Variation Of Crosslink Density Research Articles

Crosslinked polymethacrylate absorbent with phenylalanine and stearate pendants

Herein, we report a new class of oil absorbents via conventional free radical polymerization method to prepare polymethacrylate gels with a range of crosslinking density having both phenylalanine and stearate moieties in the gel matrix. Analogue crosslinked gel was also synthesized with only stearate pendants without phenylalanine. Solvent absorbing capacity of gels was studied in various solvents with different dielectric constants. The mechanical property and morphology of the absorbents were studied by rheological method and field emission scanning electron microscopy, respectively. Modulation of porosity, absorption capacity and mechanical property of the absorbent were achieved with the variation of crosslinking density. The side chain pendants in the absorbent played an important role toward the solvent uptake capabilities. The absorbent readily can uptake 66 times chloroform and 14 times petrol of its dry weight at room temperature. Overall, the solvent uptake capacity of a new class of absorbents was well studied in this report.

4D Printing of a Sodium Alginate Hydrogel with Step-Wise Shape Deformation Based on Variation of Crosslinking Density

4D Printing of a Sodium Alginate Hydrogel with Step-Wise Shape Deformation Based on Variation of Crosslinking Density

Design and Preparation of Magnetism-Driven Intelligent Hydrogel Actuators

Abstract Novel kinds of magnetism-driven poly N,N-dimethylacrylamide bilayer intelligent hydrogels with various nanofibrillated cellulose (NFC) contents were prepared successfully via one-step insitu free radical polymerization. The bilayer hydrogels possessed high mechanical strength, efficient swelling and steady magnetic response. With the increase of nanofibrillated cellulose content, the crosslinking density of the hydrogels increased, leading to the decrease of swelling rate and increase of mechanical strength and swelling bending degree of hydrogel actuators, respectively. Fe3O4 particles existed tightly on the micropore surfaces of the hydrogels, which built the function base of magnetic response of hydrogel actuators. The addition of Fe3O4 was irrelevant to the variation of crosslinking density. The bilayer structure exhibited high bonding strength. Based on intelligent responsive properties, bilayer hydrogels were designed as soft magnetism-driven actuators, realizing capture and transportation properties and provided material candidates for soft robots.

The temperature dependent extraction of polysaccharides from eucheuma and the rheological synergistic effect in their mixtures with kappa carrageenan

Water extraction (at 60, 70, 80, and 90 °C) was applied to Eucheuma to explore the relationship between the extraction temperature and the structure of the extracted polysaccharides. The extracts mainly comprised iota carrageenan. The extraction temperature affected the compositions and properties of the polysaccharide extract. The yield decreased as the temperature increased (30.33–24.49%). Higher temperature extraction resulted in lower molecular height (60–90 °C: 0.57–0.48 nm) and layer height (60–90 °C: 0.76–0.58 nm) on atomic force microscopy images, leading to a better gel-forming potential; lower temperature extraction produced more diverse polysaccharides. 60 and 80 °C extracts were selected to represent low temperature extract and high temperature extract. Though the extracts had a weakness gelling ability, they cooperated with kappa carrageenan through synergistic effect. 80 °C extracts provided even higher synergistic effect than commercial IC (iota carrageenan) with stronger gel. It can be due to the amount of crosslink density variation. Possible schematic presentation was shown accordingly to illustrate the synergy.

Improved SiRNA Loading of Cationic Nanohydrogel Particles by Variation of Crosslinking Density

AbstractThis paper deals with a concept to improve the loading capability of cationic nanohydrogel particles with siRNA. For this purpose, a new tetrafunctional crosslinker is synthesized via a peptide coupling approach using lysine and spermine derivatives. Applying this four‐arm crosslinker to the particle synthesis makes it possible to perform the crosslinking with an excess or a deficit of the crosslinker. This allows for varying numbers of cationic groups per cationic core and its crosslinking density while the stability of the carrier system remains. The obtained cationic nanohydrogel particles are narrowly distributed in size as determined by dynamic light scattering measurements. Zeta potential measurements confirm the cationic nature of these carriers (ζ > +30 mV) and the ability to form complex anionic siRNA (agarose gel electrophoresis). As a result, it becomes possible to increase the siRNA loading by a factor of four by varying the composition of the crosslinked core. Such an increased siRNA loading should lead to an improved therapeutic gene knockdown effect.

