqueous solutions at pH 5 and above, thermally crosslinked multilayers were stable against film loss and instead exhibit pH responsive swelling. The anti-fouling properties of crosslinked coatings were assessed by evaluating the resistance of PEOX-I containing multilayers to fouling by proteins, cells and bacteria. Our study of multilayers with thicknesses ranging from ∼12nm to ∼1.5μm revealed thickness dependence of surface fouling resistance to BSA. Crosslinked multilayers of ∼220nm were found to be highly effective in suppressing surface adsorption of bovine serum albumin (BSA), while thinner or thicker layers were increasingly susceptible to BSA adsorption. We further found that coatings of ∼220nm and above were all highly effective at preventing surface attachment of fibroblasts, gram-positive (S. aureus) and gram-negative (E. coli) bacteria.

Preparation of TiO2 nanoparticles modified electrospun nanocomposite membranes toward efficient dye degradation for wastewater treatment

New poly(vinyl alcohol)/poly(acrylic acid)/carboxyl-functionalized graphene oxide nanocomposite membranes were prepared using electrospinning technique with thermal treatment and modified with TiO2 nanoparticles. The thus prepared composite membranes are composed of polymeric nanofibers with carboxyl-functionalized graphene oxide sheets, which are anchored on the fibers by heat-induced crosslinking reaction. The preparation process of the designed nanocomposites is facile, eco-friendly, and controllable. The TiO2 nanoparticles were deposited uniformly and homogeneously onto the surface of the obtained composite membranes. The as-prepared composite membranes demonstrate efficient photocatalytic capacity toward dye degradation, which is primarily due to the specific surface area of electrospun membranes and the photoactivity of TiO2 nanoparticles. In addition, the composite membranes reported here are easy to regenerate. In comparison with other composite catalysts, the present preparation process is highly eco-friendly and facile to operate and regulate, which demonstrates potential large-scale applications in wastewater treatment and dye removal.

Bioinspired Polydopamine Sheathed Nanofibers Containing Carboxylate Graphene Oxide Nanosheet for High-Efficient Dyes Scavenger

New hierarchical bioinspired nanocomposite materials of poly(vinyl alcohol)/poly(acrylic acid)/carboxylate graphene oxide nanosheet@polydopamine (PVA/PAA/GO-COOH@PDA) were successfully prepared by electrospinning technique, thermal treatment, and polydopamine modification. The obtained composite membranes are composed of polymeric nanofibers with carboxylate graphene oxide nanosheets, which are anchored on the fibers by heat-induced cross-linking reaction. The preparation process demonstrate eco-friendly and controllable manner. These as-formed nanocomposites were characterized by various morphological methods and spectral techniques. Due to the unique polydopamine and graphene oxide containing structures in composites, the as-obtained composite demonstrate well efficient adsorption capacity toward dye removal, which is primarily due to the specific surface area of electrospun membranes and the active polydopamine/graphene oxide components. In addition, the composite membranes reported here are easy to re...

Key role of cysteine residues and sulfenic acids in thermal- and H2O2-mediated modification of β-lactoglobulin

Oxidation results in protein deterioration in mammals, plants, foodstuffs and pharmaceuticals, via changes in amino acid composition, fragmentation, aggregation, solubility, hydrophobicity, conformation, function and susceptibility to digestion. This study investigated whether and how individual or combined treatment with heat, a commonly encountered factor in industrial processing, and H2O2 alters the structure and composition of the major whey protein β-lactoglobulin. Thermal treatment induced reducible cross-links, with this being enhanced by low H2O2 concentrations, but decreased by high concentrations, where fragmentation was detected. Cross-linking was prevented when the single free Cys121 residue was blocked with iodoacetamide. Low concentrations of H2O2 added before heating depleted thiols, with H2O2 alone, or H2O2 added after heating, having lesser effects. A similar pattern was detected for methionine loss and methionine sulfoxide formation. Tryptophan loss was only detected with high levels of H2O2, and no other amino acid was affected, indicating that sulfur-centered amino acids are critical targets. No protection against aggregation was provided by high concentrations of the radical scavenger 5, 5-dimethyl-1-pyrroline N-oxide (DMPO), consistent with molecular oxidation, rather than radical reactions, being the major process. Sulfenic acid formation was detected by Western blotting and LC–MS/MS peptide mass-mapping of dimedone-treated protein, consistent with these species being significant intermediates in heat-induced cross-linking, especially in the presence of H2O2. Studies using circular dichroism and intrinsic fluorescence indicate that H2O2 increases unfolding during heating. These mechanistic insights provide potential strategies for modulating the extent of modification of proteins exposed to thermal and oxidant treatment.

