Low Degree Of Cross-linking Research Articles

Identifying the properties of epoxy composites filled with the solid phase of wastes from metal enterprises

The article addresses the issue related to the disposal of dust from steel industry as a reinforcing filler for epoxy composites. The polymer composition of welding that has been developed and studied includes epoxy dian oligomer, amine hardener and the filler – finely dispersed waste of metals. Polyethylene polyamine was used as a hardener in order to improve heat resistance and strength characteristics. Manganese triacetate was used in order to decrease the temperature and reduce the time of curing. The possibility was established to dispose of finely dispersed metal-containing waste from metallurgical production to be used a filler for epoxy composites of cold curing. It was revealed that the optimal content of dusts from foundries in the composite is at the level of 45–60 %. At this content, there is the highest impact resistance at the level of 40–50 MPa and a softening temperature in the range of 170–190 °С. It was established that at an increase in the amount of a filler from 40 % to 70 %, the cross-linking degree increases by 88 % to 98 %, respectively. However, at the content of the filler less than 45 % or exceeding 60 %, the impact resistance of the resulting composites decreases. At the content of a filler in the composite less than 45 %, the cause of low values of impact resistance and softening temperature could be the low cross-linking degree, less than 90 %. A decrease in these properties of composites at the content of the filler exceeding 60 % could be associated with the formation of a heterogeneous structure of filler. In the compositions with the highest performance characteristics, there is an optimized content of the filler and catalyst. Using a hardener and a curing catalyst in quantities of 3–3.5 and 1.5–2 %, respectively, makes it possible to shorten curing time by up to 2 hours. In general, the resulting epoxy composites are superior in their performance to known cold-curing analogs. The dependences of impact resistance, softening temperature, and cross-linking degree on the content of waste in the composite were derived, which make it possible to calculate the optimal formulation for composites depending on the required properties

A new approach for silane curing polyolefin elastomer/linear low density polyethylene blends by Monosil and Sioplas processes

Silane cross-linking of metallocene-based polyethylene-octene elastomer (POE)/linear low density polyethylene (LLDPE) blend was carried out using the single-step Monosil process and two-step Sioplas process in an industrial scale twin-screw extruder. The study revealed that benzoyl peroxide (BPO) is a better initiator than dicumyl peroxide (DCP) for grafting reaction of vinyl trimethoxysilane (VTMS). The optimum values of compression set, gel content and tear strength which were, respectively, found to be 40.4%, 79% and 22.6% obtained at 0.2 w% of BPO. Addition of 0.3 w% antioxidant cause the curing time decreased from 14 to 16 h to 8 h. Oit of the sample was increased linearly with increasing the antioxidant content up to 0.5 w%. The effect of dibutyltin dilaurate (DBTDL) as catalyst on the melt flow index (MFI) of the silane grafted compounds, with the aim of selecting an appropriate silane grafting process, was investigated. The results showed a MFI= 6.4 g/10 min for the silane grafted compound obtained from the Sioplas process, but, MFI= 1.9 g/10 min which was due to the presence of the catalyst in the Monosil process indicated that this method is not a promising process for silane grafting.HighlightsSilane crosslinking POE/LLDPE blend was carried out using the Monosil and Sioplas processes in a twin-screw extruder.BPO is better initiator for grafting reaction of VTMS than DCP.The curing time required for confirming hot set test was decreased from 14 to 16 h to 8 h by addition of 0.3 wt% antioxidant, Irganox 1010.OIT time increased linearly with increasing the antioxidant concentration.Sioplas process caused lower crosslinking degree as well as minimum pre-crosslinking, which prevented the formation of gell spots and rough surface.