Thermal Degradation and Water Uptake of Biodegradable Resin Prepared from Millet Flour and Wheat Gluten Crosslinked with Epoxydized Vegetable Oils

The degradation temperatures (DTs), heat stability (IPDT), degradation kinetics, and water uptake of epoxy resin were investigated using thermogravimetric analysis. Epoxy resins were prepared by crosslinking epoxydized oils with vital wheat gluten (VG) and millet flour. The reactions included three oils (cottonseed, sesame, and sunflower) and three levels of zinc chloride (ZC) (1, 2, and 3%). The apparent activation energy (Ea) was calculated using the Flynn–Wall–Ozawa method. The DT increased at higher heating rates within the same ZC level of the same oil type. Cottonseed oil exhibited the highest DT. The highest IPDT was 637°C of the sunflower oil/millet resin (3% ZC), and the least was the cottonseed/millet (1% ZC) at 479°C. The sesame-millet resin exhibited the highest Ea (622 KJ/mol) followed by sunflower-gluten (496 KJ/mol) and sesame-gluten (454 KJ/mol). The profiles of all resins point to a multistep degradation, but some of the profiles display two dominant kinetic processes, and the remaining resins showed three processes. The variation in crosslinking density between the oils is attributable to the different amounts of oxirane rings which are associated with the double bonds of the fatty acid of the oils. Like other parameters, the water uptake was affected by the ZC content, where most of the resins did not reach water uptake equilibrium. Nonetheless, the 3% ZC resin reached equilibrium after 5 days of immersion.

Thermo-oxidative ageing behavior of cerium oxide/silicone rubber

Cerium oxide/silicon rubber was prepared via mechanical blending. Mechanical and frictional properties, as well as thermal stability after thermo-oxidative ageing were investigated in this rubber composite. 3D surface profilometry, scanning electron microscopy (SEM) and thermogravimetry analysis (TGA) were used to study the friction surface characteristics, friction mechanism and thermal stability, respectively. Additionally, swelling experiments were carried out to investigate the variation of crosslinking density. After thermo-oxidative ageing, the tear strength of cerium oxide/silicon rubber decreases. However, in the early ageing stage, improvements in tensile strength, elongation at break, and frictional performance are caused by crosslinking density increments. Moreover, the addition of cerium oxide remarkably improves the re-crosslinking degree during ageing process, which in turn decreases the number of holes on the friction surface and endows the silicon rubber with better mechanical and frictional properties, as well as thermo-oxidative ageing resistance.

Cross-linking density and aging constitutive model of HTPB coating under prestrain thermal accelerated aging

In order to study the cross-linking density and aging constitutive relationship of HTPB coating during storage, the thermal accelerated aging tests at 0%, 3%, 6% and 9% prestrains were carried out. The cross-linking density of HTPB coating at different aging stages were tested using low-field 1H NMR and the variation of cross-linking density was analyzed. The aging model of cross-linking density considering the chemical aging and the physical stretching factors was established. The uniaxial tensile tests were carried out on HTPB coating at different aging stages and the cross-linking density was introduced into Ogden hyperelastic constitutive model as a characterization parameter of correction coefficient. Combined with uniaxial tensile test results, a prestrain aging constitutive model of HTPB coating was established. The results show that the cross-linking density of HTPB coating increases rapidly at first and then slowly with the increase of thermal accelerated aging time without prestrain. Under prestrain conditions, the cross-linking density of HTPB coating decreases at the early stage, and increases rapidly at first and then slowly at the middle and late stages of thermal accelerated aging. The correlation coefficients of aging model of cross-linking density and aging constitutive model with test results are R > 0.9500 and R > 0.9900 respectively, which can be used to accurately describe the cross-linking density and aging constitutive relationship of HTPB coating under prestrain accelerated thermal aging conditions.

