Effect Of Crosslink Density Research Articles

Green and sustainable carboxymethyl cellulose-chitosan composite hydrogels: Effect of crosslinker on microstructure

The toxicity level of conventional hydrogels is considerably high for most applications. To date, very few studies on hydrogels synthesized using only safe materials and simple, reproducible processes are available in the literature. Moreover, critical information on hydrogels hydrolytic behavior and stability—important parameters to determine microstructure—is missing. Herein, new composite hydrogel matrices are developed using green, sustainable materials through a simple, reproducible low-temperature chemical method. Natural polymers (carboxymethyl cellulose and chitosan), crosslinkers (fumaric acid, citric acid, tartaric acid and zinc ion), and solvent (water) are used. The hydrogels are characterized for degree of crosslinking and microstructure parameters calculated using Flory Rehner equation. Bonding strength and stability of the microstructure are further assessed. With respect to the hydrolytic properties, it was found that CMC~FU~CSN exhibits well-defined microstructure (155% swelling, 79% water absorbency and 82% gel fraction) superior to the other crosslinked hydrogels and the non-crosslinked composite, CMC~CSN. However, its gel fraction was within the same range as CMC~CSN (332% swelling, 91% water absorbency and 79% gel fraction). Other characterization results confirm that CMC~FU~CSN has the optimum microstructure (3 nm mesh size, 261 g/mol molecular weight between crosslinks and 0.01 μmol/cm3 effective crosslinking density), thermal stability (2% mass loss at 50 °C, 3% at 100 °C against 6% at 50 °C, 8% at 100 °C for CMC~CSN), semi-crystalline structure and chemical properties making it preferred over CMC~CSN as a promising green and sustainable hydrogel matrix.

Modeling the Effect of Cross-Link Density on Resins Catalytic Activities

Two mathematical models were coupled to quantitatively assess the effect of cross-link density on the catalytic activity of sulfonated styrene–divinylbenzene (DVB) resins: (I) a copolymerization mo...

Carboxymethyl cellulose-chitosan composite hydrogel: Modelling and experimental study of the effect of composition on microstructure and swelling response

Molecular recognition is essential for the advancement of functional supramolecular natural polymer-based hydrogels. First, a series of carboxymethyl cellulose (CMC)-chitosan (CSN) hydrogels crosslinked with fumaric acid are studied, where the influence of composition on microstructure and swelling is investigated using mathematical modelling and experiment and the hydrolytic properties, microstructure parameters and physicochemical properties are examined. Second, best fit values for the responses are obtained using multiple linear regression and MATLAB R2020a curve fitting and predictive models are generated. Third, the optimum microstructure is loaded with polyethylene glycol (PEG) and bismuth telluride (Bi2Te3) and coated on fabric for imparting thermal sensitivity. The results show that (1) optimum microstructure (25.65 ± 1.86 nm mesh size, 116.25 ± 0.00 μmol/cm3 effective crosslinking-density, 348.03 ± 10.81% swelling, and 62.86 ± 1.11% gel fraction) is found at CMC:CSN = 1:3 for G3; (2) the model shows good agreement with experimental data demonstrating potential for estimating hydrogel swelling and microstructure; and (3) G3/PEG and G3/PEG/Bi2Te3 enhance thermal conductivity of fabric at ambient, body, and elevated temperatures. The study demonstrates the potential of the generated model in predicting CMC-CSN swelling and G3 as an ideal host matrix for wearable textiles/devices.

Effect of Crosslink Density on Cut and Chip Resistance of 100% SBR Based Tire Tread Compound

The effect of Crosslink density on Cut and Chip resistance was affected on a typical 100 percent styrene-butadiene rubber (SBR)-based tire tread compound. In order to successfully develop products for tires used in off-road or poor roads and other demanding rubber applications, it is important to understand the C and C effect in rubber. Crosslink density varied by varying the sulphur to the accelerator ratio and also by changing the process aids. Basic polymer, filler and other ingredients such as activators and anti-degradants have remained unchanged. In the first setup, the sulphur was kept constant and the accelerator varied and the reverse was done in the second setup. It was made to achieve different crosslink density by changing the oil dosage and adding different resins. An attempt has been made to correlate Cut and Chip resistance to other physical properties. All these tests have been identified and optimized by the traditional tire tread compound.