Nanofiltration membranes with hydrophobic microfiltration substrates for robust structure stability and high water permeation flux

Traditional nanofiltration membranes (NFMs) suffer from ultrafiltration substrates with low porosity, small pore size and relatively poor solvent stability. Herein, NFMs have been fabricated on a series of hydrophobic polymer microfiltration substrates to address these issues. Polyphenol-based coatings of tannic acid/diethylenetriamine (TA/DETA) were co-deposited on the hydrophobic substrates to improve their surface wettability and to make them appropriate for interfacial polymerization. The spreading behaviors of aqueous solutions, which are of significant importance to the formation of defect-free polyamide layers, were directly visualized by laser confocal microscopy. The influences of TA/DETA coatings on the interfacial polymerization were further demonstrated by both dynamic molecular simulation and nanofiltration performance evaluation. The as-prepared NFMs exhibit higher water permeation flux compared with traditional ones because of the large pore size and high porosity of the microfiltration substrates, as well as the relatively low cross-linking degree of polyamide layers. Internal stress during the nanofiltration process was calculated by the theory of thin plates and the results claim good pressure resistance for these NFMs. Therefore, the as-prepared NFMs can be steadily used under high operation pressures even up to 0.9 MPa, which are in accordance with the theoretical calculation. Furthermore, these NFMs also present good solvent resistance since the chemical stability of the no-polar hydrophobic substrates.

The Influence of Experimental Factors on the Sorption of Vanillin by a Macroporous Highly Basic Anion Exchanger with a Low Degree of Cross-Linking

The influence of an initial form of a macroporous highly basic anion exchanger with a low degree of cross-linking on the sorption of vanillin has been studied and the effect of experimental factors on the shape of the output curve for the sorption has been determined. By varying different process parameters upon hydroxybenzaldehyde absorption by an OH-form AV-17-2P anion exchanger, it has been shown that the working and the total capacities of the sorbent depend on the concentration and flow rate of an initial solution in the studied range of these parameters: under dynamic conditions, the sorption capacity of the anion exchanger decreases as temperature is elevated. It has been found that the macroporous highly basic sorbent with a low degree of cross-linking may also be used for extraction of some other phenolic compounds under dynamic conditions.

Ambient pressure dried flexible silica aerogel for construction of monolithic shape-stabilized phase change materials

Flexible monolithic silica aerogel was prepared using an ambient pressure drying method without extra modifying and solvent-exchange process by controlling the crosslinking degree of the network and in-situ modification of methyl groups to the siloxane backbone. The introduction of the CH3-riched trimethylmethoxysilane (TMMS) molecules transforms silica clusters from originally rigid network structure with high crosslinking degree into flexible network structure with low crosslinking degree, not only improving the flexibility of the aerogel, but also endowing the hydrophobic property. The monolith feature, flexibility, hierarchical porous structure and hydrophobicity make the aerogel an ideal porous support for fabricating shape-stabilized phase change materials (PCMs). The obtained monolithic Paraffin (PA)/Aerogel composite shows high loading amount of PCMs, enhanced phase change enthalpy and good thermal stability. It is convenient to produce diverse shapes of monolithic composite PCMs to satisfy a range of specific requirements for energy storage applications.

Reuse of reclaimed tire rubber for gas-separation membranes prepared by hot-pressing

Abstract Waste tires are mainly composed of natural rubber, synthetic rubber, carbon black, steel wires, and fibers. Rubber, which is a valuable resource, is one of the raw materials used for membrane preparation. In this study, a convenient and environmentally friendly synthetic technology, hot-pressing method was developed for reusing reclaimed tire rubber to prepare gas-separation membranes. The effects of the preparation conditions, including hot-pressing temperature, pressure, and time on the membrane structure and separation performance, were investigated. Thermogravimetric analysis and field-emission scanning electron microscopy were carried out. The Thermogravimetric analysis, Fourier-transform infrared spectroscopy, and cross-linking degree results showed that the reclaimed tire rubbers used in this study consisted mainly of styrene-butadiene rubber and polybutadiene rubber. Reclaimed tire rubbers with low carbon contents and low degrees of cross-linking are beneficial for fabricating membranes by hot-pressing. The hot-pressing technique proposed in this study yielded dense gas-separation membranes from the reclaimed tire rubbers, proved to be an efficient approach for simplified recycling of waste tires and can in fact inspire the development of new recycling routes. The hot-pressed membranes derived from reclaimed tire rubber showed competitive results with a CO2/N2 selectivity of 12.8 and CO2 permeability of 161 Barrer.