Aging constitutive model of hydroxyl-terminated polybutadiene coating in solid rocket motor

To accurately describe the tensile mechanical properties of aged hydroxyl-terminated polybutadiene (HTPB) coating, the aging constitutive model was studied. The single-step and multi-step relaxation tests were performed on the unaged samples, and the tensile mechanical properties of the aged HTPB coating were tested, while the crosslink density was obtained by nuclear magnetic resonance (NMR) experiments. The model parameters were solved using the experimental data. The crosslink density was used to characterize the aging degree of the HTPB coating, and combined with the modified Arrhenius equation, a model of crosslink density variation with aging time was built. Multiply the hyper-elastic model with the aging characteristic function, an aging constitutive model of HTPB coating was established, which can be used to describe the tensile mechanical properties of aging HTPB coating. The verification tests show that the predicted value of the crosslink density under the test of 313.15 K is in good agreement with the test value. The aging constitutive model can predict the tensile mechanical behavior of HTPB coating well, which is of important engineering significance.

Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading

The aim of this paper is to demonstrate that the stress–strain behavior of natural rubber reinforced with short pineapple leaf fiber (PALF) can easily be manipulated by changing the cross-link density and the amount of carbon black (CB) primary filler. This gives more manageable control of mechanical properties than is possible with conventional particulate fillers alone. This type of hybrid rubber composite displays a very sharp rise in stress at very low strains, and then the stress levels off at medium strains before turning up again at the highest strains. The composites studied here contain a fixed amount of PALF at 10 part (by weight) per hundred rubber (phr) and varying carbon black contents from 0 to 30 phr. To change the cross-link density, the amount of sulfur was varied from 2 to 4 phr. Swelling ratio results indicate that composites prepared with greater amounts of sulfur and carbon black have greater cross-link densities. Consequently, this affects the stress–strain behavior of the composites. The greater the cross-link density, the less is the strain at which the stress upturn occurs. Variations in the rate of stress increase (although not the stress itself) in the very low strain region, while dependent on fillers, are not dependent on the crosslink density. The effect of changes in crosslinking is most obvious in the high strain region. Here, the rate of stress increase becomes larger with increasing cross-link density. Hence, we demonstrate that the use of PALF filler, along with the usual carbon primary filler, provides a convenient method for the manipulation of the stress–strain relationships of the reinforced rubber. Such composites can be prepared with a controllable, wide range of mechanical behavior for specific high performance engineering applications.

MRI monitoring and non-destructive quality measurement of polymeric patterns manufactured via stereolithography

The use of Magnetic Resonance Imaging (MRI) for monitoring, studying and performing output quality measurements of the acrylate-based polymeric patterns manufactured using stereolithography (SL) was introduced in this work. The effects of build parameters and humid environment on sample homogeneity, distribution of crosslink density, stability and defect formation were examined. The spatial resolution of the method was found to be sufficient to identify patterns according to the build parameters used and to detect specific hatch-predicted crosslink density variations. Qualitative information obtained using MRI visualisation was supplemented by quantitative measurements of Nuclear Magnetic Resonance (NMR) relaxation times and 1H NMR spectra. NMR spectroscopy confirmed the identity of the chemical composition among the patterns and showed that the crosslink density variation observed via spatially resolved T2-profiles stems from the difference of the build parameters. Different types of defects in the samples were observed and classified; some defects originated from local matrix continuity failures (partially cured resin trapping within the polymer or bubbles formation), while other defects were found in the form of bulk layering. MRI visualisation coupled with relaxometry and 1H spectroscopy of patterns during their interaction with humidity allowed tracking water distribution inside the sample and observing effects of swelling, fracturing and chemical decomposition. It was found that the initial inhomogeneous structure of the specimen has a crucial role in subsequent fracturing due to non-uniform expansion of the swollen parts. As a result, the approach presented in this work improves the output quality control and current testing techniques, provides insight how physical properties of the 3D parts are affected by different technical parameters, and eventually can help the use of SL technologies for a variety of applications.

Effects of alginate hydrogel cross-linking density on mechanical and biological behaviors for tissue engineering

An effective cross-linking of alginate gel was made through reaction with calcium carbonate (CaCO3). We used human chondrocytes as a model cell to study the effects of cross-linking density. Three different pore size ranges of cross-linked alginate hydrogels were fabricated. The morphological, mechanical, and rheological properties of various alginate hydrogels were characterized and responses of biosynthesis of cells encapsulated in each gel to the variation in cross-linking density were investigated. Desired outer shape of structure was maintained when the alginate solution was cross-linked with the applied method. The properties of alginate hydrogel could be tailored through applying various concentrations of CaCO3. The rate of synthesized GAGs and collagens was significantly higher in human chondrocytes encapsulated in the smaller pore structure than that in the larger pore structure. The expression of chondrogenic markers, including collagen type II and aggrecan, was enhanced in the smaller pore structure. It was found that proper structural morphology is a critical factor to enhance the performance and tissue regeneration.