Preparation and properties of the decomposable thermoreversible hydrogels based on novel dendritic crosslinkers derived from cystamine

Novel dendritic monomers derived from cystamine (Cy) were firstly synthesized in this research. Then, these monomers were used as crosslinkers to prepare three series of thermosensitive hydrogels with 2-(2-methoxyethoxy) ethyl methacrylate (MEMA). The effects of these crosslinkers on the hydrogel’s properties such as swelling behavior, mechanical properties, decomposability and thermoreversibility were investigated and compared with the hydrogel crosslinked by N,N′-bisacrylyl cystamine. The results showed that the equilibrium swelling ratio (SReq) of the hydrogel was affected by the hydrophilic group on the structure of the crosslinker and branch distribution of dendrimer crosslinker. More hydrophilic groups led to higher SReq; however, more complex branch distribution occupied the storage space of water and thus reduced the SReq. The prepared hydrogels all showed apparent thermoreversibility, and the critical gel transition temperature (CGTT) of these hydrogels increased with an increase of hydrophilic group in the crosslinker. Their CGTT showed a range from 26 °C (pure polymer) to 45 °C. More importantly, disulfide bonds in the synthesized hydrogel could be decomposed in the presence of cysteine. This result was confirmed by the change of swelling ratio and mechanical properties before and after they were treated with cysteine. The hydrogels decomposed by cysteine showed higher swelling ratio than the pristine hydrogels. And for mechanical properties, the shear modulus, Young’s modulus and effective crosslinking density of the decomposed hydrogels are lower than the pristine hydrogels. In summary, a thermoresponsive and cysteine-responsive hydrogel has been developed.

Mechanical and Degradation Properties of Hybrid Scaffolds for Tissue Engineered Heart Valve (TEHV).

In addition to biocompatibility, an ideal scaffold for the regeneration of valvular tissue should also replicate the natural heart valve extracellular matrix (ECM) in terms of biomechanical properties and structural stability. In our previous paper, we demonstrated the development of collagen type I and hyaluronic acid (HA)-based scaffolds with interlaced microstructure. Such hybrid scaffolds were found to be compatible with cardiosphere-derived cells (CDCs) to potentially regenerate the diseased aortic heart valve. This paper focused on the quantification of the effect of crosslinking density on the mechanical properties under dry and wet conditions as well as degradation resistance. Elastic moduli increased with increasing crosslinking densities, in the dry and wet state, for parent networks, whereas those of interlaced scaffolds were higher than either network alone. Compressive and storage moduli ranged from 35 ± 5 to 95 ± 5 kPa and 16 ± 2 kPa to 113 ± 6 kPa, respectively, in the dry state. Storage moduli, in the dry state, matched and exceeded those of human aortic valve leaflets (HAVL). Similarly, degradation resistance increased with increasing the crosslinking densities for collagen-only and HA-only scaffolds. Interlaced scaffolds showed partial degradation in the presence of either collagenase or hyaluronidase as compared to when exposed to both enzymes together. These results agree with our previous findings that interlaced scaffolds were composed of independent collagen and HA networks without crosslinking between them. Thus, collagen/HA interlaced scaffolds have the potential to fill in the niche for designing an ideal tissue engineered heart valve (TEHV).

Ambient temperature cross-linkable acrylic latexes: effect of cross-link density, glass transition temperature and application temperature difference on mechanical properties

This study focuses on the mechanical properties of ambient temperature cross-linkable acrylic latex polymers synthesized via the semibatch emulsion polymerization reaction. Various combinations of monomers including common acrylates and diacetone acrylamide (DAAM) were used to prepare functional polymers. DAAM cross-linkable monomer in 0.5, 1, 2, and 3 wt% of the monomers and theoretical glass transition temperatures at 5 °C and 15 °C (as the representative temperatures under the application temperature) were considered in designing different emulsions. Adipic dihydrazide was added to the emulsion as a cross-linking agent at the stoichiometric ratio. The mixtures were then applied on the plates and leave to dry at 25 °C. ATR-FTIR and NMR spectroscopies and dynamic light scattering (DLS) were used to characterize the latexes. DLS results revealed the storage stability of the mixtures composed of the latex and cross-linking agent in one pot even after 2 months. Mechanical performance of dry films was assessed through hardness and abrasion resistance measurements, dynamic mechanical thermal analysis, and stress–strain analysis. Mechanical assessments resulted that the higher DAAM percentage in polymer synthesis does not necessarily lead to mechanical improvement and the difference between ambient temperature and glass transition temperature (Ta–Tg) in the film formation process has more importance than the cross-link density of the dry films in the mechanical performance of the latex films.