Photoaging of human retinal pigment epithelium is accompanied by oxidative modifications of its eumelanin

Although photodegradation of the retinal pigment epithelium (RPE) melanin may contribute to the etiology of age-related macular degeneration, the molecular mechanisms of this phenomenon and the structural changes of the modified melanin remain unknown. Recently, we found that the ratio of pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) to pyrrole-2,3,5-tricarboxylic acid (PTCA) is a marker for the heat-induced cross-linking of eumelanin. In this study, we examined UVA-induced changes in synthetic eumelanins to confirm the usefulness of the PTeCA/PTCA ratio as an indicator of photo-oxidation and compared changes in various melanin markers and their ratios in human melanocytes exposed to UVA, in isolated bovine RPE melanosomes exposed to strong blue light and in human RPE cells from donors of various ages. The results indicate that the PTeCA/PTCA ratio is a sensitive marker for the oxidation of eumelanin exposed to UVA or blue light and that eumelanin and pheomelanin in human RPE cells undergo extensive structural modifications due to the life-long exposure to blue light.

Heat-Induced Cross-Linking and Degradation of Wheat Gluten, Serum Albumin, and Mixtures Thereof

Some wheat-based food systems, such as cakes, cookies, and egg noodles, contain mixtures of animal and plant (gluten) proteins and are processed under (mildly) alkaline conditions. Although changes in these proteins during processing can affect end product quality, they have seldom been studied. This study investigated protein cross-linking and degradation during heating (0-120 min, pH 8.0, 50-130 °C) of (mixtures of) wheat gluten and bovine serum albumin (BSA). The decrease in protein extractabilities in sodium dodecyl sulfate containing buffer under (non)reducing conditions and the levels of (cross-linked) amino acids were measured. No indications for polymerization at 50 °C were found. Below 100 °C, BSA polymerized more readily than wheat gluten. Above 100 °C, the opposite was observed. The kinetics of heat-induced polymerization of a 1:1 gluten-BSA mixture were similar to that of isolated gluten, implying that gluten decelerated BSA denaturation. Severe heating (130 °C, >15 min) induced degradation reactions in gluten but not in BSA. At all conditions used in this study, disulfide (SS) bonds contributed to the extractability loss. In addition, above 110 °C, β-elimination of cystine led to non-SS cross-links. Intramolecular SS bonds more often transformed in intermolecular non-SS bonds in BSA than in gluten.

A composite of quaternized and crosslinked poly(4-vinylpyridine) with processable polypyrrole for the construction of humidity sensors with improved sensing properties

Processable polypyrrole (PPy) was prepared by solution polymerization of pyrrole in the presence of poly(ionic liquid) (PIL) and subsequent anion exchange reaction. The obtained hydrophobic PIL–PPy showed good dispersion in organic solvents, and formed a composite with quaternized and crosslinked poly(4-vinylpyridine) (QC-P4VP) by simple solution mixing and heat treatment. The structure and morphology of the composite of QC-P4VP/PIL–PPy have been characterized by FT-IR and scanning electron microscopy. It was found that PIL–PPy particles were dispersed within the matrix of QC-P4VP. Thin film humidity sensors based on the composite were facilely prepared by dip-coating and heat-induced crosslinking and quaternization. Humidity sensing properties of the sensors were investigated at room temperature. It was revealed that the sensor based on the composite exhibited higher sensitivity (impedance change from 10 7 to 10 3 Ω over the range of 11–95% RH), better sensing linearity and much smaller hysteresis (∼1% RH) as compared to sensors based on QC-P4VP alone. Moreover, it was featured with fast response and good reproducibility. Attempts were made to explain the improved sensing properties of the composite.

Design of a stable and methanol resistant membrane with cross-linked multilayered polyelectrolyte complexes for direct methanol fuel cells

Sulfonated poly (arylene ether ketone) bearing carboxyl groups (SPAEK-C) membranes have been prepared as proton exchange membranes for applications in direct methanol fuel cells (DMFCs). Multilayered polyelectrolyte complexes (PECs) which applied as methanol barrier agents are prepared by alternate deposition of the oppositely charged amino-containing poly (ether ether ketone) (Am-PEEK) and the highly sulfonated SPAEK-C via a layer-by-layer method. The cross-linked PEC (c-PEC) is derived from a simple heat-induced cross-linking reaction between Am-PEEK and SPAEK-C. Fourier transform infrared spectroscopy confirms that Am-PEEK and SPAEK-C are assembled successfully in the multilayers. The morphology of the membranes is studied by scanning electron microscopy, which shows the presence of the thin layers coated on the SPAEK-C membrane. After PEC and c-PEC modification, the methanol permeability decreases obviously when compared to that of the pristine membrane. Notably, improved proton conductivities are obtained for the PEC modified membranes in comparison with the pristine membrane. Moreover, the selectivity of these modified membranes is one order of magnitude higher than that of Nafion 117. The thermal stability, oxidative stability, water uptake and swelling of PEC and c-PEC modified membranes are also investigated.