Liquid-phase synthesis of butyl levulinate with simultaneous water removal catalyzed by acid ion exchange resins

The liquid-phase synthesis of butyl levulinate by esterification of levulinic with an excess of 1-butanol (initial molar ratio 1:3) and simultaneous water removal has been studied at atmospheric pressure and at the boiling point of the reacting medium. The catalytic performance of ten commercial sulfonic resins has been compared: four gel-type and six macroreticular. For both type of resins the levulinic acid conversion was complete after 4–6h and no byproducts derived from the acid were detected. Selectivity of 1-butanol towards the ester was about 95%, di-butyl ether and butenes being the detected 1-butanol-derived byproducts. Among the tested catalysts, gel-type resins with low crosslinking degree showed the highest activity, what could be attributed to a higher accessibility to active centers in polar medium. In additional experiments where initial reactants content was at stoichiometric relation, the levulinic acid conversion was lower (82–85%), while selectivity of 1-butanol towards the ester was slightly higher, because of the lower 1-butanol concentration.

Cross-Linked Polystyrene Shells Grown on Iron Oxide Nanoparticles via Surface-Grafted AGET-ATRP in Microemulsion.

Most applications of nanoparticles require robust stabilization, for example, by surface-bound ligands or the encapsulation within polymer shells. Furthermore, for biomedical applications, the particles must be dispersible in a complex biological environment. Thus, high-quality nanoparticles synthesized in organic solvents must be transferred into aqueous media. Here, we present a novel scalable method enabling the robust hydrophilic encapsulation of non-agglomerated nanoparticles by growing polystyrene shells via AGET-ATRP in microemulsion. To demonstrate this approach, we encapsulate iron oxide nanoparticles (diameter: 13.7 ± 0.6 nm). Because the ATRP initiator is grafted onto the nanoparticles' surface, the shells are covalently attached to the iron oxide cores. By varying the amount of monomers, the shell thickness can be adjusted precisely, as indicated by the increasing hydrodynamic size from ∼22 to 26 nm (DLS, number mean) with an increasing amount of added monomers. Moreover, the degree of cross-linking can be controlled by the amount of added divinylbenzene (DVB). To evaluate the robustness of the polymer shells against ion infusion, we introduce a novel colorimetric method, which is based on the formation of the red iron thiocyanate complex. After addition of HCl, the increase in absorbance at 468 nm indicates leaching of iron ions from the polymer-encapsulated core particles. These measurements confirm that with increasing shell thickness, significantly improved shielding is achieved. Furthermore, high concentrations of added DVB [33-50% (v/v) in a monomer mixture] improve the shielding effect. However, when smaller amounts of DVB were added [10-25% (v/v)], the shielding effect was diminished, even in comparison to non-cross-linked polymer shells. This finding suggests a higher porosity of shells with a low degree of cross-linking.

Interconnected Microdomain Structure of a Cross-Linked Cellulose Nanocomposite Revealed by Micro-Raman Imaging and Its Influence on Water Permeability of a Film.

Substantial adsorption of water vapor triggered by hydrogen-bonding interactions between water molecules and cellulose chains (or nanoplates) is hard to avoid in nanocomposite films, although the addition of nanoplates can improve the oxygen (or carbon dioxide) barrier property. In the present work, an effective strategy is raised to decline adsorption by weakening hydrogen-bonding interactions via chemical cross-linking by epichlorohydrin (ECH) without sacrificing the homogeneous dispersion of nanoplates. The generated microdomain structure of the chemical cross-linking reaction via ECH is explicitly revealed by micro-Raman imaging. Unambiguously, Raman maps of scanning elucidate the distribution and morphology of physical and chemical cross-linking domains quantitatively. The chemical cross-linking domains are nearly uniformly located in the matrix at a low degree of cross-linking, while the interconnected and assembled networks are formed at a high degree of cross-linking. ECH boosts the formation of chemical cross-linking microdomains, bringing out the terrific water vapor barrier property and alleviating the interfacial interactions in penetration, consequently magnifying the water contact angle and holding back the water vapor permeability. Our methodology confers an effective and convenient strategy to obtain remarkable water vapor-resistant cellulose-based films that meet the practical application in the packaging fields.