Methanol degradation mechanisms and permeability phenomena in novolac epoxy and polyurethane coatings

On a global scale, methanol is one of the most important feedstocks and is used widely as solvent and co-solvent. However, due to the polar nature and associated ability to conduct current, the small molecule can take part in galvanic corrosion of metal storage tanks and degrade the barrier properties of protective coatings. In the present work, we investigated the degradation of two novolac epoxy coatings and a polyurethane (PU) coating exposed to methanol with the aim of quantifying the various degradation paths. Absorption and desorption rates were measured and the thermomechanical properties followed by dynamic mechanical analysis. For evaluation of the coating barrier properties (i.e., breakthrough time and steady state permeation rates of methanol), permeation cells were applied. During methanol absorption, simultaneous leaching of certain coating ingredients and bonding of methanol to the binder matrix via hydrogen bonds was evidenced. In terms of classification, the bonding of methanol took place by two types of mechanisms. In Type I, the methanol molecule forms a single hydrogen bond to the coating network, thereby acting as a plasticizer, which decreases the coating storage modulus and glass transition temperature. For Type II bonding of methanol, on the other hand, two hydrogen bonds to the coating network form per molecule, resulting in so-called physical crosslinking. The Type I mechanism boosted segmental mobility and contributed to the leaching of the plasticizer benzyl alcohol from the novolac epoxy coatings and residual solvents (i.e., naphtha and xylene) from the PU coating. Following the methanol desorption, and attributed to an increased effective crosslinking density from Type II bound methanol, the novolac epoxy and PU coatings exhibited significant increases in the glass transition temperatures. In addition, for the three coatings, a gradual decline in the permeability rate of methanol was observed over time. These enhanced (and unexpected) barrier properties result from a combination of effects ascribed to Type II bound methanol and the leaching process.

Versatile Synthetic Platform for Polymer Membrane Libraries Using Functional Networks

Poly(ethylene glycol) (PEG) diacrylate copolymer networks containing pentafluorophenyl active esters can be quantitatively substituted with a wide variety of primary amines, enabling the development of a versatile synthetic platform for the preparation of polymer hydrogel and membrane libraries. By tuning the starting network, a high degree of control over cross-linking density, water uptake, and functional group incorporation can be reproducibly achieved, which is vital for elucidating structure–property relationships in ion transporting membranes. From the same cross-linked scaffold, a diversity of basic, acidic, and solute-chelating moieties were obtained through functionalization, which allows for tailored uptake of basic and acidic organic dyes and metal chloride salts. Ion permeation and sorption measurements for a series of polymer networks with controlled cross-linking density and varied imidazole grafting densities illustrate the ability of this platform to isolate the effect of chemical functionality on ion transport from the effects of cross-linking density and water content.

Structures and impact strength variation of chemically crosslinked high-density polyethylene: effect of crosslinking density.

Impact strength of high-density polyethylene (HDPE), especially at low temperature, is crucial for its applications outdoors because of its poor impact strength. In order to improve the impact strength of HDPE, crosslinked HDPE was prepared by the addition of a peroxide crosslink agent, bis(tert-butyldioxyisopropyl)benzenehexane, and the effect of the crosslinking density on the microstructures and mechanical properties, especially impact strength between −60 °C and 23 °C, were investigated. The results show that the crosslinking density is controlled by varying the content of the crosslinking agent. It is found that, at room temperature, with increase in the content of crosslink agent from 0% to 0.5–0.7%, the impact strength increases from 4 kJ m−2 to about 80 kJ m−2 and the elongation at break increases from 20% to about 550%. With further increase in the content of crosslink agent to 1.5%, the impact strength and the elongation at break reduce to 64 kJ m−2 and 360% respectively. With increase in crosslink agent, the flexural modulus, yield strength, crystallinity, mean lamellar thickness, crystal size and spherulitic size and the brittle–ductile transition temperature (BDTT) decrease, and the gel content, impact strength of the HDPE at low temperature, intensity of β transition increase significantly. In considering both the room temperature mechanical properties and low temperature impact strength, the optimized content of the crosslink agent is about 0.7%. Overall, crosslinking significantly improves the toughness and impact strength of HDPE and extends its application, especially at low temperature.