Thermally Tailored Gradient Topography Surface on Elastomeric Thin Films

We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (L), resulting in a surface with a gradient topography with progressively varying feature heights (hF). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting.

Covalently cross-linked poly(acrylamide) brushes on gold with tunable mechanical properties via surface-initiated atom transfer radical polymerization

Versatile surface coatings, which exhibit anti-fouling properties and allow one to tune surface functionality and the state of internal (macro)molecular organization, are of central importance as platforms for the study of cell–surface interactions. To enable studies of various cell characteristics, such including cell migration, proliferation and adhesion, we investigated how the mechanical properties of poly(acrylamide) (PAAm) brushes on gold can be controlled by the varying the cross-link density. Brushes and covalently cross-linked hydrogel brushes were synthesized using self-assembled monolayers (SAMs) of ω-mercaptoundecyl bromoisobutyrate (MUBiB) on gold to initiate the surface-initiated atom transfer radical polymerization (SI-ATRP) of AAm with different fractions of (N,N′-methylenebis(acrylamide)) bisAAm in the feed. The polymer layers prepared with varied polymerization times or cross-linking densities were characterized by water contact angle measurements, grazing incidence reflection FTIR spectroscopy, ellipsometry and atomic force microscopy (AFM). It was found that during PAAm brush synthesis the film thickness increased markedly in early stages of the reaction, whereas after 60min a plateau of approximately 50nm (dry) film thickness was reached. The introduction of bisAAm into the feed resulted in reduced film thicknesses. AFM force–displacement measurements revealed that the variation of cross-linking density of polymer brush films influenced both the surface morphology and the films’ nanomechanical properties. The stiffness of the polymer brush films increased with increasing fraction of bisAAm, while the swelling ratio decreased. Hence the elastic modulus of the cross-linked PAAm brushes, as determined by AFM nanoindentation, could be varied between 20 and 700kPa, which will find application in the study of cell–surface interactions.

Electrical Treeing Characteristics in Polydimethylsiloxane-Based Organic-Inorganic Hybrid Materials

Electrical treeing characteristics of hybrid materials made from titanium alkoxide Ti(OCH2CH(C2H5)C4H9)4 and alkoxysilane-terminated polydimethylsiloxane (PDMS) in different TA/PDMS ratio was investigated. The development of AC tree of sample was mounted on a glass substrate with needle electrode was observed by CCD camera and computer. The tree inception voltage increased and tree growth speed decreased with increase in TA/PDMS ratio of the hybrid. It is suggested that the variation of crosslinking density and homogeneity of dispersion of silica clusters influenced on their tree propagation characteristic. We found that the hybrid made from the alkoxysilane-terminated PDMS and TA has good electrical insulating property compared with conventional PDMS-based hybrid materials and silicone rubber.

Biomimetic hydrogels gate transport of calcium ions across cell culture inserts

Control of the in vitro spatiotemporal availability of calcium ions is one means by which the microenvironments of hematopoietic stem cells grown in culture may be reproduced. The effects of cross-linking density on the diffusivity of calcium ions through cell culture compatible poly(2-hydroxyethyl methacrylate) [poly(HEMA)]-based bioactive hydrogels possessing 1.0 mol% 2-methacryloyloxyethyl phosphorylcholine (MPC), 5 mol% N,N-(dimethylamino)ethylmethacrylate (DMAEMA) and ca. 17 mol% n-butyl acrylate (n-BA) have been investigated to determine if varying cross-link density is a viable approach to controlling transport of calcium across hydrogel membranes. Cross-linking density was varied by changing the composition of cross-linker, tetraethyleneglycol diacrylate (TEGDA). The hydrogel membranes were formed by sandwich casting onto the external surface of track-etched polycarbonate membranes (T = 10 μm, φ = 0.4 μm pores) of cell culture inserts, polymerized in place by UV light irradiation and immersed in buffered (0.025 HEPES, pH 7.4) 0.10 M calcium chloride solution. The transport of calcium ions across the hydrogel membrane was monitored using a calcium ion selective electrode set within the insert. Degree of hydration (21.6 ± 1.0%) and void fraction were found to be constant across all cross-linking densities. Diffusion coefficients, determined using time-lag analysis, were shown to be strongly dependent on and to exponentially decrease with increasing cross-linking density. Compared to that found in buffer (2.0-2.5 × 10⁻⁶ cm²/s), diffusion coefficients ranged from 1.40 × 10⁻⁶ cm²/s to 1.80 × 10⁻⁷ cm²/s and tortuosity values ranged from 1.7 to 10.0 for the 1 and 12 mol% TEGDA cross-linked hydrogels respectively. Changes in tortuosity arising from variations in cross-link density were found to be the primary modality for controlling diffusivity through novel n-BA containing poly(HEMA)-based bioactive hydrogels.