Aluminosilicate Nanotubes Embedded Polyamide Thin Film Nanocomposite Forward Osmosis Membranes with Simultaneous Enhancement of Water Permeability and Selectivity.

Nanocomposite membranes are strongly desired to break a trade-off between permeability and selectivity. This work reports new thin film nanocomposite (TFN) forward osmosis (FO) membranes by embedding aluminosilicate nanotubes (ANTs) into a polyamide (PA) rejection layer. The surface morphology and structure of the TFN FO membranes were carefully characterized by FTIR, XPS, FESEM and AFM. The ANTs incorporated PA rejection layers exhibited many open and broad “leaf-like” folds with “ridge-and-valley” structures, high surface roughness and relatively low cross-linking degree. Compared with thin film composite (TFC) membrane without ANTs, the TFN membrane with only 0.2 w/v% ANTs loading presented significantly improved FO water permeability, selectivity and reduced structural parameters. This promising performance can be mainly contributed to the special ANTs embedded PA rejection layer, where water molecules preferentially transport through the nanochannels of ANTs. Molecular dynamic simulation further proved that water molecules have much larger flux through the nanotubes of ANTs than sodium and chloride ions, which are attributed to the intrinsic hydrophilicity of ANTs and low external force for water transport. This work shows that these TFN FO membranes with ANTs decorated PA layer are promising in desalination applications due to their simultaneously enhanced permeability and selectivity.

Substrate viscosity plays an important role in bacterial adhesion under fluid flow

Many materials used in the medical settings such as catheters and contact lenses as well as most biological tissues are not purely elastic, but rather viscoelastic. While substrate elasticity has been investigated for its influence on bacterial adhesion, the impact of substrate viscosity has not been explored. Here, the importance of considering substrate viscosity is explored by using polydimethylsiloxane (PDMS) as the substrate material, whose mechanical properties can be tuned from predominantly elastic to viscous by varying cross-linking degree. Interfacial rheology and atomic force microscopy analysis prove that PDMS with a low cross-linking degree exhibits both low stiffness and high viscosity. This degree of viscoelasticity confers to PDMS a remarkable stress relaxation, a good capability to deform and an increased adhesive force. Bacterial adhesion assays were conducted under flow conditions to study the impact of substrate viscosity on Escherichia coli adhesion. The viscous PDMS not only enhanced E. coli adhesion but also conferred greater resistance to desorption against shear stress at air/liquid interface, compared to the PDMS with high crosslinking degree. These findings highlight the importance to consider substrate viscosity while studying bacterial adhesion. The current work provides new insights to an improved understanding of how bacteria interact with complex viscoelastic environments.

Highly enhanced adsorption of Congo red onto dialdehyde cellulose-crosslinked cellulose-chitosan foam

The adsorption of Congo red (CR) was evaluated using cellulose-chitosan foam crosslinked via dialdehyde cellulose (DAC). DAC-crosslinked cellulose-chitosan foam (CCLBD) was obtained by dissolution/regeneration using a LiBr aqueous solution, followed by crosslinking between chitosan and DAC. CCLBD possessed a three-dimensional structure with 40–200 nm wide pores composed of nanofibrils with a width of 10–20 nm, resulting in a high specific surface area of 230 m2/g. CCLBD was highly stable even acidic conditions in spite of a low crosslinking degree of 10.3%, which induced a slight reduction in the amino groups that interact with CR. CCLBD showed a CR adsorption capacity of 1548.2 mg/g and the adsorption process followed the Sips isotherm and pseudo-second-order models.