Design of weakly cationic alco-cryogels consisting of poly(alkylmethacrylates)-based sulfonated terpolymer network with tunable multiresponsive assembly

Cryogels with ion-, and solvent-responsiveness have been designed by cryogelation of 2-(diethylamino)ethyl methacrylate (DEAEMA) as weakly cationic monomer, hydroxyethyl methacrylate as neutral comonomer and 2-acrylamido-2-methyl-1-propanesulfonic acid as anionic termonomer in ethanol-water mixture (50:50 v/v), which had been proved to offer a good miscibility of monomers. Influence of composition of terpolymer and polymerization temperature on the properties of terpolymer cryogels (Cgels) and hydrogels (Hgels) were evaluated by swelling kinetics, and response against rapid ionic strength change. Degree of swelling was extremely sensitive to salt concentration, cation/counterion type as well as the solvent type; acetone and monohydric alcohols with various hydrophobic alkyl groups; methanol, ethanol, and 2-propanol. Swelling of terpolymer networks in the form of Cgel and Hgel decreased with increase in the size of ions present in swelling medium and followed the order of swelling behavior Cl− > Br− > I−. Terpolymer alcogels exhibited salting-out behavior in the presence of Cl− and Br− ions, while “salting-in” behavior was observed in KI solutions of high concentrations. Sensitivity sequence of terpolymer alcogels to mono- and di-valent cations with a common anion (SO42−) on swelling was Na⁺ > K⁺ > Mg2⁺> Zn2+. Variation of elastic moduli with swelling in salt solutions was investigated and for terpolymer hydrogels, possible contribution of ionic crosslinks to effective crosslinking density νe was discussed. Increasing Na2SO4 concentration up to 0.1 M induced a 2-fold increase in the elastic modulus of terpolymer alcogels. With attention to the polarity of solvent, terpolymer gels exhibited an irregular swelling profile with a change in composition of solvent, showing the so called cosolvency phenomena. After two successive cycles of swelling and shrinking promoted by the change of ionic strength of KCl solution, the terpolymer gels were stable and dynamic swelling process was reversible. Swelling data agree well with second-order kinetic model and with increasing amount of DEAEMA in terpolymer feed, diffusion mechanism changed from non-Fickian type to Fickian diffusion. In ionic environments, the salt-sensitivity and excellent water absorption/retention capability give this intelligentized terpolymer gels wide potential applications.

Influence of coagents on Payne effect of butadiene rubber vulcanizates

Gum vulcanizates exhibit Payne effect and weak strain overshooting behavior, which is usually forgot in rubber industry. Herein the Payne effect of butadiene rubber (BR) vulcanizates is investigated for clarifying the effect of crosslinking density and sol content mediated by using coagent in conjugation with dicumyl peroxide. Toluene extraction and paraffin swelling of the vulcanizates are performed for clarifying the contribution of sol fraction and chains entanglement. The results show that the Payne effect and weak strain overshooting of gum vulcanizates are closely related to effective crosslinking density rather than network defects. This work is helpful for understanding the nonlinear rheological responses of the gum vulcanizates with defective network structure.

The Effect of Physical Aging on the Mechanical Properties of Raw, Treated and Compatibilized Coir Fibers-Based Polyisoprene Bio-Composites