Controlling Volume Shrinkage in Soft Lithography through Heat-Induced Cross-Linking of Patterned Nanofibers

When poly(isopropylidene diallylmalonate) rich in threo-disyndiotactic sequences (st(rich)-2) was utilized as a cross-linkable ink for microcontact printing, the resultant submicrometer-scale patterns featuring 700 and 300 nm wide stripes were successfully insolubilized while maintaining their high dimensional integrity by heat-induced cross-linking with elimination of CO(2) and acetone. In sharp contrast, although the thermal properties and reactivities of a polymer rich in threo-diisotactic sequences (it(rich)-2) and a polymer having low stereoregularity (2(low)) are little different from those of st(rich)-2, the patterns printed with these reference polymers collapsed considerably upon heating as a result of a volume shrinkage effect. The striking difference between st(rich)-2 and the other two polymers most likely arises from the nanofiber-forming character of st(rich)-2, where the printed stripes are porous and much less affected by the volume shrinkage of individual nanofibers.

SHG and orientation phenomena in chromophore DR1-containing polymer films

Corona poling of two different methacrylate-type co-polymers with the chromophore Disperse Red 1 methacrylate (MDR1) results in the orientation of molecular dipoles and gives a nonlinear optical response in the form of second harmonic generation (SHG). The intensity of the SHG signal was used to measure the effectiveness of poling-induced alignments, which depend also on fundamental order phenomena in the co-polymers. The two matrix polymers varied in length and polarity of their side chains, which affect strongly side chain tacticity and chromophore aggregation. In situ SHG measurements during and after poling showed that the relaxation rate in methacrylic acid (MAA) co-polymers could be lowered by heat-induced cross-linking of the polymer chains. Order parameters derived from SHG measurements were found to be greater than 4, even when no changes were recorded in the optical spectra after poling. When changes were observed in the optical spectra, it was found that these were due to a decrease in the molar extinction coefficient for hydroxy-propyl-methacrylate (HPMA) based co-polymers, and solely due to the poling-induced decrease in film thickness in methacrylic acid based co-polymers (MAA).

Terminally-crosslinked sulfonated poly(fluorenyl ether sulfone) as a highly conductive and stable proton exchange membrane

Sulfonated poly(fluorenyl ether sulfone) (PFES) with a terminally-crosslinked network structure was prepared by heat-induced crosslinking of the allyl-terminated telechelic sulfone polymers using a bisazide. The crosslinked polymer membrane with a sulfofluorenyl moiety of 70% (PFES-70) showed excellent hydrolytic, dimensional, and mechanical stability. The crosslinked PFES-70 membrane revealed a proton conductivity of 0.04 S/cm at 20 °C, which increased significantly with increasing temperature. The conductivity of 0.38 S/cm at 100 °C for PFES-70 was higher than that of both non-crosslinked (0.33 S/cm) and Nafion® (0.17 S/cm). In addition, the crosslinked PFES-70 showed a methanol permeability of only 1.4% (3.9×10−8 cm2/s) compared to Nafion®.

Heat-induced cross-linking of nylon-like weak polyelectrolyte multilayer films: UV–visible studies of methylene blue loading and rates of release

Heat-induced cross-linking of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) films was studied as a potential method for controlling the rate of release of an absorbed dye, methylene blue (MB), from the film. Alternating layers of PAA and PAH films were prepared, loaded with MB and cured at different temperatures/times. Our studies identified the conditions best controlling both the amount of MB loading and its rate of release from PAA/PAH films to be curing at 150 °C for 10 min. In general, curing temperatures both above and below 150 °C showed slower rates of release as well as lower amounts of MB loading.

Low water swelling and high methanol resistant proton exchange membrane fabricated by cross-linking of multilayered polyelectrolyte complexes

Multilayered polyelectrolyte complexes (PECs) modified proton exchange membranes are successfully prepared from alternating deposition of oppositely charged polyelectrolyte chitosan (CS) and sulfonated poly (aryl ether ketone) bearing carboxyl groups (SPAEK-C) on the surface of Nafion ®117 via the layer-by-layer (LBL) method. The cross-linked PEC-coated Nafion ® membranes (c-PEC-coated Nafion ® membranes) are prepared from heat-induced cross-linking between CS and SPAEK-C via formation of amide bonds. Fourier transform infrared spectroscopy (FT-IR) verifies that cross-linking between ammonium groups of CS and carboxyl groups of SPAEK-C occurs by the formation of amide bonds. The morphology of the membranes is studied in detail by SEM. The c-PEC-coated Nafion ® membranes exhibit low water uptake (23.5%, 80 °C), water swelling (14.0%, 80 °C) and methanol permeability (2.7 × 10 −7 cm 2 s −1). Proton conductivity of the c-PEC-coated Nafion ® membranes increase at elevated temperature, reaching the value of 0.131 S cm −1 at 80 °C. These results show that cross-linking of layered CS/SPAEK-C films can effectively reduce water uptake, water swelling and methanol permeability while improve the dimensional stability and maintain high proton conductivity.