Changes of structural and physical properties of semi-gel from Alaska pollock surimi during 4 °C storage

This study compared the semi-gel system of Alaska pollock surimi formed at a low temperature with a heat-induced gel system. The cold semi-gel and heat-induced gel were found to have obvious differences in their degrees of protein unfolding, aggregation, and gel network structures. Microscopic observations and an analysis of their fractal dimensions revealed different degrees of compactness and uniformity in the two network structures. Rheological parameters, including the least permanent deformation, equilibrium stress, and frequency exponent, confirmed the non-total aggregation and lower degree of cross-linking in the cold semi-gel. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the myosin heavy chain (MHC) value gradually decreased, but the light meromyosin (LMM) value remained stable during cold storage. Furthermore, a decrease in the α-helix and an increase in the β-sheet and random coil values, as revealed by Fourier-transform infrared (FT-IR) spectroscopy, indicated that the proteins unfolded differently in the two gel systems. By comparing the times required for the cold semi-gel and heat-induced gel to form, we concluded that the formation process of heat-induced gel is intertwined while it is temporally separated in cold storage. Overall, myosin was selected as the starting point for establishing a schema chart to characterize the gelation processes of the cold semi-gel and heat-induced gel. These findings are helpful for understanding the gelation process of surimi in refrigerated storage.

Biomimetic microgels with controllable deformability improve healing outcomes.

Platelets mediate hemostasis by aggregating and binding to fibrin to promote clotting. Over time, platelets contract the fibrin network to induce clot retraction, which contributes to wound healing outcomes by increasing clot stability and improving blood flow to ischemic tissue. In this study, we describe the development of hollow platelet-like particles (PLPs) that mimic the native platelet function of clot retraction in a controlled manner and demonstrate that clot retraction-inducing PLPs promote healing in vivo. PLPs are created by coupling fibrin-binding antibodies to CoreShell (CS) or hollow N-isopropylacrylamide (NIPAm) microgels with varying degrees of shell crosslinking. We demonstrate that hollow microgels with loosely crosslinked shells display a high degree of deformability and mimic activated platelet morphology, while intact CS microgels and hollow microgels with increased crosslinking in the shell do not. When coupled to a fibrin-binding antibody to create PLPs, hollow particles with low degrees of shell crosslinking cause fibrin clot collapse in vitro, recapitulating the clot retraction function of platelets, while other particle types do not. Furthermore, hollow PLPs with low degrees of shell crosslinking improve some wound healing outcomes in vivo.

Morphology evolution and gelation mechanism of alkali induced konjac glucomannan hydrogel

A molecular-level mechanism of alkali induced konjac glucomannan (KGM) hydrogel gelation processing is considered with the application of nuclear magnetic resonance (NMR) spectroscopy and atomic force microscopy (AFM) as complementary methods to diffusive wave spectroscopy (DWS) microrheology and thermoanalysis. It is shown that deacetylation of KGM chains occurs immediately upon mixing with Na2CO3, inducing self-packaging. Partial unfolding of the packed loose structure of dehydrated KGM is observed upon heating. The configuration transition from random coils to self-assembling filament networks takes place before KGM aggregating to form large irreversible bundles with a lower degree of cross-linking. The gelation is not fulfilled until the temperature is increased to above 70 °C when the significant agglomeration is initiated among transitional fibrils to form junction zones essentially composed of acetyl-free portions. This suggests the intermolecular aggregation of KGM chains not simply regarding to hydrogen bonds, but essentially relating to hydrophobic interactions.

Characterization of thermally crosslinkable polyester films by thermomechanical analysis: a versatile and very sensitive technique for the evaluation of low crosslinking degree in polymers

AbstractThermomechanical analysis (TMA) is an effective technique for the characterization of small samples such as thin films of micrometric thickness, allowing the determination of thermal events such as glass transition, softening temperature and thermal expansion. Here we describe use of the TMA technique to investigate the drying and curing properties of polyester thin films ranging from 40 to 70 μm thick. The main purpose of this study was to investigate the effects of residual solvent and crosslinking on the glass transition and softening temperatures of the polyester films subjected to thermal cycling. Despite the fact that the analysed films were thinner than 100 μm, TMA proved to be very sensitive to small changes in residual solvent content and to the effects of the degree of crosslinking on the glass transition and softening temperatures of the polymer films. The most important finding is the possibility of detecting very small degrees of crosslinking in an easy, fast and quantitative way. Other methods such as swelling were not sensitive enough to differentiate the crosslinked samples and data for thicker films are not comparable to the data for thin films. © 2018 Society of Chemical Industry