Abstract The production of bio-composites based on vulcanized rubber reinforced by natural fibers has been widely studied. However, the aging phenomenon occurring during the long-term storage generating irreversible damage is limiting the performance of this kind of composites which has not been studied in details. So the objective of this work was to characterize and understand the structure-mechanical property relationships in these bio-composites and to correlate the effective crosslinking density with physical aging as a function of fiber content (0, 20, 30 and 40 wt.%) and fiber surface modification (alkali treatment) for the system polyisoprene (PI) and coir fibers. Furthermore, interfacial adhesion was improved by using maleic anhydride grafted polyisoprene as a coupling agent. From the samples produced, a complete set of morphological (scanning electron microscopy) and mechanical (shear and tension) characterization was performed before and after natural aging. The results showed that the tensile modulus increased with fiber content for the range of conditions studied and the presence of a coupling agent always gave higher tensile strength due to better interfacial stress transfer. However, important elasticity loss (strain at break) was observed due to the low fiber elasticity. Finally, different physical aging mechanisms led to changes in the molecular configuration and a reduction of free volume caused by a post-curing phenomenon of the polyisoprene matrix and low environmental stability of the fibers. These phenomena were shown to increase the Young’s modulus of the neat matrix by eight times compared to the unaged neat polyisoprene, but a smaller variation (five times) was observed for the bio-composites which was related to different crosslinking density and coir fibers state of dispersion in the vulcanized matrix

Attributing the crosslinking density to water vapor transmission rate of an acrylic-melamine automotive clearcoat

In this study, the effect of crosslinking density of an acrylic-melamine clearcoat on water vapor transmission rate was evaluated. Four types of acrylic resins with different OH contents were used as the main polymeric backbone of the acrylic-melamine clearcoat. Also, three different acrylics to melamine–formaldehyde ratios were used in the coating formulations to obtain various crosslinking densities. It was observed that other film properties such as surface hydrophilicity and molecular homogeneity had a crucial impact on the clearcoat water vapor transmission rate together with the crosslinking density. Surprisingly, results showed that samples with acrylic resin with less OH content and lower melamine–formaldehyde crosslinker had the lowest water vapor transmission rate among other samples. These results were in agreement with the water contact angle and the data deduced from dynamic mechanical thermal analysis (DMTA) results of clearcoats.

Cytoskeleton-inspired artificial protein design to enhance polymer network elasticity.

Reducing topological network defects to enhance elasticity in polymeric materials remains a grand challenge. Efforts to control network topology, primarily focused on crosslinking junctions, continue to underperform compared to theoretical estimations from idealized networks using affine and phantom network theories. Here, artificial protein technology was adapted for the design of polymer-network hydrogels with precisely defined coil-like and rod-like strands to observe the impact of strand rigidity on the mechanical properties of polymeric materials. Cytoskeleton-inspired polymer-network hydrogels incorporated with rod-like protein strands nearly tripled the gel shear elastic modulus and relaxation time compared to coil-like protein strands, indicating an enhanced effective crosslinking density. Furthermore, asymmetric rod-coil protein designs in network strands with an optimal rod:coil ratio improved the hydrogel relaxation time, enhancing the stability of physical macromolecular associations by modulating crosslinker mobility. The careful design of strand rigidity presents a new direction to reduce topological defects for optimizing polymeric materials.

Diffusion Behavior of Drug Molecules in Acrylic Pressure-Sensitive Adhesive.

Acrylic pressure-sensitive adhesive (PSA) is widely used in transdermal drug delivery systems, while the diffusion behavior of drug molecules in PSA is of great importance. In this paper, PSAs with different cross-link densities were prepared by adjusting the ratio of cross-linkers. The effects of cross-link density and temperature on the diffusion of drugs in PSA were investigated by Fourier transform infrared attenuated total reflectance and molecular dynamics simulation. The consistency between the experimental and simulation results demonstrated that molecular dynamics simulation could be used to predict the diffusion behavior of drugs in PSA. The results showed that free volume and the wriggling of polymer chains are positively related to the diffusion coefficient of drug molecules, while hydrogen bonds hinder drug diffusion.

Synthesis of degradable hyperbranched epoxy resins with high tensile, elongation, modulus and low-temperature resistance