Thermo-oxidative degradation of a carbon black compounded EPDM rubber hose

A method to measure the crosslink density of carbon black was used to analyze the aging behavior of a radiator hose made of carbon black filled EPDM (ethylene-propylene diene monomer) rubber under thermo-oxidative stresses. The crosslink density and the strength of the skin of the rubber specimens initially decreased slightly but then increased over time. The elongation at the break considerably reduced and did not show any similarity to the behaviors of the crosslink density. A possible cause of this reaction might be from the formation of rigid sulphoxide crosslinks and the crosslinks produced by the reaction of unvalcunized sulfurs. Aging the rubber specimens at 180°C caused a slight increase in the crosslink density and a large decrease in tensile strength and elongation. The aging experiment also caused a rapid increase of microhardness in the surface skin region. The formation of carbonyl groups in EPDM molecule chains and the formation of rigid sulphoxide crosslink, rather than the crosslink density variation, may have had a large influence. The changing behaviours of the mechanical properties, molecule chains and crosslinks showed significantly different characteristics in the skin and the interior with aging temperature and time.

Rapid Determination of Cure Rate and Direct Identification of Spatial Variations in Cross Link Density

Transition temperature microscopy (TTM) is a novel local thermal analysis technique that maps spatial variations in thermal properties on length scales from millimeters to nanometers. Traditional bulk thermal analysis provides a sample-averaged result and cannot generally supply sufficient information about complex structures or heterogeneities within polymeric systems. There currently exists a nanoscale thermal analysis (nanoTA) technique in which a nanoscale thermal probe heats a localized region on the sample surface to measure its thermal properties, including thermal transition temperatures like cure, crystalline melting points and glass transitions. TTM enables these nanoTA measurements to be carried out rapidly at a succession of points, thus creating automated high-resolution spatial maps of the thermal properties of a sample.

Variations in cross-link density with deposition pressure in ultrathin plasma polymerized benzene and octafluorocyclobutane films

Neutron reflectometry (NR) measurements of ultrathin films from octafluorocyclobutane (OFCB) and benzene (B) precursors deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) at two pressures (0.6 and 0.05 torr) reveal that under both deposition conditions there are a 7 nm-thick surface layer and an approximately 1 nm-thick transition layer next to the substrate which have structures different than those in the middle of the film. NR measurements of films swollen with solvent reveal that the density of cross-linking next to the substrate is lower than that in the middle of the film or the region adjacent to the surface of the film for both precursors. Variations in the cross-link density with processing pressure are much stronger for PP-B films than for PP-OFCB films.

Controlling of silver nanoparticles structure by hydrogel networks

Silver nanoparticles are the most widely used antibacterial agents with a number of advantages. The higher degree of biocompatibility and long-term antibacterial activity can be achieved with hydrogel-silver nanoparticles. In this work, a simple and facile synthetic strategy is developed to control the size and shape of the silver nanoparticles within the hydrogel networks. The variation in cross-link density of the polymer network has been found not only to control the size of the nanoparticles between 1 and 10nm, but it also regulates shape of the nanostructures such as nanorods, nanocubes, etc. This approach takes the advantage of the existing free-space between the networks of hydrogels that not only acts as a template for nucleation of particles but also provides long term stability. Further, nanoparticles can be recovered at any time from the hydrogel networks. These hybrid nanocomposites release nanoparticles with time which can eventually promote for antibacterial application. It can be inferred from the study that fine tuning of the hydrogel synthetic parameters will enhance the possibilities of desired nano-product tailor made for particular applications.