Photocrosslinking of low band-gap conjugated polymers using alkyl chloride sidechains: Toward high-efficiency, thermally stable polymer solar cells

Abstract

Sesbania gum based hydrogel as platform for sustained drug delivery: An ‘in vitro’ study of 5-Fu release

The purpose of this study is to fabricate 5-Fluorouracil sustained release matrix based on a novel, nontoxic, eco-friendly modified biopolymer. The sesbania gum based hydrogel has been prepared by microwave assisted method using acrylamide as a monomer and N,N Methylenebisacrylamide as a crosslinker. The crosslink copolymerization has been confirmed by several modern techniques such as FTIR, SEM, XRD, TGA, DSC, elemental analysis etc. The bioactive 5-Fluorouracil has been encapsulated via solvent swelling method and its release rate has been investigated in various pH dissolution medium through USP standard protocol.The synthesized hydrogel with higher degree of crosslinking exhibited slower release rate than that of hydrogel having lower degree of crosslinking. Thus, resulting higher t25 value, the release rate increases with increase in pH of the medium. Release kinetics suggests the non-Fickian release behaviour of the hydrogel.

Scientific investigation into the water sensitivity of twentieth century oil paints

In order to develop a better understanding at the molecular level of water sensitivity in twentieth century oil paintings, water sensitive Winsor & Newton oil paint swatches and twentieth century oil paintings were characterised using gas chromatography–mass spectrometry (GC–MS) and direct injection electrospray ionisation mass spectrometry (ESI-MS), and the data were analysed using principal component analysis. Liquid chromatography coupled with tandem mass spectrometry based on quadrupole and time of flight mass detectors and electrospray interface (HPLC-ESI-Q-ToF) was also used to obtain a better insight into the molecular composition of a selection of samples. The study highlights a strong relationship between the molecular composition of the binding medium and the type of pigment present in the paint, which relates to water sensitivity. Consistently non-water sensitive lead white, titanium white, and zinc white paints [all containing zinc oxide] contained a relatively low proportion of extractable diacids, and a relatively high proportion of extractable short chain monoacids. These paints also contained a relatively low level of unsaturated and hydroxylated glycerides. Water sensitive iron oxide and ultramarine paints are associated with both a relatively high degree of oxidation and a high proportion of extractable diacids, as well as a relatively high content of unsaturated and hydroxylated glycerides. Water sensitive cadmium red, yellow and orange paints were generally not highly oxidised, but they also contained a relatively high content of unsaturated and hydroxylated glycerides. It is hypothesised that water sensitivity relates to a low degree of saponification and crosslinking and possibly, on the relative content of dicarboxylic acids.

Performance enhancement of polybenzimidazole based high temperature proton exchange membranes with multifunctional crosslinker and highly sulfonated polyaniline

Polybenzimidazole (PBI) based high temperature proton exchange membranes were prepared by covalent cross-linking with triglycidylisocyanurate (TGIC) and doping with highly sulfonated polyaniline (SPAN). Insoluble SPAN with high degree of sulfonation was synthesized as the proton conductor. SPAN was homogeneously dispersed in the PBI-TGIC matrix. The composite membranes presented good thermal, dimensional, mechanical and oxidative stability for fuel cell applications. Since TGIC was a tri-functional cross-linker, with a relatively low cross-linking degree (CLD), the mechanical strength, dimensional stability and oxidative resistance of the composite membranes were improved. The low CLD also allowed high doping level of SPAN and consequently high proton conductivity. The proton conductivity of PBI-TGIC(5%)/SPAN(50%) and PBI-TGIC(10%)/SPAN(50%) at 180°C and 100% RH was 0.13 and 0.12Scm−1, respectively. At 180°C and 50% RH, the proton conductivity of these two PEMs was 0.064 and 0.058Scm−1, respectively. At 180°C and 0% RH, the proton conductivity of them was 0.018 and 0.016Scm−1, respectively. The methanol resistance and membrane selectivity of PBI-TGIC/SPAN were also good and thus the membranes could be well applied in direct methanol fuel cells.