Hyperbranched epoxy resins (HERs) have attracted much interest due to improving homogeneously the ductility and strength of popular diglycidyl ether of bisphenol-A (DGEBA), but the preparation of HERs/DGEBA composites with low-temperature resistance and rapid degradation is still a challenge in the sustainable development of thermoset field. Here we represent self-curable hyperbranched epoxy resins (DSEHP-n, n = 3, 5, 7, 11) to modify DGEBA. DSEHP-n not only increases simultaneously the tensile strength, flexural strength, impact strength, modulus, and elongation by 64.8%, 38.1%, 93.7%, 70.4%, and 50%, respectively, and but also improve remarkably the low-temperature resistance and degradation degree. The improvement of mechanical properties is caused by the combined effect of crosslinking density, free volume, intermolecular cavity, hyperbranched topological structure, and good compatibility, the account of an in-situ reinforcing and toughening mechanism which substantiated by dynamic light scattering (DLS), SEM, DMA, and free volume analysis. The detailed analysis of degradation products from GC-MS spectra showed that the degradation of the composites was attributed to the cleavage of the C–N and ester bonds. This paper will bring a novel method to prepare highly-efficient degradable thermosets and composites.

Silica-Filled Composites from Epoxidized Natural Oils

Thermosetting cast resins, filled with varying levels of silica, were prepared from epoxidized soybean oil (ESO) and epoxidized triolein (ETO) in order to study matrix-filler interactions and effects of cross-linking density on physical properties. While ESO is characterized by a highly heterogeneous composition, ETO had very regular structure but lower epoxy content. Room temperature ring opening polymerization initiated with butanediol was catalyzed by fluoroboric acid. Epoxy matrices were almost entirely made from natural oils, with cross-link densities correlated with epoxy oxygen content in the epoxidized oils. In spite of high cross-link density, glass transitions of ESO and ETO matrices were below room temperature, which was attributed partially to the plasticizing effect of “dangling chains” and side reactions (cyclization). Ground silica particles increased strength and modulus but did not shift glass transition in a predictable way. The impact of filler on mechanical properties was more pronounced in composites prepared with lower functionality ETO. These rapidly setting materials were prepared by a simple process of mixing and casting. The resulting inexpensive epoxy composites displayed relatively low polarity and low dielectric loss, particularly in the glassy state, and therefore may show utility in electrical insulation applications.

Effect of Loop Defects on the High Strain Rate Behavior of PEGDA Hydrogels: A Molecular Dynamics Study.

The high strain rate behavior of nonideal poly(ethylene glycol) diacrylate hydrogels under uniaxial tension and transient-state shear deformations is investigated using molecular dynamics (MD) simulations. This work specifically focuses on the influence of first-order loop defects, including their effect on topological evolutions. Two approaches are proposed to systematically introduce first-order loops, allowing separate and controllable investigations of effective cross-link functionality and cross-link density. MD simulations confirm that first-order loop defects are elastically inactive, but the topological disruptions caused by the presence of loop defects influence mechanical behavior. For decreasing effective cross-link functionality but constant cross-link density, a weaker tensile stress-strain response and decreasing shear-thickening behavior are observed. This is due to nonaffine translation of cross-link junction positions during deformation. Hydrogels with lower cross-link density but constant effective functionality show a stronger stress-strain response and an earlier transition between entropic and enthalpic deformation regimes. This behavior is correlated to changes in mesh size caused by the introduction of loops within an elastically active network. However, the resulting range of cross-link densities is not sufficient to cause measurable changes in shear-thickening behavior. To conclude, reductions in effective cross-link functionality are more important to high strain rate behavior than reductions in cross-link density.

Effect of crosslinking density on thermoresponsive nanogels: A study on the size control and the kinetics release of biomacromolecules

Thermoresponsive nanogels emerged as a robust and efficient platform for the delivery of a number of therapeutic agents. The need to deliver a wide range of biomolecules from different sizes requires a structural system with the ability to be tuned according to the specific applications. In this study, we design a library of nanogels using dendritic polyglycerol (dPG) as a crosslinker and poly (N-isopropyl acrylamide) as connecting thermoresponsive chains. Nanogels with diameters in a range of 70–210 nm were obtained by precipitation polymerization with low polydispersity. We provide a detailed study on how the flexibility of the network and size of the nanogels can be tuned by the modification of functionalities of the dPG, monomer feed and the size of the crosslinker. Additionally, we use different molecular weight proteins (6–66 kDa) as an indicator of the density of the structure by encapsulation and release experiments. We prove that the versatility of the system relays on the correct design of the nanogels and the careful selection of the starting materials and its final proportion in the reaction. By controlling the network density, a wider range of application can be achieved from a system with a short-term release triggered by temperature to a long-term release that could serve as reservoir platform.